602 lines
19 KiB
Plaintext
602 lines
19 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/array.sgml,v 1.34 2003/11/29 19:51:36 pgsql Exp $ -->
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<sect1 id="arrays">
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<title>Arrays</title>
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<indexterm>
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<primary>array</primary>
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</indexterm>
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<para>
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<productname>PostgreSQL</productname> allows columns of a table to be
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defined as variable-length multidimensional arrays. Arrays of any
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built-in type or user-defined type can be created.
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</para>
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<sect2>
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<title>Declaration of Array Types</title>
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<para>
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To illustrate the use of array types, we create this table:
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<programlisting>
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CREATE TABLE sal_emp (
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name text,
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pay_by_quarter integer[],
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schedule text[][]
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);
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</programlisting>
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As shown, an array data type is named by appending square brackets
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(<literal>[]</>) to the data type name of the array elements. The
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above command will create a table named
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<structname>sal_emp</structname> with a column of type
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<type>text</type> (<structfield>name</structfield>), a
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one-dimensional array of type <type>integer</type>
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(<structfield>pay_by_quarter</structfield>), which represents the
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employee's salary by quarter, and a two-dimensional array of
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<type>text</type> (<structfield>schedule</structfield>), which
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represents the employee's weekly schedule.
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</para>
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<para>
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The syntax for <command>CREATE TABLE</command> allows the exact size of
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arrays to be specified, for example:
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<programlisting>
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CREATE TABLE tictactoe (
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squares integer[3][3]
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);
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</programlisting>
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However, the current implementation does not enforce the array size
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limits --- the behavior is the same as for arrays of unspecified
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length.
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</para>
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<para>
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Actually, the current implementation does not enforce the declared
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number of dimensions either. Arrays of a particular element type are
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all considered to be of the same type, regardless of size or number
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of dimensions. So, declaring number of dimensions or sizes in
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<command>CREATE TABLE</command> is simply documentation, it does not
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affect runtime behavior.
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</para>
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<para>
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An alternative, SQL99-standard syntax may be used for one-dimensional arrays.
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<structfield>pay_by_quarter</structfield> could have been defined as:
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<programlisting>
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pay_by_quarter integer ARRAY[4],
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</programlisting>
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This syntax requires an integer constant to denote the array size.
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As before, however, <productname>PostgreSQL</> does not enforce the
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size restriction.
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</para>
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</sect2>
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<sect2>
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<title>Array Value Input</title>
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<indexterm>
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<primary>array</primary>
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<secondary>constant</secondary>
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</indexterm>
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<para>
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To write an array value as a literal constant, enclose the element
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values within curly braces and separate them by commas. (If you
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know C, this is not unlike the C syntax for initializing
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structures.) You may put double quotes around any element value,
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and must do so if it contains commas or curly braces. (More
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details appear below.) Thus, the general format of an array
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constant is the following:
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<synopsis>
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'{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
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</synopsis>
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where <replaceable>delim</replaceable> is the delimiter character
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for the type, as recorded in its <literal>pg_type</literal> entry.
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(For all built-in types, this is the comma character
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<quote><literal>,</literal></>.) Each
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<replaceable>val</replaceable> is either a constant of the array
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element type, or a subarray. An example of an array constant is
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<programlisting>
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'{{1,2,3},{4,5,6},{7,8,9}}'
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</programlisting>
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This constant is a two-dimensional, 3-by-3 array consisting of
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three subarrays of integers.
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</para>
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<para>
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(These kinds of array constants are actually only a special case of
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the generic type constants discussed in <xref
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linkend="sql-syntax-constants-generic">. The constant is initially
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treated as a string and passed to the array input conversion
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routine. An explicit type specification might be necessary.)
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</para>
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<para>
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Now we can show some <command>INSERT</command> statements.
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<programlisting>
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INSERT INTO sal_emp
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VALUES ('Bill',
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'{10000, 10000, 10000, 10000}',
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'{{"meeting", "lunch"}, {}}');
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INSERT INTO sal_emp
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VALUES ('Carol',
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'{20000, 25000, 25000, 25000}',
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'{{"talk", "consult"}, {"meeting"}}');
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</programlisting>
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</para>
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<para>
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A limitation of the present array implementation is that individual
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elements of an array cannot be SQL null values. The entire array
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can be set to null, but you can't have an array with some elements
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null and some not.
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</para>
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<para>
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This can lead to surprising results. For example, the result of the
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previous two inserts looks like this:
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<programlisting>
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SELECT * FROM sal_emp;
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name | pay_by_quarter | schedule
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-------+---------------------------+--------------------
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Bill | {10000,10000,10000,10000} | {{meeting},{""}}
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Carol | {20000,25000,25000,25000} | {{talk},{meeting}}
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(2 rows)
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</programlisting>
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Because the <literal>[2][2]</literal> element of
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<structfield>schedule</structfield> is missing in each of the
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<command>INSERT</command> statements, the <literal>[1][2]</literal>
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element is discarded.
