The usual comparison operators are available, as shown in Table 9-1.
Table 9-1. Comparison Operators
|<=||less than or equal to|
|>=||greater than or equal to|
|<> or !=||not equal|
Note: The != operator is converted to <> in the parser stage. It is not possible to implement != and <> 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 3).
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.
Table 9-2. Comparison Predicates
|a BETWEEN x AND y||between|
|a NOT BETWEEN x AND y||not between|
|a BETWEEN SYMMETRIC x AND y||between, after sorting the comparison values|
|a NOT BETWEEN SYMMETRIC x AND y||not between, after sorting the comparison values|
|a IS DISTINCT FROM b||not equal, treating null like an ordinary value|
|a IS NOT DISTINCT FROM b||equal, treating null like an ordinary value|
|expression IS NULL||is null|
|expression IS NOT NULL||is not null|
|expression ISNULL||is null (nonstandard syntax)|
|expression NOTNULL||is not null (nonstandard syntax)|
|boolean_expression IS TRUE||is true|
|boolean_expression IS NOT TRUE||is false or unknown|
|boolean_expression IS FALSE||is false|
|boolean_expression IS NOT FALSE||is true or unknown|
|boolean_expression IS UNKNOWN||is unknown|
|boolean_expression IS NOT UNKNOWN||is true or false|
The BETWEEN predicate simplifies range tests:
a BETWEEN x AND y
is equivalent to
a >= x AND a <= y
Notice that 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 b a 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 NULL expression IS NOT NULL
or the equivalent, but nonstandard, predicates:
expression ISNULL expression 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.)
Tip: 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.
If the 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 row IS DISTINCT FROM NULL or row 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 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.