The usual comparison operators are shown in Table 8-1.
Table 8-1. 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.
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.
The ordinary comparison operators yield null (signifying "unknown") when either input is null. Another way to do comparisons is with the IS DISTINCT FROM construct:
expression IS DISTINCT FROM expression
For non-null inputs this 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. Thus it effectively acts 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.