What is this operator <=> in MySQL?
It is the NULL-safe equal to operator
<=> Operator is used to compare NULL values with the fields. If normal =(equals) Operators return NULL if one of the comparison value is NULL. With <=> operator returns true or false. <=> Operator is same as IS NULL.
From the manual:-
<=>
performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
-> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
-> 1, NULL, NULL
Edit:-(Although very late to add one important side note mentioning NOT <=> as well)
On a side note:-
NOT <=>
There is one more point NOT <=> which is used to compare NULL values with the fields. If normal != or <> (not equals) Operators return NULL if one of the comparison value is NULL. With NOT applied to <=> operator returns true or false. NOT applied to <=> Operator is same as IS NOT NULL.
Example:-
SELECT NULL != NULL, //--Result is NULL
NOT NULL <=> NULL, //--Result is 0
NULL IS NOT NULL; //--Result is 0
is <=> NULL-safe equal to operator
This operator performs an equality comparison like the = operator, but returns 1 rather than NULL if both operands are NULL, and 0 rather than NULL if one operand is NULL.
See here for the documentation
Sample :
you should use IS NOT NULL. (The comparison operators = and <> both give UNKNOWN with NULL on either side of the expression.)
SELECT *
FROM table
WHERE YourColumn IS NOT NULL;
can also negate the null safe equality operator but this is not standard SQL.
SELECT *
FROM table
WHERE NOT (YourColumn <=> NULL);
TL;DR
It's the NULL
safe equal operator.
Like the regular =
operator, two values are compared and the result is either 0
(not equal) or 1
(equal); in other words: 'a' <=> 'b'
yields 0
and 'a' <=> 'a'
yields 1
.
Unlike the regular =
operator, values of NULL
don't have a special meaning and so it never yields NULL
as a possible outcome; so: 'a' <=> NULL
yields 0
and NULL <=> NULL
yields 1
.
Usefulness
This can come in useful when both operands may contain NULL
and you need a consistent comparison result between two columns.
Another use-case is with prepared statements, for example:
... WHERE col_a <=> ? ...
Here, the placeholder can be either a scalar value or NULL
without having to change anything about the query.
Related operators
Besides <=>
there are also two other operators that can be used to compare against NULL
, namely IS NULL
and IS NOT NULL
; they're part of the ANSI standard and therefore supported on other databases, unlike <=>
, which is MySQL-specific.
You can think of them as specialisations of MySQL's <=>
:
'a' IS NULL ==> 'a' <=> NULL
'a' IS NOT NULL ==> NOT('a' <=> NULL)
Based on this, your particular query (fragment) can be converted to the more portable:
WHERE p.name IS NULL
Support
The SQL:2003 standard introduced a predicate for this, which works exactly like MySQL's <=>
operator, in the following form:
IS [NOT] DISTINCT FROM
The following is universally supported, but is relative complex:
CASE WHEN (a = b) or (a IS NULL AND b IS NULL)
THEN 1
ELSE 0
END = 1