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</para>
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<note>
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<para>
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Fixing this is on the to-do list.
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</para>
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</note>
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<para>
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The <literal>ARRAY</literal> expression syntax may also be used:
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<programlisting>
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INSERT INTO sal_emp
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VALUES ('Bill',
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ARRAY[10000, 10000, 10000, 10000],
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ARRAY[['meeting', 'lunch'], ['','']]);
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INSERT INTO sal_emp
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VALUES ('Carol',
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ARRAY[20000, 25000, 25000, 25000],
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ARRAY[['talk', 'consult'], ['meeting', '']]);
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SELECT * FROM sal_emp;
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name | pay_by_quarter | schedule
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-------+---------------------------+-------------------------------
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Bill | {10000,10000,10000,10000} | {{meeting,lunch},{"",""}}
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Carol | {20000,25000,25000,25000} | {{talk,consult},{meeting,""}}
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(2 rows)
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</programlisting>
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Note that with this syntax, multidimensional arrays must have matching
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extents for each dimension. A mismatch causes an error report, rather than
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silently discarding values as in the previous case.
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For example:
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<programlisting>
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INSERT INTO sal_emp
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VALUES ('Carol',
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ARRAY[20000, 25000, 25000, 25000],
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ARRAY[['talk', 'consult'], ['meeting']]);
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ERROR: multidimensional arrays must have array expressions with matching dimensions
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</programlisting>
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Also notice that the array elements are ordinary SQL constants or
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expressions; for instance, string literals are single quoted, instead of
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double quoted as they would be in an array literal. The <literal>ARRAY</>
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expression syntax is discussed in more detail in <xref
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linkend="sql-syntax-array-constructors">.
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</para>
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</sect2>
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<sect2>
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<title>Accessing Arrays</title>
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<para>
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Now, we can run some queries on the table.
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First, we show how to access a single element of an array at a time.
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This query retrieves the names of the employees whose pay changed in
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the second quarter:
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<programlisting>
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SELECT name FROM sal_emp WHERE pay_by_quarter[1] <> pay_by_quarter[2];
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name
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-------
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Carol
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(1 row)
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</programlisting>
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The array subscript numbers are written within square brackets.
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By default <productname>PostgreSQL</productname> uses the
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one-based numbering convention for arrays, that is,
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an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
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ends with <literal>array[<replaceable>n</>]</literal>.
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</para>
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<para>
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This query retrieves the third quarter pay of all employees:
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<programlisting>
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SELECT pay_by_quarter[3] FROM sal_emp;
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pay_by_quarter
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----------------
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10000
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25000
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(2 rows)
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</programlisting>
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</para>
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<para>
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We can also access arbitrary rectangular slices of an array, or
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subarrays. An array slice is denoted by writing
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<literal><replaceable>lower-bound</replaceable>:<replaceable>upper-bound</replaceable></literal>
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for one or more array dimensions. For example, this query retrieves the first
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item on Bill's schedule for the first two days of the week:
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<programlisting>
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SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
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schedule
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--------------------
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{{meeting},{""}}
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(1 row)
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</programlisting>
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We could also have written
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<programlisting>
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SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
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</programlisting>
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with the same result. An array subscripting operation is always taken to
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represent an array slice if any of the subscripts are written in the form
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<literal><replaceable>lower</replaceable>:<replaceable>upper</replaceable></literal>.
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A lower bound of 1 is assumed for any subscript where only one value
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is specified, as in this example:
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<programlisting>
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SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
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schedule
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---------------------------
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{{meeting,lunch},{"",""}}
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(1 row)
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</programlisting>
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</para>
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<para>
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The current dimensions of any array value can be retrieved with the
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<function>array_dims</function> function:
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<programlisting>
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SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';
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array_dims
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------------
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[1:2][1:1]
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(1 row)
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</programlisting>
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<function>array_dims</function> produces a <type>text</type> result,
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which is convenient for people to read but perhaps not so convenient
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for programs. Dimensions can also be retrieved with
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<function>array_upper</function> and <function>array_lower</function>,
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which return the upper and lower bound of a
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specified array dimension, respectively.
