The usual comparison operators are available, as shown inTable 9.1.
Table 9.1. Comparison Operators
||less than or equal to|
||greater than or equal to|
!=operator is converted to
<>in the parser stage. It is not possible to implement
<>operators that do different things.
Comparison operators are available for all relevant data types. All comparison operators are binary operators that return values of type
boolean; expressions like
1 < 2 < 3are not valid (because there is no
<operator to compare a Boolean value with
There are also some comparison predicates, as shown inTable 9.2. These behave much like operators, but have special syntax mandated by the SQL standard.
Table 9.2. Comparison Predicates
||between, after sorting the comparison values|
||not between, after sorting the comparison values|
||not equal, treating null like an ordinary value|
||equal, treating null like an ordinary value|
||is not null|
||is null (nonstandard syntax)|
||is not null (nonstandard syntax)|
||is false or unknown|
||is true or unknown|
||is true or false|
BETWEENpredicate simplifies range tests:
a BETWEEN x AND y
is equivalent to
a > = x AND a < = y
BETWEENtreats the endpoint values as included in the range.
NOT BETWEENdoes the opposite comparison:
a NOT BETWEEN x AND y
is equivalent to
a < x OR a > y
BETWEEN SYMMETRICis like
BETWEENexcept there is no requirement that the argument to the left of
ANDbe less than or equal to the argument on the right. If it is not, those two arguments are automatically swapped, so that a nonempty range is always implied.
Ordinary comparison operators yield null (signifying“unknown”), not true or false, when either input is null. For example,
7 = NULLyields null, as does
7 <> NULL. When this behavior is not suitable, use the
IS [NOT] DISTINCT FROMpredicates:
a IS DISTINCT FROM b a IS NOT DISTINCT FROM b
For non-null inputs,
IS DISTINCT FROMis the same as the
<>operator. However, if both inputs are null it returns false, and if only one input is null it returns true. Similarly,
IS NOT DISTINCT FROMis identical to
=for non-null inputs, but it returns true when both inputs are null, and false when only one input is null. Thus, these predicates effectively act as though null were a normal data value, rather than“unknown”.
To check whether a value is or is not null, use the predicates:
expression IS NULL expression IS NOT NULL
or the equivalent, but nonstandard, predicates:
expression ISNULL expression NOTNULL
NULLis not“equal to”
NULL. (The null value represents an unknown value, and it is not known whether two unknown values are equal.)
Some applications might expect that
expression= NULLreturns true if_
expression_evaluates to the null value. It is highly recommended that these applications be modified to comply with the SQL standard. However, if that cannot be done thetransform_null_equalsconfiguration variable is available. If it is enabled,PostgreSQLwill convert
x = NULLclauses to
x IS NULL.
expression_is row-valued, then
IS NULLis true when the row expression itself is null or when all the row's fields are null, while
IS NOT NULLis true when the row expression itself is non-null and all the row's fields are non-null. Because of this behavior,
IS NOT NULLdo not always return inverse results for row-valued expressions; in particular, a row-valued expression that contains both null and non-null fields will return false for both tests. In some cases, it may be preferable to write
IS DISTINCT FROM NULLor
IS NOT DISTINCT FROM NULL, which will simply check whether the overall row value is null without any additional tests on the row fields.
Boolean values can also be tested using the predicates
boolean_expression IS TRUE boolean_expression IS NOT TRUE boolean_expression IS FALSE boolean_expression IS NOT FALSE boolean_expression IS UNKNOWN boolean_expression IS NOT UNKNOWN
These will always return true or false, never a null value, even when the operand is null. A null input is treated as the logical value“unknown”. Notice that
IS NOT UNKNOWNare effectively the same as
IS NOT NULL, respectively, except that the input expression must be of Boolean type.
Some comparison-related functions are also available, as shown inTable 9.3.
Table 9.3. Comparison Functions
||returns the number of non-null arguments||
||returns the number of null arguments||