9.2. 比較函式及運算子

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

Table 9.1. Comparison Operators

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 typeboolean; expressions like1 < 2 < 3are not valid (because there is no<operator to compare a Boolean value with3).

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.

Table 9.2. Comparison Predicates

TheBETWEENpredicate simplifies range tests:

a
 BETWEEN 
x
 AND 
y

is equivalent to

a
>
= 
x
 AND 
a
<
= 
y

Notice thatBETWEENtreats the endpoint values as included in the range.NOT BETWEENdoes the opposite comparison:

a
 NOT BETWEEN 
x
 AND 
y

is equivalent to

a
<
x
 OR 
a
>
y

BETWEEN SYMMETRICis likeBETWEENexcept there is no requirement that the argument to the left ofANDbe 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 = NULLyields null, as does7 <> NULL. When this behavior is not suitable, use theIS [NOT] DISTINCT FROMpredicates:

a
 IS DISTINCT FROM 
b
a
 IS NOT DISTINCT FROM 
b

For non-null inputs,IS DISTINCT FROMis 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 FROMis 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 NULL

expression
 IS NOT NULL

or the equivalent, but nonstandard, predicates:

expression
 ISNULL

expression
 NOTNULL

Donot_writeexpression_= NULLbecauseNULLis 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 thatexpression= NULLreturns 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 thetransform_null_equalsconfiguration variable is available. If it is enabled,PostgreSQLwill convertx = NULLclauses tox IS NULL.

If theexpression_is row-valued, thenIS NULLis true when the row expression itself is null or when all the row's fields are null, whileIS NOT NULLis true when the row expression itself is non-null and all the row's fields are non-null. Because of this behavior,IS NULLandIS NOT NULLdo 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 NULLorrow_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 predicates

boolean_expression
 IS TRUE

boolean_expression
 IS NOT TRUE

boolean_expression
 IS FALSE

boolean_expression
 IS NOT FALSE

boolean_expression
 IS UNKNOWN

boolean_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 UNKNOWNandIS NOT UNKNOWNare effectively the same asIS NULLandIS 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.

Table 9.3. Comparison Functions