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<programlisting>
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SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol';
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array_upper
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-------------
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2
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(1 row)
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</programlisting>
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</para>
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</sect2>
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<sect2>
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<title>Modifying Arrays</title>
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<para>
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An array value can be replaced completely:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
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WHERE name = 'Carol';
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</programlisting>
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or using the <literal>ARRAY</literal> expression syntax:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000]
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WHERE name = 'Carol';
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</programlisting>
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An array may also be updated at a single element:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter[4] = 15000
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WHERE name = 'Bill';
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</programListing>
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or updated in a slice:
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<programlisting>
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UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
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WHERE name = 'Carol';
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</programlisting>
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</para>
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<para>
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A stored array value can be enlarged by assigning to an element adjacent to
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those already present, or by assigning to a slice that is adjacent
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to or overlaps the data already present. For example, if array
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<literal>myarray</> currently has 4 elements, it will have five
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elements after an update that assigns to <literal>myarray[5]</>.
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Currently, enlargement in this fashion is only allowed for one-dimensional
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arrays, not multidimensional arrays.
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</para>
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<para>
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Array slice assignment allows creation of arrays that do not use one-based
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subscripts. For example one might assign to <literal>myarray[-2:7]</> to
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create an array with subscript values running from -2 to 7.
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</para>
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<para>
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New array values can also be constructed by using the concatenation operator,
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<literal>||</literal>.
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<programlisting>
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SELECT ARRAY[1,2] || ARRAY[3,4];
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?column?
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-----------
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{1,2,3,4}
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(1 row)
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SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]];
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?column?
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---------------------
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{{5,6},{1,2},{3,4}}
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(1 row)
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</programlisting>
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</para>
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<para>
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The concatenation operator allows a single element to be pushed on to the
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beginning or end of a one-dimensional array. It also accepts two
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<replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
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and an <replaceable>N+1</>-dimensional array.
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</para>
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<para>
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When a single element is pushed on to the beginning of a one-dimensional
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array, the result is an array with a lower bound subscript equal to
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the right-hand operand's lower bound subscript, minus one. When a single
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element is pushed on to the end of a one-dimensional array, the result is
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an array retaining the lower bound of the left-hand operand. For example:
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<programlisting>
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SELECT array_dims(1 || ARRAY[2,3]);
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array_dims
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------------
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[0:2]
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(1 row)
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SELECT array_dims(ARRAY[1,2] || 3);
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array_dims
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------------
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[1:3]
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(1 row)
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</programlisting>
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</para>
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<para>
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When two arrays with an equal number of dimensions are concatenated, the
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result retains the lower bound subscript of the left-hand operand's outer
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dimension. The result is an array comprising every element of the left-hand
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operand followed by every element of the right-hand operand. For example:
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<programlisting>
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SELECT array_dims(ARRAY[1,2] || ARRAY[3,4,5]);
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array_dims
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------------
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[1:5]
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(1 row)
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SELECT array_dims(ARRAY[[1,2],[3,4]] || ARRAY[[5,6],[7,8],[9,0]]);
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array_dims
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------------
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[1:5][1:2]
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(1 row)
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</programlisting>
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</para>
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<para>
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When an <replaceable>N</>-dimensional array is pushed on to the beginning
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or end of an <replaceable>N+1</>-dimensional array, the result is
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analogous to the element-array case above. Each <replaceable>N</>-dimensional
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sub-array is essentially an element of the <replaceable>N+1</>-dimensional
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array's outer dimension. For example:
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<programlisting>
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SELECT array_dims(ARRAY[1,2] || ARRAY[[3,4],[5,6]]);
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array_dims
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------------
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[0:2][1:2]
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(1 row)
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</programlisting>
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</para>
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<para>
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An array can also be constructed by using the functions
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<function>array_prepend</function>, <function>array_append</function>,
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or <function>array_cat</function>. The first two only support one-dimensional
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arrays, but <function>array_cat</function> supports multidimensional arrays.
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Note that the concatenation operator discussed above is preferred over
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direct use of these functions. In fact, the functions are primarily for use
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in implementing the concatenation operator. However, they may be directly
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useful in the creation of user-defined aggregates. Some examples:
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<programlisting>
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SELECT array_prepend(1, ARRAY[2,3]);
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array_prepend
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---------------
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{1,2,3}
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(1 row)
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SELECT array_append(ARRAY[1,2], 3);
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array_append
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--------------
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{1,2,3}
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(1 row)
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SELECT array_cat(ARRAY[1,2], ARRAY[3,4]);
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array_cat
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-----------
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{1,2,3,4}
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(1 row)
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SELECT array_cat(ARRAY[[1,2],[3,4]], ARRAY[5,6]);
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array_cat
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---------------------
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{{1,2},{3,4},{5,6}}
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(1 row)
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SELECT array_cat(ARRAY[5,6], ARRAY[[1,2],[3,4]]);
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array_cat
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---------------------
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{{5,6},{1,2},{3,4}}
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</programlisting>
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</para>
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</sect2>
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<sect2>
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<title>Searching in Arrays</title>
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<para>
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To search for a value in an array, you must check each value of the
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array. This can be done by hand, if you know the size of the array.
