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, but also letters with
diacritical marks and non-Latin letters) or an 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
PostgreSQL, 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 PostgreSQL:
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
PostgreSQL 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, and PostgreSQL is
consistent with SQL9x in this regard.
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 and no exponent. 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. At least one digit must follow the exponent delimiter
(e) if that field is present.
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
PostgreSQL type notation:
REAL '1.23' -- string style
'1.23'::REAL -- PostgreSQL (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 string's text 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 ( 'string' )
but not all type names may be used in this way; see for details.
The ::, CAST(), and
function-call syntaxes can also be used to specify run-time type
conversions of arbitrary expressions, as discussed in . But the form
type 'string'
can only be used to specify the type of a literal constant.
Another restriction on
type 'string'
is that it does not work for array types; use ::
or CAST() to specify the type of an array 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 subarray. 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
subarrays of integers.
Individual array elements can be placed between double-quote
marks (") to avoid ambiguity
problems with respect to whitespace. Without quote marks, the
array-value parser will skip leading whitespace.
(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
PostgreSQL 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
PostgreSQL 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.
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
table/column name separator
::
left
PostgreSQL-style typecast
[ ]
left
array element selection
-
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.
Schemas and naming conventions
schemas
search path
namespaces
A PostgreSQL database cluster (installation)
contains one or more named databases. Users and groups of users are
shared across the entire cluster, but no other data is shared across
databases. Any given client connection to the server can access
only the data in a single database, the one specified in the connection
request.
Users of a cluster do not necessarily have the privilege to access every
database in the cluster. Sharing of user names means that there
cannot be different users named, say, joe> in two databases
in the same cluster; but the system can be configured to allow
joe> access to only some of the databases.
A database contains one or more named schemas>, which
in turn contain tables. Schemas also contain other kinds of named
objects, including datatypes, functions, and operators. The same
object name can be used in different schemas without conflict; for
example, both schema1> and myschema> may
contain tables named mytable>. Unlike databases, schemas
are not rigidly separated: a user may access objects in any of the
schemas in the database he is connected to, if he has privileges
to do so.
qualified names
names
qualified
To name a table precisely, write a qualified name> consisting
of the schema name and table name separated by a dot:
schema>.>table>
Actually, the even more general syntax
database>.>schema>.>table>
can be used too, but at present this is just for pro-forma compliance
with the SQL standard; if you write a database name it must be the
same as the database you are connected to.
unqualified names
names
unqualified
Qualified names are tedious to write, and it's often best not to
wire a particular schema name into applications anyway. Therefore
tables are often referred to by unqualified names>,
which consist of just the table name. The system determines which table
is meant by following a search path>, which is a list
of schemas to look in. The first matching table in the search path
is taken to be the one wanted. If there is no match in the search
path, an error is reported, even if matching table names exist
in other schemas in the database.
The first schema named in the search path is called the current schema.
Aside from being the first schema searched, it is also the schema in
which new tables will be created if the CREATE TABLE>
command does not specify a schema name.
The search path works in the same way for datatype names, function names,
and operator names as it does for table names. Datatype and function
names can be qualified in exactly the same way as table names. If you
need to write a qualified operator name in an expression, there is a
special provision: you must write
OPERATOR(>schema>.>operator>)>
This is needed to avoid syntactic ambiguity. An example is
SELECT 3 OPERATOR(pg_catalog.+) 4;
In practice one usually relies on the search path for operators,
so as not to have to write anything so ugly as that.
The standard search path in PostgreSQL
contains first the schema having the same name as the session user
(if it exists), and second the schema named public>
(if it exists, which it does by default). This arrangement allows
a flexible combination of private and shared tables. If no per-user
schemas are created then all user tables will exist in the shared
public> schema, providing behavior that is backwards-compatible
with pre-7.3 PostgreSQL releases.
There is no concept of a public> schema in the SQL standard.
To achieve closest conformance to the standard, the DBA should
create per-user schemas for every user, and not use (perhaps even
remove) the public> schema.
In addition to public> and user-created schemas, each database
contains a
pg_catalog> schema, which contains the system tables
and all the built-in datatypes, functions, and operators.
pg_catalog> is always effectively part of the search path.
If it is not named explicitly in the path then it is implicitly searched
before> searching the path's schemas. This ensures that
built-in names will always be findable. However, you may explicitly
place pg_catalog> at the end of your search path if you
prefer to have user-defined names override built-in names.
Reserved names
reserved names
names
reserved
There are several restrictions on the names that can be chosen for
user-defined database objects. These restrictions vary depending
on the kind of object. (Note that these restrictions are
separate from whether the name is a key word or not; quoting a
name will not allow you to escape these restrictions.)
Schema names beginning with pg_> are reserved for system
purposes and may not be created by users.
