|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
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7.2 Comparison Operators
The usual comparison operators are available, shown in Table 7-1.
|| less than
|| greater than
|| less than or equal to
|| greater than or equal to
!=operator is converted to
<>in the parser stage. It is not possible to implement
<>operators that do different things.
Comparison operators are available for all data types where this
makes sense. All comparison operators are binary operators that
return values of type
boolean; expressions like
1 < 2 < 3 are not valid (because there is
< operator to compare a Boolean value with
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
a NOT BETWEEN x AND y
is equivalent to
a < x OR a > y
There is no difference between the two respective forms apart from
the CPU cycles required to rewrite the first one
into the second one internally.
BETWEEN SYMMETRIC is the same as
except there is no requirement that the argument to the left of
AND be less than
or equal to the argument on the right; the proper range is automatically determined.
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
NULL is not “equal to”
NULL. (The null value represents an unknown value,
and it is not known whether two unknown values are equal.) This
behavior conforms to the SQL standard.
Tip: Some applications may expect that
expression = NULLreturns 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_equalsconfiguration variable is available. If it is enabled, PostgreSQL will convert
x = NULLclauses to
x IS NULL. This was the default behavior in PostgreSQL releases 6.5 through 7.1.
Note: If the expression is row-valued, then
IS NULLis true when the row expression itself is null or when all the row's fields are null, while
IS NOT NULLis true when the row expression itself is non-null and all the row's fields are non-null. This definition conforms to the SQL standard, and is a change from the inconsistent behavior exhibited by PostgreSQL versions prior to 8.2.
The ordinary comparison operators yield null (signifying “unknown”)
when either input is null. Another way to do comparisons is with the
IS [ NOT ] DISTINCT FROM construct:
expression IS DISTINCT FROM expression expression IS NOT DISTINCT FROM expression
For non-null inputs,
IS DISTINCT FROM is
the same as the
<> operator. However, when both
inputs are null it will return false, and when just one input is
null it will return 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”.
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”.
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.
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|