256 lines
8.4 KiB
Plaintext
256 lines
8.4 KiB
Plaintext
<!-- doc/src/sgml/dml.sgml -->
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<chapter id="dml">
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<title>Data Manipulation</title>
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<remark>
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This chapter is still quite incomplete.
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</remark>
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<para>
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The previous chapter discussed how to create tables and other
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structures to hold your data. Now it is time to fill the tables
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with data. This chapter covers how to insert, update, and delete
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table data. The chapter
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after this will finally explain how to extract your long-lost data
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from the database.
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</para>
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<sect1 id="dml-insert">
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<title>Inserting Data</title>
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<indexterm zone="dml-insert">
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<primary>inserting</primary>
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</indexterm>
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<indexterm zone="dml-insert">
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<primary>INSERT</primary>
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</indexterm>
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<para>
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When a table is created, it contains no data. The first thing to
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do before a database can be of much use is to insert data. Data is
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conceptually inserted one row at a time. Of course you can also
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insert more than one row, but there is no way to insert less than
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one row. Even if you know only some column values, a
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complete row must be created.
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</para>
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<para>
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To create a new row, use the <xref linkend="sql-insert">
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command. The command requires the
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table name and column values. For
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example, consider the products table from <xref linkend="ddl">:
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<programlisting>
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CREATE TABLE products (
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product_no integer,
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name text,
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price numeric
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);
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</programlisting>
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An example command to insert a row would be:
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<programlisting>
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INSERT INTO products VALUES (1, 'Cheese', 9.99);
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</programlisting>
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The data values are listed in the order in which the columns appear
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in the table, separated by commas. Usually, the data values will
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be literals (constants), but scalar expressions are also allowed.
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</para>
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<para>
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The above syntax has the drawback that you need to know the order
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of the columns in the table. To avoid this you can also list the
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columns explicitly. For example, both of the following commands
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have the same effect as the one above:
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<programlisting>
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INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', 9.99);
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INSERT INTO products (name, price, product_no) VALUES ('Cheese', 9.99, 1);
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</programlisting>
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Many users consider it good practice to always list the column
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names.
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</para>
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<para>
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If you don't have values for all the columns, you can omit some of
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them. In that case, the columns will be filled with their default
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values. For example:
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<programlisting>
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INSERT INTO products (product_no, name) VALUES (1, 'Cheese');
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INSERT INTO products VALUES (1, 'Cheese');
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</programlisting>
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The second form is a <productname>PostgreSQL</productname>
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extension. It fills the columns from the left with as many values
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as are given, and the rest will be defaulted.
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</para>
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<para>
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For clarity, you can also request default values explicitly, for
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individual columns or for the entire row:
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<programlisting>
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INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', DEFAULT);
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INSERT INTO products DEFAULT VALUES;
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</programlisting>
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</para>
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<para>
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You can insert multiple rows in a single command:
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<programlisting>
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INSERT INTO products (product_no, name, price) VALUES
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(1, 'Cheese', 9.99),
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(2, 'Bread', 1.99),
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(3, 'Milk', 2.99);
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</programlisting>
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</para>
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<tip>
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<para>
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When inserting a lot of data at the same time, considering using
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the <xref linkend="sql-copy"> command.
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It is not as flexible as the <xref linkend="sql-insert">
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command, but is more efficient. Refer
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to <xref linkend="populate"> for more information on improving
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bulk loading performance.
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</para>
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</tip>
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</sect1>
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<sect1 id="dml-update">
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<title>Updating Data</title>
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<indexterm zone="dml-update">
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<primary>updating</primary>
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</indexterm>
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<indexterm zone="dml-update">
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<primary>UPDATE</primary>
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</indexterm>
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<para>
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The modification of data that is already in the database is
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referred to as updating. You can update individual rows, all the
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rows in a table, or a subset of all rows. Each column can be
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updated separately; the other columns are not affected.
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</para>
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<para>
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To update existing rows, use the <xref linkend="sql-update">
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command. This requires
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three pieces of information:
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<orderedlist spacing="compact">
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<listitem>
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<para>The name of the table and column to update</para>
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</listitem>
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<listitem>
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<para>The new value of the column</para>
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</listitem>
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<listitem>
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<para>Which row(s) to update</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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Recall from <xref linkend="ddl"> that SQL does not, in general,
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provide a unique identifier for rows. Therefore it is not
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always possible to directly specify which row to update.
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Instead, you specify which conditions a row must meet in order to
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be updated. Only if you have a primary key in the table (independent of
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whether you declared it or not) can you reliably address individual rows
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by choosing a condition that matches the primary key.
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Graphical database access tools rely on this fact to allow you to
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update rows individually.
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</para>
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<para>
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For example, this command updates all products that have a price of
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5 to have a price of 10:
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<programlisting>
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UPDATE products SET price = 10 WHERE price = 5;
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</programlisting>
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This might cause zero, one, or many rows to be updated. It is not
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an error to attempt an update that does not match any rows.
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</para>
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<para>
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Let's look at that command in detail. First is the key word
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<literal>UPDATE</literal> followed by the table name. As usual,
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the table name can be schema-qualified, otherwise it is looked up
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in the path. Next is the key word <literal>SET</literal> followed
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by the column name, an equal sign, and the new column value. The
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new column value can be any scalar expression, not just a constant.
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For example, if you want to raise the price of all products by 10%
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you could use:
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<programlisting>
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UPDATE products SET price = price * 1.10;
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</programlisting>
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As you see, the expression for the new value can refer to the existing
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value(s) in the row. We also left out the <literal>WHERE</literal> clause.
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If it is omitted, it means that all rows in the table are updated.
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If it is present, only those rows that match the
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<literal>WHERE</literal> condition are updated. Note that the equals
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sign in the <literal>SET</literal> clause is an assignment while
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the one in the <literal>WHERE</literal> clause is a comparison, but
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this does not create any ambiguity. Of course, the
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<literal>WHERE</literal> condition does
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not have to be an equality test. Many other operators are
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available (see <xref linkend="functions">). But the expression
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needs to evaluate to a Boolean result.
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</para>
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<para>
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You can update more than one column in an
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<command>UPDATE</command> command by listing more than one
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assignment in the <literal>SET</literal> clause. For example:
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<programlisting>
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UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a > 0;
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</programlisting>
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</para>
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</sect1>
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<sect1 id="dml-delete">
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<title>Deleting Data</title>
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<indexterm zone="dml-delete">
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<primary>deleting</primary>
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</indexterm>
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<indexterm zone="dml-delete">
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<primary>DELETE</primary>
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</indexterm>
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<para>
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So far we have explained how to add data to tables and how to
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change data. What remains is to discuss how to remove data that is
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no longer needed. Just as adding data is only possible in whole
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rows, you can only remove entire rows from a table. In the
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previous section we explained that SQL does not provide a way to
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directly address individual rows. Therefore, removing rows can
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only be done by specifying conditions that the rows to be removed
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have to match. If you have a primary key in the table then you can
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specify the exact row. But you can also remove groups of rows
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matching a condition, or you can remove all rows in the table at
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once.
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</para>
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<para>
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You use the <xref linkend="sql-delete">
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command to remove rows; the syntax is very similar to the
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<command>UPDATE</command> command. For instance, to remove all
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rows from the products table that have a price of 10, use:
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<programlisting>
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DELETE FROM products WHERE price = 10;
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</programlisting>
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</para>
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<para>
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If you simply write:
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<programlisting>
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DELETE FROM products;
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</programlisting>
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then all rows in the table will be deleted! Caveat programmer.
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</para>
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</sect1>
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</chapter>
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