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The usual comparison operators are available, shown in Table 9-1 .
Table 9-1. Comparison Operators
| Operator | Description |
|---|---|
| < | less than |
| > | greater than |
| <= | less than or equal to |
| >= | greater than or equal to |
| = | equal |
| <> 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 data types where this makes sense. 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
Similarly,
a
NOT BETWEEN
x
AND
y
is equivalent to
a
<
x
OR
a
>
y
There is no difference between the two respective forms apart from the CPU cycles required to rewrite the first one into the second one internally. 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; the proper range is automatically determined.
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.) This behavior conforms to the SQL standard.
Tip: Some applications may 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 . This was the default behavior in PostgreSQL releases 6.5 through 7.1.
Note: 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. This definition conforms to the SQL standard, and is a change from the inconsistent behavior exhibited by PostgreSQL versions prior to 8.2.
The ordinary comparison operators yield null (signifying "unknown" ) when either input is null. Another way to do comparisons is with the IS [ NOT ] DISTINCT FROM construct:
expression
IS DISTINCT FROM
expression
expression
IS NOT DISTINCT FROM
expression
For non-null inputs, IS DISTINCT FROM is the same as the <> operator. However, when both inputs are null it will return false, and when just one input is null it will return 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" .
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.