SQL Syntax
syntax
SQL
This chapter describes the syntax of SQL.
Lexical Structure
SQL input consists of a sequence of
commands. A command is composed of a
sequence of tokens, terminated by a
semicolon (;
). The end of the input stream also
terminates a command. Which tokens are valid depends on the syntax
of the particular command.
A token can be a key word, an
identifier, a quoted
identifier, a literal (or
constant), or a special character symbol. Tokens are normally
separated by whitespace (space, tab, newline), but need not be if
there is no ambiguity (which is generally only the case if a
special character is adjacent to some other token type).
Additionally, comments can occur in SQL
input. They are not tokens, they are effectively equivalent to
whitespace.
For example, the following is (syntactically) valid SQL input:
SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
INSERT INTO MY_TABLE VALUES (3, 'hi there');
This is a sequence of three commands, one per line (although this
is not required; more than one command can be on a line, and
commands can usefully be split across lines).
The SQL syntax is not very consistent regarding what tokens
identify commands and which are operands or parameters. The first
few tokens are generally the command name, so in the above example
we would usually speak of a SELECT
, an
UPDATE
, and an INSERT
command. But
for instance the UPDATE command always requires
a SET token to appear in a certain position, and
this particular variation of INSERT also
requires a VALUES in order to be complete. The
precise syntax rules for each command are described in the
Reference Manual.
Identifiers and Key Words
identifiers
key words
syntax
Tokens such as SELECT, UPDATE, or
VALUES in the example above are examples of
key words, that is, words that have a fixed
meaning in the SQL language. The tokens MY_TABLE
and A are examples of
identifiers. They identify names of
tables, columns, or other database objects, depending on the
command they are used in. Therefore they are sometimes simply
called names
. Key words and identifiers have the
same lexical structure, meaning that one cannot know whether a
token is an identifier or a key word without knowing the language.
A complete list of key words can be found in .
SQL identifiers and key words must begin with a letter
(a-z) or underscore
(_). Subsequent characters in an identifier or
key word can be letters, digits
(0-9), or underscores,
although the SQL standard will not define a key word that contains
digits or starts or ends with an underscore.
The system uses no more than NAMEDATALEN-1
characters of an identifier; longer names can be written in
commands, but they will be truncated. By default,
NAMEDATALEN is 32 so the maximum identifier length
is 31 (but at the time the system is built,
NAMEDATALEN can be changed in
src/include/postgres_ext.h).
case sensitivity
SQL commands
Identifier and key word names are case insensitive. Therefore
UPDATE MY_TABLE SET A = 5;
can equivalently be written as
uPDaTE my_TabLE SeT a = 5;
A convention often used is to write key words in upper
case and names in lower case, e.g.,
UPDATE my_table SET a = 5;
quotes
and identifiers
There is a second kind of identifier: the delimited
identifier or quoted
identifier. It is formed by enclosing an arbitrary
sequence of characters in double-quotes
("). A delimited
identifier is always an identifier, never a key word. So
"select" could be used to refer to a column or
table named select
, whereas an unquoted
select would be taken as a key word and
would therefore provoke a parse error when used where a table or
column name is expected. The example can be written with quoted
identifiers like this:
UPDATE "my_table" SET "a" = 5;
Quoted identifiers can contain any character other than a double
quote itself. This allows constructing table or column names that
would otherwise not be possible, such as ones containing spaces or
ampersands. The length limitation still applies.
Quoting an identifier also makes it case-sensitive, whereas
unquoted names are always folded to lower case. For example, the
identifiers FOO, foo and
"foo" are considered the same by
Postgres, but "Foo"
and "FOO" are different from these three and
each other.
The folding of unquoted names to lower case in PostgreSQL>
is incompatible with the SQL standard, which says that unquoted
names should be folded to upper case. Thus, foo
should be equivalent to "FOO" not
"foo" according to the standard. If you want to
write portable applications you are advised to always quote a particular
name or never quote it.
Constants
constants
There are four kinds of implicitly typed
constants in Postgres:
strings, bit strings, integers, and floating point numbers.
Constants can also be specified with explicit types, which can
enable more accurate representation and more efficient handling by
the system. The implicit constants are described below; explicit
constants are discussed afterwards.
String Constants
character strings
constants
quotes
escaping
A string constant in SQL is an arbitrary sequence of characters
bounded by single quotes ('
), e.g., 'This
is a string'. SQL allows single quotes to be embedded
in strings by typing two adjacent single quotes (e.g.,
'Dianne''s horse'). In
Postgres single quotes may
alternatively be escaped with a backslash (\
,
e.g., 'Dianne\'s horse').
