I'm working on code written by a previous developer and in a query it says,
WHERE p.name <=> NULL
What does <=>
mean in this query? Is it something equal to =
? Or is it a syntax error?
But it is not showing any errors or exceptions. I already know that <>
= !=
in MySQL.
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
.
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.
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
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