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NULL-safe equal
http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#operator_equal-to
SELECT 1=1,1=NULL,0=NULL,NULL=NULL;
1 NULL NULL NULL
SELECT 1<=>1,1<=>NULL,0<=>NULL,NULL<=>NULL;
1 0 0 1
<=>
NULL
-safe equal. 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
.
The <=>
operator is equivalent to the standard SQL IS NOT DISTINCT FROM
operator.
For row comparisons, (a, b) <=> (x, y)
is equivalent to:
(a <=> x) AND (b <=> y)
NULL-safe equal
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