# 9.2. 比較函式及運算子 The usual comparison operators are available, as shown in [Table 9.1](https://www.postgresql.org/docs/12/functions-comparison.html#FUNCTIONS-COMPARISON-OP-TABLE). #### **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](https://www.postgresql.org/docs/12/functions-comparison.html#FUNCTIONS-COMPARISON-PRED-TABLE). 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 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](https://www.postgresql.org/docs/12/runtime-config-compatible.html#GUC-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](https://www.postgresql.org/docs/12/functions-comparison.html#FUNCTIONS-COMPARISON-FUNC-TABLE). #### **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` | --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.postgresql.tw/13/the-sql-language/functions-and-operators/comparison-functions-and-operators.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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