MySQL中的<=>
是什么意思和做什么?
The manual says it all:
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.
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)
mysql>
It's the NULL-safe equal operator .
The difference between <=> and = is when one or both of the operands are NULL values. For example:
NULL <=> NULL gives True
NULL = NULL gives NULL
Here is the full table for the <=>
comparison of values 1, 2 and NULL:
| 1 2 NULL -----+------------------- 1 | True False False 2 | False True False NULL | False False True
Compare to the ordinary equality operator:
| 1 2 NULL -----+------------------- 1 | True False NULL 2 | False True NULL NULL | NULL NULL NULL
NULL-safe equal to operator
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_equal-to
<=>
is a so called NULL
-safe-equality operator .
SELECT 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
-> 1, 1, 0
SELECT 1 = 1, NULL = NULL, 1 = NULL;
-> 1, NULL, NULL
它与SQL标准关键字DISTINCT相同
SELECT * FROM somewhere WHERE `address1` is not distinct from `address2`
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