The usual comparison operators are available, 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).
In addition to the comparison operators, the special BETWEEN construct is available:
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 the same as 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 constructs:
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 constructs 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 constructs:
expression IS NULL expression IS NOT NULL
or the equivalent, but nonstandard, constructs:
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 constructs
expression IS TRUE expression IS NOT TRUE expression IS FALSE expression IS NOT FALSE expression IS UNKNOWN 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.