C-style backslash escapes are also available:
\b is a backspace, \f is a
form feed, \n is a newline,
\r is a carriage return, \t
is a tab, and \xxx,
where xxx is an octal number, is the
character with the corresponding ASCII code. Any other character
following a backslash is taken literally. Thus, to include a
backslash in a string constant, type two backslashes.
The character with the code zero cannot be in a string constant.
Two string constants that are only separated by whitespace
with at least one newline are concatenated
and effectively treated as if the string had been written in one
constant. For example:
SELECT 'foo'
'bar';
is equivalent to
SELECT 'foobar';
but
SELECT 'foo' 'bar';
is not valid syntax.
Bit String Constants
bit strings
constants
Bit string constants look like string constants with a
B (upper or lower case) immediately before the
opening quote (no intervening whitespace), e.g.,
B'1001'. The only characters allowed within
bit string constants are 0 and
1. Bit string constants can be continued
across lines in the same way as regular string constants.
Integer Constants
Integer constants in SQL are sequences of decimal digits (0
though 9) with no decimal point. The range of legal values
depends on which integer data type is used, but the plain
integer type accepts values ranging from -2147483648
to +2147483647. (The optional plus or minus sign is actually a
separate unary operator and not part of the integer constant.)
Floating Point Constants
floating point
constants
Floating point constants are accepted in these general forms:
digits.digitse+-digits
digits.digitse+-digits
digitse+-digits
where digits is one or more decimal
digits. At least one digit must be before or after the decimal
point, and after the e if you use that option.
Thus, a floating point constant is distinguished from an integer
constant by the presence of either the decimal point or the
exponent clause (or both). There must not be a space or other
characters embedded in the constant.
These are some examples of valid floating point constants:
3.5
4.
.001
5e2
1.925e-3
Floating point constants are of type DOUBLE
PRECISION. REAL can be specified explicitly
by using SQL string notation or
Postgres type notation:
REAL '1.23' -- string style
'1.23'::REAL -- Postgres (historical) style
Constants of Other Types
data types
constants
A constant of an arbitrary type can be
entered using any one of the following notations:
type 'string'
'string'::type
CAST ( 'string' AS type )
The value inside the string is passed to the input conversion
routine for the type called type. The
result is a constant of the indicated type. The explicit type
cast may be omitted if there is no ambiguity as to the type the
constant must be (for example, when it is passed as an argument
to a non-overloaded function), in which case it is automatically
coerced.
It is also possible to specify a type coercion using a function-like
syntax:
typename ( value )
although this only works for types whose names are also valid as
function names. (For example, double precision
can't be used this way --- but the equivalent float8
can.)
The ::, CAST(), and
function-call syntaxes can also be used to specify the type of
arbitrary expressions, but the form
type
'string' can only be used to specify
the type of a literal constant.
Array constants
arrays
constants
The general format of an array constant is the following:
'{ val1 delim val2 delim ... }'
where delim is the delimiter character
for the type, as recorded in its pg_type
entry. (For all built-in types, this is the comma character
,>.) Each val is either a constant
of the array element type, or a sub-array. An example of an
array constant is
'{{1,2,3},{4,5,6},{7,8,9}}'
This constant is a two-dimensional, 3 by 3 array consisting of three
sub-arrays of integers.
Individual array elements can be placed between double-quote
marks (") to avoid ambiguity
problems with respect to white space. Without quote marks, the
array-value parser will skip leading white space.
(Array constants are actually only a special case of the generic
type constants discussed in the previous section. The constant
is initially treated as a string and passed to the array input
conversion routine. An explicit type specification might be
necessary.)
Operators
operators
syntax
An operator is a sequence of up to NAMEDATALEN-1
(31 by default) characters from the following list:
+ - * / < > = ~ ! @ # % ^ & | ` ? $
There are a few restrictions on operator names, however:
$> (dollar) cannot be a single-character operator, although it
can be part of a multiple-character operator name.
-- and /* cannot appear
anywhere in an operator name, since they will be taken as the
start of a comment.
A multiple-character operator name cannot end in +> or ->,
unless the name also contains at least one of these characters:
~ ! @ # % ^ & | ` ? $
For example, @- is an allowed operator name,
but *- is not. This restriction allows
Postgres to parse SQL-compliant
queries without requiring spaces between tokens.
When working with non-SQL-standard operator names, you will usually
need to separate adjacent operators with spaces to avoid ambiguity.
For example, if you have defined a left-unary operator named @,
you cannot write X*@Y; you must write
X* @Y to ensure that
Postgres reads it as two operator names
not one.
Special Characters
Some characters that are not alphanumeric have a special meaning
that is different from being an operator. Details on the usage can
be found at the location where the respective syntax element is
described. This section only exists to advise the existence and
summarize the purposes of these characters.
