表示select的类型，常见的取值，如下表所示：

select_type 含义
SIMPLE 简单的select查询，查询中不包含子查询或者UNION
PRIMARY 查询中若包含任何复杂的子查询，最外层查询标记为该标识
SUBQUERY 在SELECT或WHERE列表中包含了子查询
DERIVED 在FROM列表中包含的子查询，被标记为DERIVED（衍生）MYSQL会递归执行这些子查询，把结果放在临时表中
UNION 若第二个SELECT出现在UNION之后，则标记为UNION；若UNION包含在FROM子句的子查询中，外层SELECT将被标记为：DERIVED
UNION
 从UNION表获取结果的SELECT

SIMPLE：

  1. explain select * from user where id = 11001;
mysql> explain select * from user where id = 11001;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-----------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra                             |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-----------------------------------+
|  1 | SIMPLE      | user  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Directly search via Primary Index |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-----------------------------------+
1 row in set (0.01 sec)

PRIMARY:

explain select * from user where id = (select user_id from user_vip where user_id = 11025);

+----+-------------+----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table    | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | user     | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | user_vip | NULL       | ref   | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set (0.01 sec)