In PostgreSQL versions before 7.3, table
names beginning with pg_> were reserved. This is no longer
true: you may create such a table name if you wish, in any non-system
schema. However, it's best to continue to avoid such names,
to ensure that you won't suffer a conflict if some future version
defines a system catalog named the same as your table. (With the
default search path, an unqualified reference to your table name
would be resolved as the system catalog instead.) System catalogs will
continue to follow the convention of having names beginning with
pg_>, so that they will not conflict with unqualified
user-table names so long as users avoid the pg_> prefix.
Every table has several system columns> that are
implicitly defined by the system. Therefore, these names cannot
be used as names of user-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 PostgreSQL to all table rows (unless
the table was created WITHOUT OIDS, in which case this column is
not present). See for more info.
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 physical location of the tuple within its table.
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.
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.
A positional parameter reference, in the body of a function declaration.
An operator invocation.
A function call.
An aggregate expression.
A type cast.
A scalar subquery.
( expression )
Parentheses are used to group subexpressions and override precedence.
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 the name of a
table (possibly qualified), or an alias for a table defined by means of a
FROM clause, or
the key words 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 and separating dot may 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 or elements in the array. If no subscript is provided, then the
whole array is selected. (See for more about
arrays.)
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.
Operator Invocations
There are three possible syntaxes for an operator invocation:
expression operator expression (binary infix operator)
operator expression (unary prefix operator)
expression operator (unary postfix operator)
where the operator token follows the syntax
rules of , or is one of the
keywords AND, OR, and
NOT, or is a qualified operator name
OPERATOR(>schema>.>operatorname>)>
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.
Function Calls
The syntax for a function call is the name of a function
(possibly qualified with a schema name), 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 (possibly a qualified name), and
expression is
any value 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.
Type Casts
data types
type casts
A type cast specifies a conversion from one data type to another.
PostgreSQL accepts two equivalent syntaxes
for type casts:
CAST ( expression AS type )
expression::type
The CAST> syntax conforms to SQL92; the syntax with
:: is historical PostgreSQL
usage.
When a cast is applied to a value expression of a known type, it
represents a run-time type conversion. The cast will succeed only
if a suitable type conversion function is available. Notice that this
is subtly different from the use of casts with constants, as shown in
. A cast applied to an
unadorned string literal represents the initial assignment of a type
to a literal constant value, and so it will succeed for any type
(if the contents of the string literal are acceptable input syntax for the
data type).
An explicit type cast may usually be omitted if there is no ambiguity as
to the type that a value expression must produce (for example, when it is
assigned to a table column); the system will automatically apply a
type cast in such cases. However, automatic casting is only done for
cast functions that are marked okay to apply implicitly>
in the system catalogs. Other cast functions must be invoked with
explicit casting syntax. This restriction is intended to prevent
surprising conversions from being applied silently.
It is also possible to specify a type cast using a function-like
syntax:
typename ( expression )
However, 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. Also, the names interval>, time>, and
timestamp> can only be used in this fashion if they are
double-quoted, because of syntactic conflicts. Therefore, the use of
the function-like cast syntax leads to inconsistencies and should
probably be avoided in new applications.
Scalar Subqueries
A scalar subquery is an ordinary
SELECT in parentheses that returns exactly one
row with one column. The SELECT query is executed
and the single returned value is used in the surrounding value expression.
It is an error to use a query that
returns more than one row or more than one column as a scalar subquery.
(But if, during a particular execution, the subquery returns no rows,
there is no error; the scalar result is taken to be NULL.)
The subquery can refer to variables from the surrounding query,
which will act as constants during any one evaluation of the subquery.
See also .
For example, the following finds the largest city population in each
state:
SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
FROM states;
Expression Evaluation
The order of evaluation of subexpressions is not defined. In
particular, the inputs of an operator or function are not necessarily
evaluated left-to-right or in any other fixed order.
Furthermore, if the result of an expression can be determined by
evaluating only some parts of it, then other subexpressions
might not be evaluated at all. For instance, if one wrote
SELECT true OR somefunc();
then somefunc() would (probably) not be called
at all. The same would be the case if one wrote
SELECT somefunc() OR true;
Note that this is not the same as the left-to-right
short-circuiting
of Boolean operators that is found
in some programming languages.
As a consequence, it is unwise to use functions with side effects
as part of complex expressions. It is particularly dangerous to
rely on side effects or evaluation order in WHERE and HAVING clauses,
since those clauses are extensively reprocessed as part of
developing an execution plan. Boolean
expressions (AND/OR/NOT combinations) in those clauses may be reorganized
in any manner allowed by the laws of Boolean algebra.
When it is essential to force evaluation order, a CASE construct may
be used. For example, this is an untrustworthy way of trying to
avoid division by zero in a WHERE clause:
SELECT ... WHERE x <> 0 AND y/x > 1.5;
but this is safe:
SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
A CASE construct used in this fashion will defeat optimization attempts,
so it should only be done when necessary.