1 一道面試題
新建兩張表

mysql> create table m
    -> (
    -> mid int,
    -> hid int,
    -> gid int,
    -> mres varchar(10),
    -> matime date
    -> )engine myisam charset utf8;
Query OK, 0 rows affected, 1 warning (0.79 sec)

mysql> create table t
    -> (
    -> tid int,
    -> tname varchar(20)
    -> )engine myisam charset utf8;
Query OK, 0 rows affected, 1 warning (0.43 sec)
 

插入如下資料

要求：

思考:

1

mysql> select hid,mres,gid,matime from m;

2

mysql> select t.tname, mres,gid,matime from m
    -> left join t
    -> on
    -> m.hid = t.tid;

可以把

m left join t on m.hid = t.tid

這一部分再當成一個表來看

3 初步思路

mysql> select t.tname,mres,gid,matime from
    -> (m left join t on m.hid = t.tid) left join t
    -> on m.gid = t.tid;
 

報錯

ERROR 1066 (42000): Not unique table/alias: 't'

4 修改
錯誤的原因是 m t t相連，兩張t表，名字衝突，起個別名就可以解決

mysql> select t1.tname,mres,t2.tname,matime from
    -> (m left join t as t1 on m.hid = t1.tid) left join t
    -> as t2
    -> on m.gid = t2.tid;

5 進一步

mysql> select t1.tname,mres,t2.tname,matime from
    -> (m left join t as t1 on m.hid = t1.tid) left join t
    -> as t2
    -> on m.gid = t2.tid
    -> where
    -> matime
    -> between '2006-06-01' and '2006-07-01';