## 9.2. Comparison Functions and Operators

The usual comparison operators are available, as shown in Table 9.1, “Comparison Operators”.

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, “Comparison Predicates”. 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 configuration variable is available. If it is enabled, Postgres Pro 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, “Comparison Functions”.

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`