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
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
-> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
-> 1, NULL, NULL