Table of Contents
This chapter describes the syntax of SQL. It forms the foundation for understanding the following chapters which will go into detail about how the SQL commands are applied to define and modify data.
We also advise users who are already familiar with SQL to read this chapter carefully because there are several rules and concepts that are implemented inconsistently among SQL databases or that are specific to PostgreSQL.
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 Part VI, “Reference”.
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 Appendix C, SQL Key Words.
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, underscores, digits
(0
-9
), or dollar signs
($
). Note that dollar signs are not allowed in identifiers
according to the letter of the SQL standard, so their use may render
applications less portable.
The SQL standard will not define a key word that contains
digits or starts or ends with an underscore, so identifiers of this
form are safe against possible conflict with future extensions of the
standard.
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 64 so the maximum identifier
length is 63. If this limit is problematic, it can be raised by
changing the NAMEDATALEN
constant in
src/include/postgres_ext.h
.
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;
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, except the character with code zero. (To include a double quote, write two double quotes.) 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.)
There are three kinds of implicitly-typed constants in PostgreSQL: strings, bit strings, and numbers. Constants can also be specified with explicit types, which can enable more accurate representation and more efficient handling by the system. These alternatives are discussed in the following subsections.
A string constant in SQL is an arbitrary sequence of characters
bounded by single quotes ('
), for example
'This is a string'
. To include
a single-quote character within a string constant,
write two adjacent single quotes, e.g.
'Dianne''s horse'
.
Note that this is not the same as a double-quote
character ("
).
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 as one constant. For example:
SELECT 'foo' 'bar';
is equivalent to
SELECT 'foobar';
but
SELECT 'foo' 'bar';
is not valid syntax. (This slightly bizarre behavior is specified by SQL; PostgreSQL is following the standard.)
PostgreSQL also accepts “escape”
string constants, which are an extension to the SQL standard.
An escape string constant is specified by writing the letter
E
(upper or lower case) just before the opening single
quote, e.g. E'foo'
. (When continuing an escape string
constant across lines, write E
only before the first opening
quote.)
Within an escape string, a backslash character (\
) begins a
C-like backslash escape sequence, in which the combination
of backslash and following character(s) represents a special byte value.
\b
is a backspace,
\f
is a form feed,
\n
is a newline,
\r
is a carriage return,
\t
is a tab.
Also supported are
\
, where
digits
digits
represents an octal byte value, and
\x
, where
hexdigits
hexdigits
represents a hexadecimal byte value.
(It is your responsibility that the byte sequences you create are
valid characters in the server character set encoding.) Any other
character following a backslash is taken literally. Thus, to
include a backslash character, write two backslashes (\\
).
Also, a single quote can be included in an escape string by writing
\'
, in addition to the normal way of ''
.
If the configuration parameter
standard_conforming_strings is off
,
then PostgreSQL recognizes backslash escapes
in both regular and escape string constants. This is for backward
compatibility with the historical behavior, in which backslash escapes
were always recognized.
Although standard_conforming_strings
currently defaults to
off
, the default will change to on
in a future
release for improved standards compliance. Applications are therefore
encouraged to migrate away from using backslash escapes. If you need
to use a backslash escape to represent a special character, write the
constant with an E
to be sure it will be handled the same
way in future releases.
In addition to standard_conforming_strings
, the configuration
parameters escape_string_warning and
backslash_quote govern treatment of backslashes
in string constants.
The character with the code zero cannot be in a string constant.
While the standard syntax for specifying string constants is usually
convenient, it can be difficult to understand when the desired string
contains many single quotes or backslashes, since each of those must
be doubled. To allow more readable queries in such situations,
PostgreSQL provides another way, called
“dollar quoting”, to write string constants.
A dollar-quoted string constant
consists of a dollar sign ($
), an optional
“tag” of zero or more characters, another dollar
sign, an arbitrary sequence of characters that makes up the
string content, a dollar sign, the same tag that began this
dollar quote, and a dollar sign. For example, here are two
different ways to specify the string “Dianne's horse”
using dollar quoting:
$$Dianne's horse$$ $SomeTag$Dianne's horse$SomeTag$
Notice that inside the dollar-quoted string, single quotes can be used without needing to be escaped. Indeed, no characters inside a dollar-quoted string are ever escaped: the string content is always written literally. Backslashes are not special, and neither are dollar signs, unless they are part of a sequence matching the opening tag.
It is possible to nest dollar-quoted string constants by choosing different tags at each nesting level. This is most commonly used in writing function definitions. For example:
$function$ BEGIN RETURN ($1 ~ $q$[\t\r\n\v\\]$q$); END; $function$
Here, the sequence $q$[\t\r\n\v\\]$q$
represents a
dollar-quoted literal string [\t\r\n\v\\]
, which will
be recognized when the function body is executed by
PostgreSQL. But since the sequence does not match
the outer dollar quoting delimiter $function$
, it is
just some more characters within the constant so far as the outer
string is concerned.
The tag, if any, of a dollar-quoted string follows the same rules
as an unquoted identifier, except that it cannot contain a dollar sign.
Tags are case sensitive, so $tag$String content$tag$
is correct, but $TAG$String content$tag$
is not.
A dollar-quoted string that follows a keyword or identifier must be separated from it by whitespace; otherwise the dollar quoting delimiter would be taken as part of the preceding identifier.
