9.2. Comparison Functions and Operators
The usual comparison operators are available, as shown in Table 9.1.
Table 9.1. Comparison Operators
|Less than or equal to|
|Greater than or equal to|
<> 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
<> 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
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
Between (inclusive of the range endpoints).
Not between (the negation of
Between, after sorting the two endpoint values.
Not between, after sorting the two endpoint values.
Not equal, treating null as a comparable value.
Equal, treating null as a comparable value.
Test whether value is null.
Test whether value is not null.
Test whether value is null (nonstandard syntax).
Test whether value is not null (nonstandard syntax).
Test whether boolean expression yields true.
Test whether boolean expression yields false or unknown.
Test whether boolean expression yields false.
Test whether boolean expression yields true or unknown.
Test whether boolean expression yields unknown.
Test whether boolean expression yields true or false.
BETWEEN predicate simplifies range tests:
is equivalent to
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.
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:
aIS DISTINCT FROM
aIS NOT DISTINCT FROM
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:
expressionIS NOT NULL
or the equivalent, but nonstandard, predicates:
Do not write
expression = NULL
NULL is not “equal to”
NULL. (The null value represents an unknown value, and it is not known whether two unknown values are equal.)
Some applications might expect that
returns true if
expression = NULL
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.
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
IS DISTINCT FROM NULL or
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_expressionIS NOT TRUE
boolean_expressionIS NOT FALSE
boolean_expressionIS 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