MySQL中的"<=>"是什么意思？

Question

<=>MySQL中的含义是什么意思？

Answer 1

该手册说明了一切:

NULL安全相等.此运算符执行与=运算符类似的相等比较,但如果两个操作数均为NULL,则返回1而不是NULL;如果一个操作数为NULL,则返回0而不是NULL.

mysql> select NULL <=> NULL;
+---------------+
| NULL <=> NULL |
+---------------+
|             1 |
+---------------+
1 row in set (0.00 sec)

mysql> select NULL = NULL;
+-------------+
| NULL = NULL |
+-------------+
|        NULL |
+-------------+
1 row in set (0.00 sec)

mysql> select NULL <=> 1;
+------------+
| NULL <=> 1 |
+------------+
|          0 |
+------------+
1 row in set (0.00 sec)

mysql> select NULL = 1;
+----------+
| NULL = 1 |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

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