# 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 `<` 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 relevant data types. 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`).

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 `a` `BETWEEN` `x` `AND` `y` between `a` `NOT BETWEEN` `x` `AND` `y` not between `a` `BETWEEN SYMMETRIC` `x` `AND` `y` between, after sorting the comparison values `a` `NOT BETWEEN SYMMETRIC` `x` `AND` `y` not between, after sorting the comparison values `a` `IS DISTINCT FROM` `b` not equal, treating null like an ordinary value `a` `IS NOT DISTINCT FROM` `b` equal, treating null like an ordinary value `expression` `IS NULL` is null `expression` `IS NOT NULL` is not null `expression` `ISNULL` is null (nonstandard syntax) `expression` `NOTNULL` is not null (nonstandard syntax) `boolean_expression` `IS TRUE` is true `boolean_expression` `IS NOT TRUE` is false or unknown `boolean_expression` `IS FALSE` is false `boolean_expression` `IS NOT FALSE` is true or unknown `boolean_expression` `IS UNKNOWN` is unknown `boolean_expression` `IS NOT UNKNOWN` is true or false

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. `NOT BETWEEN` does the opposite comparison:

`a NOT BETWEEN x AND y`

is equivalent to

`a < x OR a > y`

`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.

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 ba 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 NULLexpression IS NOT NULL`

or the equivalent, but nonstandard, predicates:

`expression ISNULLexpression 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 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 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 Example Result `num_nonnulls(VARIADIC "any")` returns the number of non-null arguments `num_nonnulls(1, NULL, 2)` `2` `num_nulls(VARIADIC "any")` returns the number of null arguments `num_nulls(1, NULL, 2)` `1`
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