9.2. 比較函式及運算子
The usual comparison operators are available, as shown inTable 9.1.
Table 9.1. Comparison Operators
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 typeboolean
; expressions like1 < 2 < 3
are not valid (because there is no<
operator to compare a Boolean value with3
).
There are also some comparison predicates, as shown inTable 9.2. These behave much like operators, but have special syntax mandated by the SQL standard.
Table 9.2. Comparison Predicates
TheBETWEEN
predicate simplifies range tests:
is equivalent to
Notice thatBETWEEN
treats the endpoint values as included in the range.NOT BETWEEN
does the opposite comparison:
is equivalent to
BETWEEN SYMMETRIC
is likeBETWEEN
except there is no requirement that the argument to the left ofAND
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 does7 <> NULL
. When this behavior is not suitable, use theIS [NOT] DISTINCT FROM
predicates:
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:
or the equivalent, but nonstandard, predicates:
Donot_writeexpression
_= NULLbecauseNULL
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 thatexpression
= 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 thetransform_null_equalsconfiguration variable is available. If it is enabled,PostgreSQLwill convertx = NULL
clauses tox IS NULL
.
If theexpression
_is row-valued, thenIS NULL
is true when the row expression itself is null or when all the row's fields are null, whileIS 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
andIS 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 writerowIS DISTINCT FROM NULL
orrow
_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
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 thatIS UNKNOWN
andIS NOT UNKNOWN
are effectively the same asIS NULL
andIS NOT NULL
, respectively, except that the input expression must be of Boolean type.
Some comparison-related functions are also available, as shown inTable 9.3.
Table 9.3. Comparison Functions
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