# 9.2. Comparison Operators

The usual comparison operators are available, 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`).

In addition to the comparison operators, the special `BETWEEN` construct is available:

```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 the same as `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` constructs:

```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 constructs 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 constructs:

```expression IS NULL
expression IS NOT NULL
```

or the equivalent, but nonstandard, constructs:

```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 constructs

```expression IS TRUE
expression IS NOT TRUE
expression IS FALSE
expression IS NOT FALSE
expression IS UNKNOWN
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.

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https://www.postgresql.org/docs/9.5/static/functions-comparison.html

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