mysql - IFNULL 和 IF 的区别

Question

以下两种说法是否有区别：

mysql> EXPLAIN SELECT IF(arms IS NULL, 'asdf', arms) FROM limbs;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | limbs | ALL  | NULL          | NULL | NULL    | NULL |   12 |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT IF(arms IS NULL, 'asdf', arms) FROM limbs;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | limbs | ALL  | NULL          | NULL | NULL    | NULL |   12 |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

一个性能更好还是比另一个更好？还是它们相同？

Answer 1

IFNULL只是ANSI SQL 的语法糖，IF它是 ANSI SQL 的语法糖CASE。

因此，请使用您更喜欢的，以及适合您特定需求的。

PS：EXPLAIN不依赖SELECT