A dollar sign ($) followed by digits is used
to represent the positional parameters in the body of a function
definition. In other contexts the dollar sign may be part of an
operator name.
Parentheses (()) have their usual meaning to
group expressions and enforce precedence. In some cases
parentheses are required as part of the fixed syntax of a
particular SQL command.
Brackets ([]) are used to select the elements
of an array. See for more information
on arrays.
Commas (,) are used in some syntactical
constructs to separate the elements of a list.
The semicolon (;) terminates an SQL command.
It cannot appear anywhere within a command, except within a
string constant or quoted identifier.
The colon (:) is used to select
slices
from arrays. (See .) In certain SQL dialects (such as Embedded
SQL), the colon is used to prefix variable names.
The asterisk (*) has a special meaning when
used in the SELECT command or with the
COUNT aggregate function.
The period (.) is used in floating point
constants, and to separate table and column names.
Columns
A column
is either a user-defined column of a given table or one of the
following system-defined columns:
columns
system columns
oid>
OID
The object identifier (object ID) of a row. This is a serial number
that is automatically added by Postgres to all table rows (unless
the table was created WITHOUT OIDS, in which case this column is
not present).
tableoid>
The OID of the table containing this row. This attribute is
particularly handy for queries that select from inheritance
hierarchies, since without it, it's difficult to tell which
individual table a row came from. The
tableoid can be joined against the
oid column of
pg_class to obtain the table name.
xmin>
The identity (transaction ID) of the inserting transaction for
this tuple. (Note: a tuple is an individual state of a row;
each UPDATE of a row creates a new tuple for the same logical row.)
cmin>
The command identifier (starting at zero) within the inserting
transaction.
xmax>
The identity (transaction ID) of the deleting transaction,
or zero for an undeleted tuple. It is possible for this field
to be nonzero in a visible tuple: that usually indicates that the
deleting transaction hasn't committed yet, or that an attempted
deletion was rolled back.
cmax>
The command identifier within the deleting transaction, or zero.
ctid>
The tuple ID of the tuple within its table. This is a pair
(block number, tuple index within block) that identifies the
physical location of the tuple. Note that although the ctid
can be used to locate the tuple very quickly, a row's ctid
will change each time it is updated or moved by VACUUM
FULL>.
Therefore ctid is useless as a long-term row identifier.
The OID, or even better a user-defined serial number, should
be used to identify logical rows.
OIDs are 32-bit quantities and are assigned from a single cluster-wide
counter. In a large or long-lived database, it is possible for the
counter to wrap around. Hence, it is bad practice to assume that OIDs
are unique, unless you take steps to ensure that they are unique.
Recommended practice when using OIDs for row identification is to create
a unique index on the OID column of each table for which the OID will be
used. Never assume that OIDs are unique across tables; use the
combination of tableoid> and row OID if you need a database-wide
identifier. (Future releases of Postgres are likely to use a separate
OID counter for each table, so that tableoid> must> be
included to arrive at a globally unique identifier.)
Transaction identifiers are 32-bit quantities. In a long-lived
database it is possible for transaction IDs to wrap around. This
is not a fatal problem given appropriate maintenance procedures;
see the Administrator's Guide> for details. However, it is
unwise to depend on uniqueness of transaction IDs over the long term
(more than one billion transactions).
Command identifiers are also 32-bit quantities. This creates a hard
limit of 2^32 (4 billion) SQL commands within a single transaction.
In practice this limit is not a problem --- note that the limit is on
number of SQL queries, not number of tuples processed.
For further information on the system attributes consult
.
Value Expressions
Value expressions are used in a variety of contexts, such
as in the target list of the SELECT command, as
new column values in INSERT or
UPDATE, or in search conditions in a number of
commands. The result of a value expression is sometimes called a
scalar, to distinguish it from the result of
a table expression (which is a table). Value expressions are
therefore also called scalar expressions (or
even simply expressions). The expression
syntax allows the calculation of values from primitive parts using
arithmetic, logical, set, and other operations.
A value expression is one of the following:
A constant or literal value; see .
A column reference
An operator invocation:
expression operator expression (binary infix operator)
operator expression (unary prefix operator)
expression operator (unary postfix operator)
where operator follows the syntax
rules of or is one of the
tokens AND, OR, and
NOT. Which particular operators exist and whether
they are unary or binary depends on what operators have been
defined by the system or the user.
describes the built-in operators.
( expression )
Parentheses are used to group subexpressions and override precedence.
A positional parameter reference, in the body of a function declaration.
