Chapter 4. SQL Syntax
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Chapter 4. SQL Syntax
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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.
4.1. 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 Part VI, “Reference”.
4.1.1. Identifiers and Key Words
Tokens such as
, 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 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. (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.
220.127.116.11. String Constants
A string constant in SQL is an arbitrary sequence of characters
bounded by single quotes ('), for example 'This is a string'. The standard-compliant way of writing a single-quote character within a string constant is to write two adjacent single quotes, e.g. 'Dianne''s horse'. PostgreSQL also allows single quotes to be escaped with a backslash (\'). However, future versions of PostgreSQL will not allow this, so applications using backslashes should convert to the standard-compliant method outlined above.
Another PostgreSQL extension is that C-style backslash escapes are available: \b is a backspace, \f is a form feed, \n is a newline, \r is a carriage return, \t is a tab. Also supported is \digits, where digits represents an octal byte value, and \xhexdigits, where 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 in a string constant, write two backslashes.
While ordinary strings now support C-style backslash escapes, future versions will generate warnings for such usage and eventually treat backslashes as literal characters to be standard-conforming. The proper way to specify escape processing is to use the escape string syntax to indicate that escape processing is desired. Escape string syntax is specified by writing the letter E (upper or lower case) just before the string, e.g. E'\041'. This method will work in all future versions of PostgreSQL.
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:
is equivalent to
SELECT 'foo' 'bar';
is not valid syntax. (This slightly bizarre behavior is specified by SQL; PostgreSQL is following the standard.)
18.104.22.168. Dollar-Quoted String Constants
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:
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:
RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
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.
22.214.171.124. Bit-String Constants
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.
126.96.36.199. Numeric Constants
Numeric constants are accepted in these general forms:
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:
A numeric constant that contains neither a decimal point nor an
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.
188.8.131.52. Constants of Other Types
A constant of an arbitrary type can be entered using any one of the following notations:
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 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 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 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.
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:
~ ! @ # % ^ & | ` ?
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.
4.1.4. 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 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.
4.1.6. Lexical Precedence
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 [mdash ] until it is too late [mdash ] 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)
Note that the operator precedence rules also apply to user-defined
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().