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The usual comparison operators are available, as shown inTable 9.1.

These comparison operators are available for all built-in data types that have a natural ordering, including numeric, string, and date/time types. In addition, arrays, composite types, and ranges can be compared if their component data types are comparable.

It is usually possible to compare values of related data types as well; for exampleinteger>bigint will work. Some cases of this sort are implemented directly by“cross-type” comparison operators, but if no such operator is available, the parser will coerce the less-general type to the more-general type and apply the latter's comparison operator.

As shown above, all comparison operators are binary operators that return values of typeboolean. Thus, expressions like1 < 2 < 3 are not valid (because there is no< operator to compare a Boolean value with3). Use theBETWEEN predicates shown below to perform range tests.

There are also some comparison predicates, as shown inTable 9.2. These behave much like operators, but have special syntax mandated by the SQL standard.

TheBETWEEN predicate simplifies range tests:

a BETWEENx ANDy

is equivalent to

a >=x ANDa <=y

Notice thatBETWEEN treats the endpoint values as included in the range.BETWEEN SYMMETRIC is likeBETWEEN 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.

The various variants ofBETWEEN are implemented in terms of the ordinary comparison operators, and therefore will work for any data type(s) that can be compared.

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 predicates:

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 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 NULLexpression IS NOT NULL

or the equivalent, but nonstandard, predicates:

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. For example:

SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');SELECT ROW(table.*) IS NULL FROM table;  -- detect all-null rowsSELECT ROW(table.*) IS NOT NULL FROM table;  -- detect all-non-null rowsSELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in rows

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 predicates

boolean_expression IS TRUEboolean_expression IS NOT TRUEboolean_expression IS FALSEboolean_expression IS NOT FALSEboolean_expression IS UNKNOWNboolean_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 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.

Some comparison-related functions are also available, as shown inTable 9.3.