MySQL left join 左连接详解介绍

表a

1 aid adate

2 1 a1

3 2 a2

4 3 a3
表b

1 bid bdate

2 1 b1

3 2 b2

4 4 b4

代码如下	复制代码
select * from a left join b on a.aid = b.bid

首先取出a表中所有数据，然后再加上与a、b匹配的的数据。

此时的取出的是：

1 1 a1 b1

2 2 a2 b2

3 3 a3 空字符
同样的也有right join

指的是首先取出b表中所有数据，然后再加上与a、b匹配的的数据。

此时的取出的是：

1 1 a1 b1

2 2 a2 b2

left join性能

1例子

代码如下	复制代码
select distinct count('goods_id') as num , g.goods_id,g.goods_name from ms_goods as g left join ms_order_goods as og on g.goods_id = og.goods_id group by goods_id

显示行 0 - 29 (1,475 总计, 查询花费 0.0167 秒)

2例子

代码如下	复制代码
select distinct count('goods_id') as num , g.goods_id,g.goods_name from ms_goods as g , ms_order_goods as og where g.goods_id = og.goods_id group by goods_id

显示行 0 - 29 (257 总计, 查询花费 0.0088 秒)

实例

表结构如下：

MDate      StoreCode GoodsCode ToStoreCode GoodsAmount GoodsFlag
2005-12-12   001             101                  888                         2            3
2005-12-13   001             101                     3                          1
2005-12-15   001             101                     1                          2

三条记录的意思分别为：
2005-12-12从001移到888 2件
2005-12-13从001销售     3件
2005-12-15仓店001进货   2件

出报表
StoreCode GoodsCode StockAmount   InAmount     OutAmount    SellAmount
001              101                       3                  0                      2             1
注：StockAmount进货数量   InAmount移入数量     OutAmount移出数量    SellAmount销售数量

代码如下	复制代码
select StoreCode, GoodsCode, StockAmount = sum(case GoodsFlag when 2 then GoodsAmount else 0 end), InAmount = sum(case GoodsFlag when 4 then GoodsAmount else 0 end), OutAmount = sum(case GoodsFlag when 3 then GoodsAmount else 0 end), SellAmount = sum(case GoodsFlag when 1 then GoodsAmount else 0 end) from 表 group by StoreCode,GoodsCode

例子3
po_order_det 表
ID   MA_ID     QTY
01   #21钢     30
02   #22钢     40
03   #23铝     30

ST_CONVER 表
ID QTY
01 20
02 10
要求返回集 QTY = po_order_det.QTY - ST_CONVER.QTY AND po_order_det.ID=ST_CONVER.ID

ID     MA_ID   QTY
01     #21钢   10
02     #22钢   30
03     #23铝    30

代码如下	复制代码
select a.order_id,a.id,a.ma_id,a.qty,isnull(b.qty,0) qtyy, isnull(a.qty - b.qty ,0) qtyx from po_order_det a left join st_conver b on a.id=b.id and a.filid=b.filid and a.id=b.id and a.order_id=b.order_id Where a.filid='S'

总结

SQL 中只出现一个左连接：

代码如下

复制代码

select a.*, b.*, c.* from b, a

left join c on a.id = c.id

where b.id = a.iid

注意表a 要和最近的一个关联的leftjoin挨着。

SQL中出现了2个或者多个左连接：

代码如下

复制代码

select a.*, b.* c.*, d.*, e.*

from ((

c, b, a

)

left join d on d.id = a.id

) left join e on e.id = b.id

where c.id = b.id

and b.id = a.id

leftjoin总和最近的一个表挨着，同时需要用借助于括号

补充：数据库,mysql教程