左连接查询

左连接查询语句
表1 left join 表2 on 条件； 然后where，having，group等语句可以照常使用
以下例子用两次左连接进行匹配
select t1.tname,mres,t2.tname,matime from m left join t as t1 on t1.tid=m.hid left join t as t2 on t2.tid=m.gid;


mysql> set names gbk;
Query OK, 0 rows affected (0.02 sec)

mysql> use qq;
Database changed
mysql> create table m(
    -> mid int not null primary key,
    -> hid int ,
    -> gid int ,
    -> mres varchar(20),
    -> matime date
    -> )engine myisam charset utf8;
Query OK, 0 rows affected (0.11 sec)

mysql> 
mysql> create table t(
    -> tid int ,
    -> tname varchar(20)
    -> )engine myisam charset utf8;
Query OK, 0 rows affected (0.03 sec)

mysql> 
mysql> insert into m
    -> values
    -> (1,1,2,'2:0','2006-05-21'),
    -> (2,2,3,'1:2','2006-06-21'),
    -> (3,3,1,'2:5','2006-05-25'),
    -> (4,2,1,'3:2','2006-07-21');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into t
    -> values
    -> (1,'国安'),
    -> (2,'申花'),
    -> (3,'公益联队');
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select $ from m;
ERROR 1054 (42S22): Unknown column '$' in 'field list'
mysql> select * from m;
+-----+------+------+------+------------+
| mid | hid  | gid  | mres | matime     |
+-----+------+------+------+------------+
|   1 |    1 |    2 | 2:0  | 2006-05-21 |
|   2 |    2 |    3 | 1:2  | 2006-06-21 |
|   3 |    3 |    1 | 2:5  | 2006-05-25 |
|   4 |    2 |    1 | 3:2  | 2006-07-21 |
+-----+------+------+------+------------+
4 rows in set (0.03 sec)

mysql> #将所有队列序号改为名字
mysql> select t1.tname,mres,t2.tname,matime from
    -> m left join t as t1 on t1.tid=m.hid
    -> left join t as t2 on t2.tid=m.gid;
+----------+------+----------+------------+
| tname    | mres | tname    | matime     |
+----------+------+----------+------------+
| 国安         | 2:0  | 申花       | 2006-05-21 |
| 申花       | 1:2  | 公益联队         | 2006-06-21 |
| 公益联队         | 2:5  | 国安         | 2006-05-25 |
| 申花       | 3:2  | 国安         | 2006-07-21 |
+----------+------+----------+------------+
4 rows in set (0.01 sec)