The usual comparison operators are available, as shown in Table 9.1.
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 < 3 are not valid (because there is no
< operator to compare a Boolean value with
There are also some comparison predicates, as shown in Table 9.2. These behave much like operators, but have special syntax mandated by the SQL standard.
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
BETWEEN predicate simplifies range tests:
a BETWEEN x AND y
is equivalent to
a >= x AND a <= y
BETWEEN treats the endpoint values as included in the range.
NOT BETWEEN does the opposite comparison:
a NOT BETWEEN x AND y
is equivalent to
a < x OR a > y
BETWEEN SYMMETRIC is like
BETWEEN except there is no requirement that the argument to the left of
AND be 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 = NULL yields null, as does
7 <> NULL. When this behavior is not suitable, use the
IS [ NOT ] DISTINCT FROM predicates:
a IS DISTINCT FROM ba IS NOT DISTINCT FROM b
For non-null inputs,
IS DISTINCT FROM is 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 FROM is 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 NULLexpression IS NOT NULL
or the equivalent, but nonstandard, predicates:
expression ISNULLexpression NOTNULL
Do not write
expression = NULL because
NULL is 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 = NULL returns 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 the transform_null_equals configuration variable is available. If it is enabled, PostgreSQL will convert
x = NULL clauses to
x IS NULL.
expression is row-valued, then
IS NULL is true when the row expression itself is null or when all the row's fields are null, while
IS NOT NULL is true when the row expression itself is non-null and all the row's fields are non-null. Because of this behavior,
IS NULL and
IS NOT NULL do 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 NULL or
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 TRUEboolean_expression IS NOT TRUEboolean_expression IS FALSEboolean_expression IS NOT FALSEboolean_expression IS UNKNOWNboolean_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 UNKNOWN and
IS NOT UNKNOWN are effectively the same as
IS NULL and
IS NOT NULL, respectively, except that the input expression must be of Boolean type.
Some comparison-related functions are also available, as shown in Table 9.3.
returns the number of non-null arguments
returns the number of null arguments