A function call
An aggregate expression
A scalar subquery. This is an ordinary
SELECT in parentheses that returns exactly one
row with one column. It is an error to use a subquery that
returns more than one row or more than one column in the context
of a value expression.
In addition to this list, there are a number of constructs that can
be classified as an expression but do not follow any general syntax
rules. These generally have the semantics of a function or
operator and are explained in the appropriate location in . An example is the IS NULL
clause.
We have already discussed constants in . The following sections discuss
the remaining options.
Column References
A column can be referenced in the form:
correlation.columnname `['subscript`]'
correlation is either the name of a
table, an alias for a table defined by means of a FROM clause, or
the keyword NEW or OLD.
(NEW and OLD can only appear in the action portion of a rule,
while other correlation names can be used in any SQL statement.)
The correlation name can be omitted if the column name is unique
across all the tables being used in the current query. If
column is of an array type, then the
optional subscript selects a specific
element in the array. If no subscript is provided, then the whole
array is selected. Refer to the description of the particular
commands in the PostgreSQL Reference Manual
for the allowed syntax in each case.
Positional Parameters
A positional parameter reference is used to indicate a parameter
in an SQL function. Typically this is used in SQL function
definition statements. The form of a parameter is:
$number
For example, consider the definition of a function,
dept, as
CREATE FUNCTION dept (text) RETURNS dept
AS 'SELECT * FROM dept WHERE name = $1'
LANGUAGE 'sql';
Here the $1 will be replaced by the first
function argument when the function is invoked.
Function Calls
The syntax for a function call is the name of a function
(which is subject to the syntax rules for identifiers of ), followed by its argument list
enclosed in parentheses:
function (expression , expression ... )
For example, the following computes the square root of 2:
sqrt(2)
The list of built-in functions is in .
Other functions may be added by the user.
Aggregate Expressions
aggregate functions
An aggregate expression represents the
application of an aggregate function across the rows selected by a
query. An aggregate function reduces multiple inputs to a single
output value, such as the sum or average of the inputs. The
syntax of an aggregate expression is one of the following:
aggregate_name (expression)
aggregate_name (ALL expression)
aggregate_name (DISTINCT expression)
aggregate_name ( * )
where aggregate_name is a previously
defined aggregate, and expression is
any expression that does not itself contain an aggregate
expression.
The first form of aggregate expression invokes the aggregate
across all input rows for which the given expression yields a
non-NULL value. (Actually, it is up to the aggregate function
whether to ignore NULLs or not --- but all the standard ones do.)
The second form is the same as the first, since
ALL is the default. The third form invokes the
aggregate for all distinct non-NULL values of the expression found
in the input rows. The last form invokes the aggregate once for
each input row regardless of NULL or non-NULL values; since no
particular input value is specified, it is generally only useful
for the count() aggregate function.
For example, count(*) yields the total number
of input rows; count(f1) yields the number of
input rows in which f1 is non-NULL;
count(distinct f1) yields the number of
distinct non-NULL values of f1.
The predefined aggregate functions are described in . Other aggregate functions may be added
by the user.
Lexical Precedence
operators
precedence
The precedence and associativity of the operators is hard-wired
into the parser. Most operators have the same precedence and are
left-associative. This may lead to non-intuitive behavior; for
example the Boolean operators <> and >> have a different
precedence than the Boolean operators <=> and >=>. Also,
you will sometimes need to add parentheses when using combinations
of binary and unary operators. For instance
SELECT 5 ! - 6;
will be parsed as
SELECT 5 ! (- 6);
because the parser has no idea -- until it is too late -- that
! is defined as a postfix operator, not an infix one.
To get the desired behavior in this case, you must write
SELECT (5 !) - 6;
This is the price one pays for extensibility.
Operator Precedence (decreasing)
Operator/Element
Associativity
Description
::
left
Postgres-style typecast
[ ]
left
array element selection
.
left
table/column name separator
-
right
unary minus
^
left
exponentiation
* / %
left
multiplication, division, modulo
+ -
left
addition, subtraction
IS
test for TRUE, FALSE, UNKNOWN, NULL
ISNULL
test for NULL
NOTNULL
test for NOT NULL
(any other)
left
all other native and user-defined operators
IN
set membership
BETWEEN
containment
OVERLAPS
time interval overlap
LIKE ILIKE
string pattern matching
< >
less than, greater than
=
right
equality, assignment
NOT
right
logical negation
AND
left
logical conjunction
OR
left
logical disjunction
Note that the operator precedence rules also apply to user-defined
operators that have the same names as the built-in operators
mentioned above. For example, if you define a
+
operator for some custom data type it will have
the same precedence as the built-in +
operator, no
matter what yours does.