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MySQLleftjoin左连接详解介绍

时间:2021-07-01 10:21:17 帮助过:41人阅读

LEFT JOIN 关键字会从左表 (table_name1) 那里返回所有的行,即使在右表 (table_name2) 中没有匹配的行。left join 左连接——以左边的数据表为基准,若右边的数据表没有对应的数据则显示空白

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


1

代码如下
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

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

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

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

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

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

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

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

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