IFNULL和IF之间的差异

Question

以下两个陈述之间是否存在差异：

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)

一个人表现得更好还是优于另一个？或者它们是否相同？

Answer 1

IFNULL只是IF的{​​{3}}，这是ANSI SQL CASE的语法糖。

因此，您可以根据自己的特殊需求使用自己喜欢的任何一种。

PS：EXPLAIN不依赖SELECT