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The usual comparison operators are available, shown inTable 9.1.

Comparison operators are available for all relevant data types. All comparison operators are binary operators that return values of typeboolean; expressions like1 < 2 < 3 are not valid (because there is no< operator to compare a Boolean value with3).

In addition to the comparison operators, the specialBETWEEN construct is available:

a BETWEENx ANDy

is equivalent to

a >=x ANDa <=y

Notice thatBETWEEN treats the endpoint values as included in the range.NOT BETWEEN does the opposite comparison:

a NOT BETWEENx ANDy

is equivalent to

a <x ORa >y

BETWEEN SYMMETRIC is the same asBETWEEN except there is no requirement that the argument to the left ofAND 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 does7 <> NULL. When this behavior is not suitable, use theIS [ NOT] DISTINCT FROM constructs:

a IS DISTINCT FROMba IS NOT DISTINCT FROMb

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 NULLexpression IS NOT NULL

or the equivalent, but nonstandard, constructs:

expression ISNULLexpression NOTNULL

Donot writeexpression = NULL becauseNULL is not“equal to”NULL. (The null value represents an unknown value, and it is not known whether two unknown values are equal.)

If theexpression is row-valued, thenIS NULL is true when the row expression itself is null or when all the row's fields are null, whileIS 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 andIS 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 writerowIS DISTINCT FROM NULL orrowIS 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 TRUEexpression IS NOT TRUEexpression IS FALSEexpression IS NOT FALSEexpression IS UNKNOWNexpression 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 thatIS UNKNOWN andIS NOT UNKNOWN are effectively the same asIS NULL andIS NOT NULL, respectively, except that the input expression must be of Boolean type.