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 the
PostgreSQL 7.3 Reference Manual.
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 B.
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 64 so the maximum identifier length
is 63 (but at the time PostgreSQL is built,
NAMEDATALEN can be changed 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 other than a double
quote itself. 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.
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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. The implicit constants are described below; explicit
constants are discussed afterwards.
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. (This slightly bizarre behavior is specified
by SQL; PostgreSQL is
following the standard.)
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.
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.
Numeric constants are accepted in these general forms:
digits
digits.[digits][e[+-]digits]
[digits].digits[e[+-]digits]
digitse[+-]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
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 Section 1.2.6 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 1.2.6. 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.
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.)
An operator 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:
$ (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.
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 Section 5.12 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 5.12.) 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 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 SQL92 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 SQL99 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 1-1 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 1-1. 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 | | 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 1-1 for "any other" operator. This is true no matter
which specific operator name appears inside OPERATOR().
