半连接是MySQL 5.6.5引入的，多在子查询exists中使用，对外部row source的每个键值，查找到内部row source匹配的第一个键值后就返回，如果找到就不用再查找内部row source其他的键值了。

测试环境

mysql> desc class;+------------+-------------+------+-----+---------+-------+| Field      | Type        | Null | Key | Default | Extra |+------------+-------------+------+-----+---------+-------+| class_num  | int(11)     | NO   | PRI | NULL    |       || class_name | varchar(20) | YES  |     | NULL    |       |+------------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)mysql> desc roster;+-------------+---------+------+-----+---------+-------+| Field       | Type    | Null | Key | Default | Extra |+-------------+---------+------+-----+---------+-------+| class_num   | int(11) | YES  |     | NULL    |       || student_num | int(11) | YES  |     | NULL    |       |+-------------+---------+------+-----+---------+-------+2 rows in set (0.00 sec)

roster表中记录的是学生的学号以及对应的教室，多个学生可能在同一个教室，所以字段class_num有重复值

class表中记录的是教室及对应的班级名，字段class_num为唯一值

如果要查询存在学生的班级有哪些

mysql>  SELECT class.class_num, class.class_name FROM class INNER JOIN roster WHERE class.class_num = roster.class_num;       +-----------+------------+| class_num | class_name |+-----------+------------+|         2 | class 2    ||         3 | class 3    ||         3 | class 3    |+-----------+------------+3 rows in set (0.00 sec)

可以通过distinct去除重复值，但这样做影响性能，所以通过子查询来得出结果

mysql>  SELECT class_num, class_name FROM class WHERE class_num IN (SELECT class_num FROM roster);       +-----------+------------+| class_num | class_name |+-----------+------------+|         2 | class 2    ||         3 | class 3    |+-----------+------------+2 rows in set (0.00 sec)

优化器实际上是将子查询改写为了半连接

mysql> explain SELECT class_num, class_name FROM class WHERE class_num IN (SELECT class_num FROM roster);+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                                             |+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+|  1 | SIMPLE      | roster | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Start temporary                                                   ||  1 | SIMPLE      | class  | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL |    4 |    25.00 | Using where; End temporary; Using join buffer (Block Nested Loop) |+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------------------------------------------------------------+2 rows in set, 1 warning (0.00 sec)mysql> show warnings;+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Level | Code | Message                                                                                                                                                                                                                      |+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Note  | 1003 | /* select#1 */ select `test`.`class`.`class_num` AS `class_num`,`test`.`class`.`class_name` AS `class_name` from `test`.`class` semi join (`test`.`roster`) where (`test`.`class`.`class_num` = `test`.`roster`.`class_num`) |+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)

Start temporary 和 End temporary的使用表明使用了临时表来去除重复值 如果 select_type 的值为 MATERIALIZED 并且 字段 rows的输出是 
    
      则表明临时表用于了物化表 select_type value of MATERIALIZED and rows with a table value of 
     
      .
     
    

 

如果子查询符合准则（参考文献：http://dev.mysql.com/doc/refman/5.7/en/subquery-optimization.html#semi-joins），MySQL将其转化为semi-join并从以下策略中作出基于cost的选择

• Convert the subquery to a join, or use table pullout and run the query as an inner join between subquery tables and outer tables. Table pullout pulls a table out from the subquery to the outer query.

• Duplicate Weedout: Run the semi-join as if it was a join and remove duplicate records using a temporary table.

• FirstMatch: When scanning the inner tables for row combinations and there are multiple instances of a given value group, choose one rather than returning them all. This "shortcuts" scanning and eliminates production of unnecessary rows.

• LooseScan: Scan a subquery table using an index that enables a single value to be chosen from each subquery's value group.

• Materialize the subquery into a temporary table with an index and use the temporary table to perform a join. The index is used to remove duplicates. The index might also be used later for lookups when joining the temporary table with the outer tables; if not, the table is scanned

     不确定的内容不敢随意翻译，摘出来原汁原味的文献内容

系统变量中的semi join 标签控制着半连接是否可用，5.6默认是开启的