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 0rather 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

原文：http://www.cnblogs.com/yuanjiangw/p/6234565.html