官网描述
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.
NULL-安全相等。此运算符执行类似于相等比较 =运算符，差别是：
如果两个操作数都是NULL则返回1，而不是NULL，
如果只一个操作数是NULL则返回0，而不是NULL。
mysql> SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
-> 1, 1, 0
mysql> SELECT 1 = 1, NULL = NULL, 1 = NULL;
-> 1, NULL, NULL
数据库三值逻辑