4.1. Lexical Structure

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) represent a special byte value, as shown in Table 4.1 .

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 '' .

It is your responsibility that the byte sequences you create, especially when using the octal or hexadecimal escapes, compose valid characters in the server character set encoding. A useful alternative is to use Unicode escapes or the alternative Unicode escape syntax, explained in Section 4.1.2.3 ; then the server will check that the character conversion is possible.

The character with the code zero cannot be in a string constant.

PostgreSQL also supports another type of escape syntax for strings that allows specifying arbitrary Unicode characters by code point. A Unicode escape string constant starts with U& (upper or lower case letter U followed by ampersand) immediately before the opening quote, without any spaces in between, for example U&'foo' . (Note that this creates an ambiguity with the operator & . Use spaces around the operator to avoid this problem.) Inside the quotes, Unicode characters can be specified in escaped form by writing a backslash followed by the four-digit hexadecimal code point number or alternatively a backslash followed by a plus sign followed by a six-digit hexadecimal code point number. For example, the string 'data' could be written as

U&'d\0061t\+000061'

The following less trivial example writes the Russian word " slon " (elephant) in Cyrillic letters:

U&'\0441\043B\043E\043D'

If a different escape character than backslash is desired, it can be specified using the UESCAPE clause after the string, for example:

U&'d!0061t!+000061' UESCAPE '!'

The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character.

To include the escape character in the string literally, write it twice.

Either the 4-digit or the 6-digit escape form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 6-digit form technically makes this unnecessary. (Surrogate pairs are not stored directly, but are combined into a single code point.)

If the server encoding is not UTF-8, the Unicode code point identified by one of these escape sequences is converted to the actual server encoding; an error is reported if that's not possible.

Also, the Unicode escape syntax for string constants only works when the configuration parameter standard_conforming_strings is turned on. This is because otherwise this syntax could confuse clients that parse the SQL statements to the point that it could lead to SQL injections and similar security issues. If the parameter is set to off, this syntax will be rejected with an error message.

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]
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 cannot 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:

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' AS type )

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 can 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 can be used in this way; see Section 4.2.9 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 4.2.9 . To avoid syntactic ambiguity, the type ' string ' syntax can only be used to specify the type of a simple literal constant. Another restriction on the type ' string ' syntax is that it does not work for array types; use :: or CAST() to specify the type of an array constant.

The CAST() syntax conforms to SQL. The type ' string ' 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 :: is historical PostgreSQL usage, as is the function-call syntax.