9.2. 比較函式及運算子

The usual comparison operators are available, as shown in Table 9.1.

Table 9.1. Comparison Operators

Operator	Description
datatype < datatype → boolean	Less than
datatype > datatype → boolean	Greater than
datatype <= datatype → boolean	Less than or equal to
datatype >= datatype → boolean	Greater than or equal to
datatype = datatype → boolean	Equal
datatype <> datatype → boolean	Not equal
datatype != datatype → boolean	Not equal

Note

<> is the standard SQL notation for “not equal”. != is an alias, which is converted to <> at a very early stage of parsing. Hence, it is not possible to implement != and <> operators that do different things.

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 example integer > 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 type boolean. Thus, expressions like 1 < 2 < 3 are not valid (because there is no < operator to compare a Boolean value with 3). Use the BETWEEN predicates shown below to perform range tests.

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

Table 9.2. Comparison Predicates

Predicate Description Example(s)
datatype BETWEEN datatype AND datatype → boolean Between (inclusive of the range endpoints). 2 BETWEEN 1 AND 3 → t 2 BETWEEN 3 AND 1 → f
datatype NOT BETWEEN datatype AND datatype → boolean Not between (the negation of BETWEEN). 2 NOT BETWEEN 1 AND 3 → f
datatype BETWEEN SYMMETRIC datatype AND datatype → boolean Between, after sorting the two endpoint values. 2 BETWEEN SYMMETRIC 3 AND 1 → t
datatype NOT BETWEEN SYMMETRIC datatype AND datatype → boolean Not between, after sorting the two endpoint values. 2 NOT BETWEEN SYMMETRIC 3 AND 1 → f
datatype IS DISTINCT FROM datatype → boolean Not equal, treating null as a comparable value. 1 IS DISTINCT FROM NULL → t (rather than NULL) NULL IS DISTINCT FROM NULL → f (rather than NULL)
datatype IS NOT DISTINCT FROM datatype → boolean Equal, treating null as a comparable value. 1 IS NOT DISTINCT FROM NULL → f (rather than NULL) NULL IS NOT DISTINCT FROM NULL → t (rather than NULL)
datatype IS NULL → boolean Test whether value is null. 1.5 IS NULL → f
datatype IS NOT NULL → boolean Test whether value is not null. 'null' IS NOT NULL → t
datatype ISNULL → boolean Test whether value is null (nonstandard syntax).
datatype NOTNULL → boolean Test whether value is not null (nonstandard syntax).
boolean IS TRUE → boolean Test whether boolean expression yields true. true IS TRUE → t NULL::boolean IS TRUE → f (rather than NULL)
boolean IS NOT TRUE → boolean Test whether boolean expression yields false or unknown. true IS NOT TRUE → f NULL::boolean IS NOT TRUE → t (rather than NULL)
boolean IS FALSE → boolean Test whether boolean expression yields false. true IS FALSE → f NULL::boolean IS FALSE → f (rather than NULL)
boolean IS NOT FALSE → boolean Test whether boolean expression yields true or unknown. true IS NOT FALSE → t NULL::boolean IS NOT FALSE → t (rather than NULL)
boolean IS UNKNOWN → boolean Test whether boolean expression yields unknown. true IS UNKNOWN → f NULL::boolean IS UNKNOWN → t (rather than NULL)
boolean IS NOT UNKNOWN → boolean Test whether boolean expression yields true or false. true IS NOT UNKNOWN → t NULL::boolean IS NOT UNKNOWN → f (rather than NULL)

The BETWEEN predicate simplifies range tests:

a BETWEEN x AND y

is equivalent to

a >= x AND a <= y

Notice that BETWEEN treats the endpoint values as included in the range. BETWEEN SYMMETRIC is like 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. If it is not, those two arguments are automatically swapped, so that a nonempty range is always implied.

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

Note

The use of AND in the BETWEEN syntax creates an ambiguity with the use of AND as a logical operator. To resolve this, only a limited set of expression types are allowed as the second argument of a BETWEEN clause. If you need to write a more complex sub-expression in BETWEEN, write parentheses around the sub-expression.

Ordinary comparison operators yield null (signifying “unknown”), not true or false, when either input is null. For example, 7 = NULL yields null, as does 7 <> NULL. When this behavior is not suitable, use the IS [ NOT ] DISTINCT FROM predicates:

a IS DISTINCT FROM b
a IS NOT DISTINCT FROM b

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 NULL
expression IS NOT NULL

or the equivalent, but nonstandard, predicates:

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.)

Tip

Some applications might 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.

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. Because of this behavior, IS NULL and IS 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 write row IS DISTINCT FROM NULL or row 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 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.

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

Table 9.3. Comparison Functions

Function Description Example(s)
num_nonnulls ( VARIADIC "any" ) → integer Returns the number of non-null arguments. num_nonnulls(1, NULL, 2) → 2
num_nulls ( VARIADIC "any" ) → integer Returns the number of null arguments. num_nulls(1, NULL, 2) → 1