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For example:
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<programlisting>
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SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
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pay_by_quarter[2] = 10000 OR
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pay_by_quarter[3] = 10000 OR
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pay_by_quarter[4] = 10000;
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</programlisting>
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However, this quickly becomes tedious for large arrays, and is not
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helpful if the size of the array is uncertain. An alternative method is
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described in <xref linkend="functions-comparisons">. The above
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query could be replaced by:
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<programlisting>
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SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);
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</programlisting>
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In addition, you could find rows where the array had all values
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equal to 10000 with:
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<programlisting>
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SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);
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</programlisting>
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</para>
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<tip>
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<para>
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Arrays are not sets; searching for specific array elements
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may be a sign of database misdesign. Consider
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using a separate table with a row for each item that would be an
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array element. This will be easier to search, and is likely to
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scale up better to large numbers of elements.
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</para>
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</tip>
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</sect2>
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<sect2>
|
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<title>Array Input and Output Syntax</title>
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<para>
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The external text representation of an array value consists of items that
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are interpreted according to the I/O conversion rules for the array's
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element type, plus decoration that indicates the array structure.
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The decoration consists of curly braces (<literal>{</> and <literal>}</>)
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around the array value plus delimiter characters between adjacent items.
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|
The delimiter character is usually a comma (<literal>,</>) but can be
|
|
something else: it is determined by the <literal>typdelim</> setting
|
|
for the array's element type. (Among the standard data types provided
|
|
in the <productname>PostgreSQL</productname> distribution, type
|
|
<literal>box</> uses a semicolon (<literal>;</>) but all the others
|
|
use comma.) In a multidimensional array, each dimension (row, plane,
|
|
cube, etc.) gets its own level of curly braces, and delimiters
|
|
must be written between adjacent curly-braced entities of the same level.
|
|
You may write whitespace before a left brace, after a right
|
|
brace, or before any individual item string. Whitespace after an item
|
|
is not ignored, however: after skipping leading whitespace, everything
|
|
up to the next right brace or delimiter is taken as the item value.
|
|
</para>
|
|
|
|
<para>
|
|
As shown previously, when writing an array value you may write double
|
|
quotes around any individual array
|
|
element. You <emphasis>must</> do so if the element value would otherwise
|
|
confuse the array-value parser. For example, elements containing curly
|
|
braces, commas (or whatever the delimiter character is), double quotes,
|
|
backslashes, or leading white space must be double-quoted. To put a double
|
|
quote or backslash in a quoted array element value, precede it with a
|
|
backslash.
|
|
Alternatively, you can use backslash-escaping to protect all data characters
|
|
that would otherwise be taken as array syntax or ignorable white space.
|
|
</para>
|
|
|
|
<para>
|
|
The array output routine will put double quotes around element values
|
|
if they are empty strings or contain curly braces, delimiter characters,
|
|
double quotes, backslashes, or white space. Double quotes and backslashes
|
|
embedded in element values will be backslash-escaped. For numeric
|
|
data types it is safe to assume that double quotes will never appear, but
|
|
for textual data types one should be prepared to cope with either presence
|
|
or absence of quotes. (This is a change in behavior from pre-7.2
|
|
<productname>PostgreSQL</productname> releases.)
|
|
</para>
|
|
|
|
<note>
|
|
<para>
|
|
Remember that what you write in an SQL command will first be interpreted
|
|
as a string literal, and then as an array. This doubles the number of
|
|
backslashes you need. For example, to insert a <type>text</> array
|
|
value containing a backslash and a double quote, you'd need to write
|
|
<programlisting>
|
|
INSERT ... VALUES ('{"\\\\","\\""}');
|
|
</programlisting>
|
|
The string-literal processor removes one level of backslashes, so that
|
|
what arrives at the array-value parser looks like <literal>{"\\","\""}</>.
|
|
In turn, the strings fed to the <type>text</> data type's input routine
|
|
become <literal>\</> and <literal>"</> respectively. (If we were working
|
|
with a data type whose input routine also treated backslashes specially,
|
|
<type>bytea</> for example, we might need as many as eight backslashes
|
|
in the command to get one backslash into the stored array element.)
|
|
</para>
|
|
</note>
|
|
|
|
<tip>
|
|
<para>
|
|
The <literal>ARRAY</> constructor syntax is often easier to work with
|
|
than the array-literal syntax when writing array values in SQL commands.
|
|
In <literal>ARRAY</>, individual element values are written the same way
|
|
they would be written when not members of an array.
|
|
</para>
|
|
</tip>
|
|
</sect2>
|
|
|
|
</sect1>
|