PostgreSQL lexical structure and syntax

This topic describes the lexical structure used to create PostgreSQL statements and defines the syntax of the lexical elements, called tokens, of this structure.

A PostgreSQL statement comprises a series of tokens. Tokens include identifiers, quoted identifiers, keywords, literals, operators, and special characters. You can separate tokens with whitespace (for example, space, tab, newline) or comments.

Identifiers and quoted identifiers

Identifiers are names that are associated with columns, tables, and other database objects. They can be unquoted or quoted.

  • The maximum identifier length is 63 characters. For more information about identifier lengths, see Quotas & limits. The PostgreSQL parser truncates identifiers that are longer than 63 characters.
  • Unquoted identifiers must begin with a letter or an underscore character. Subsequent characters can be letters, numbers, or underscores.
  • Quoted identifiers must be enclosed by double quote (") characters.
    • Quoted identifiers can contain any character, such as spaces or symbols.
    • Quoted identifiers cannot be empty.
    • Quoted identifiers support the same escape sequences as string literals.
    • A keyword must be a quoted identifier if it is a standalone keyword or the first component of a path expression. It may be unquoted as the second or later component of a path expression.

Examples

These are valid identifiers (unquoted identifiers are converted to lower case):

Customers5
"5Customers"
dataField
_dataField1
ADGROUP
"tableName~"
"GROUP"

These path expressions contain valid identifiers:

foo."GROUP"
foo.GROUP

These are invalid identifiers:

5Customers
_dataField!
GROUP

5Customers begins with a number, not a letter or underscore. _dataField! contains the special character "!" which is not a letter, number, or underscore. GROUP is a keyword, and therefore cannot be used as an identifier without being enclosed by double quote characters.

Case sensitivity

PostgreSQL is case sensitive. Quoted identifiers are case preserving. Unquoted identifiers are not. Unquoted identifiers are all converted to lower case before being compared.

Examples

CREATE TABLE Foo (a int primary key);  select * from foo;    -- works
CREATE TABLE Foo (a int primary key);  select * from "foo";  -- works
CREATE TABLE "Foo" (a int primary key);  select * from foo;  -- fails
CREATE TABLE Foo (a int primary key);  select * from "Foo";  -- fails

Keywords

In the following example, the tokens SELECT, UPDATE, and VALUES are examples of keywords, that is, words that have a fixed meaning in the SQL language.

SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
INSERT INTO MY_TABLE VALUES (3, 'hi there');

Keywords and identifiers have the same lexical structure, meaning that one can't know whether a token is an identifier or a keyword without knowing the language.

Literals (constants)

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.

String constants

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, for example '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.

String constants with C-style escapes

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, for example 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 the following table.

Backslash Escape Sequence Interpretation
\b backspace
\f form feed
\n newline
\r carriage return
\t tab
\o, \oo, \ooo (o = 0–7) octal byte value
\xh, \xhh (h = 0–9, A–F) hexadecimal byte value
\uxxxx, \Uxxxxxxxx (x = 0–9, A–F) 16 or 32-bit hexadecimal Unicode character value

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 writing ''.

Ensure that the byte sequences that 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; then the server will check that the character conversion is possible.

Note that Cloud Spanner supports only UTF-8 for strings.

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

String Constants With Unicode Escapes

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 the following:

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.

Dollar-Quoted String Constants

Although 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, because 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's possible to nest dollar-quoted string constants by choosing different tags at each nesting level.

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.

Numeric 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 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:

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 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 bigint value to be treated as type numeric by writing:

NUMERIC '123'
123::numeric

These are actually just special cases of the general casting notations discussed in the next section.

Constants Of Other Types

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.

The :: and CAST() syntaxes can also be used to specify run-time type conversions of arbitrary expressions. To avoid syntactic ambiguity, the type 'string' syntax can only be used to specify the type of a simple literal 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.

Operators

An operator name is a sequence of 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.

Operator Precedence

The following table 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. Add parentheses if you want an expression with multiple operators to be parsed in some other way than what the precedence rules imply.

Operator Precedence (highest to lowest)

Operator/Element Associativity Description
. left table/column name separator
:: left PostgreSQL-style typecast
+ - right unary plus, unary minus
^ left exponentiation
* / % left multiplication, division, modulo
+ - left addition, subtraction
(any other operator) left all other native and user-defined operators
BETWEEN LIKE   range containment, set membership, string matching
< > = <= >= <>   comparison operators
IS ISNULL NOTNULL   IS TRUE, IS FALSE, IS NULL, IS DISTINCT FROM, etc
NOT right logical negation
AND left logical conjunction
OR left logical disjunction

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 a positional parameter in the body of a prepared statement. In other contexts the dollar sign can 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.

  • 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 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 table, and column names.

Comments

A comment is a sequence of characters beginning with double dashes and extending to the end of the line. For example:

-- 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 might contain existing block comments.

A comment is removed from the input stream before further syntax analysis and is effectively replaced by whitespace. The exception is a comment that contains hints, which are preserved for interpretation by the query planner.