Dollar quoting is not part of the SQL standard, but it is often a more convenient way to write complicated string literals than the standard-compliant single quote syntax. It is particularly useful when representing string constants inside other constants, as is often needed in procedural function definitions. With single-quote syntax, each backslash in the above example would have to be written as four backslashes, which would be reduced to two backslashes in parsing the original string constant, and then to one when the inner string constant is re-parsed during function execution.
Bit-string constants look like regular 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
.
Alternatively, bit-string constants can be specified in hexadecimal
notation, using a leading X
(upper or lower case),
e.g., X'1FF'
. This notation is equivalent to
a bit-string constant with four binary digits for each hexadecimal digit.
Both forms of bit-string constant can be continued across lines in the same way as regular string constants. Dollar quoting cannot be used in a bit-string constant.
Numeric constants are accepted in these general forms:
digits
digits
.[digits
][e[+-]digits
] [digits
].digits
[e[+-]digits
]digits
e[+-]digits
where digits
is one or more decimal
digits (0 through 9). At least one digit must be before or after the
decimal point, if one is used. At least one digit must follow the
exponent marker (e
), if one is present.
There may not be any spaces or other characters embedded in the
constant. Note that any leading plus or minus sign is not actually
considered part of the constant; it is an operator applied to the
constant.
These are some examples of valid numeric constants:
42
3.5
4.
.001
5e2
1.925e-3
A numeric constant that contains neither a decimal point nor an
exponent is initially presumed to be type integer
if its
value fits in type integer
(32 bits); otherwise it is
presumed to be type bigint
if its
value fits in type bigint
(64 bits); otherwise it is
taken to be type numeric
. Constants that contain decimal
points and/or exponents are always initially presumed to be type
numeric
.
The initially assigned data type of a numeric constant is just a
starting point for the type resolution algorithms. In most cases
the constant will be automatically coerced to the most
appropriate type depending on context. When necessary, you can
force a numeric value to be interpreted as a specific data type
by casting it.
For example, you can force a numeric value to be treated as type
real
(float4
) by writing
REAL '1.23' -- string style 1.23::REAL -- PostgreSQL (historical) style
These are actually just special cases of the general casting notations discussed next.
A constant of an arbitrary type can be entered using any one of the following notations:
type
'string
' 'string
'::type
CAST ( 'string
' AStype
)
The string constant'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 assigned directly to a
table column), in which case it is automatically coerced.
The string constant can be written using either regular SQL notation or dollar-quoting.
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 Section 4.2.8, “Type Casts” for details.
The ::
, CAST()
, and
function-call syntaxes can also be used to specify run-time type
conversions of arbitrary expressions, as discussed in Section 4.2.8, “Type Casts”. But the form
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 type
'string
'::
or CAST()
to specify the type of an array constant.
The CAST()
syntax conforms to SQL. The
syntax is a generalization of the standard: SQL specifies this syntax only
for a few data types, but PostgreSQL allows it
for all types. The syntax with
type
'string
'::
is historical PostgreSQL
usage, as is the function-call syntax.
An operator name is a sequence of up to NAMEDATALEN
-1
(63 by default) characters from the following list:
+ - * / < > = ~ ! @ # % ^ & | ` ?
There are a few restrictions on operator names, however:
--
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.
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 a positional parameter in the body of a function
definition or a prepared statement. In other contexts the
dollar sign may be part of an identifier or a dollar-quoted string
constant.
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 Section 8.10, “Arrays” 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 Section 8.10, “Arrays”.) In certain SQL dialects (such as Embedded
SQL), the colon is used to prefix variable names.
The asterisk (*
) is used in some contexts to denote
all the fields of a table row or composite value. It also
has a special meaning when used as the argument of an
aggregate function, namely that the aggregate does not require
any explicit parameter.
The period (.
) is used in numeric
constants, and to separate schema, table, and column names.
A comment is an arbitrary sequence of characters beginning with double dashes and extending to the end of the line, e.g.:
-- This is a standard SQL comment
Alternatively, C-style block comments can be used:
/* multiline comment * with nesting: /* nested block comment */ */
where the comment begins with /*
and extends to
the matching occurrence of */
. These block
comments nest, as specified in the SQL standard but unlike C, so that one can
comment out larger blocks of code that may contain existing block
comments.
A comment is removed from the input stream before further syntax analysis and is effectively replaced by whitespace.
Table 4.1, “Operator Precedence (decreasing)” shows the precedence and
associativity of the operators in PostgreSQL.
Most operators have the same precedence and are left-associative.
The precedence and associativity of the operators is hard-wired
into the parser. 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.
Table 4.1. 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 |
IS TRUE , IS FALSE , IS UNKNOWN , IS NULL
|
|
ISNULL |
test for null | |
NOTNULL |
test for not null | |
(any other) | left | all other native and user-defined operators |
IN |
set membership | |
BETWEEN |
range containment | |
OVERLAPS |
time interval overlap | |
LIKE ILIKE SIMILAR
|
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.
When a schema-qualified operator name is used in the
OPERATOR
syntax, as for example in
SELECT 3 OPERATOR(pg_catalog.+) 4;
the OPERATOR
construct is taken to have the default precedence
shown in Table 4.1, “Operator Precedence (decreasing)” for “any other” operator. This is true no matter
which specific operator name appears inside OPERATOR()
.