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 | 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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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' |
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总结
SQL 中只出现一个左连接：
| 代码如下 | 复制代码 |
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select a.*, b.*, c.* from b, a left join c on a.id = c.id where b.id = a.iid |
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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 |
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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
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leftjoin总和最近的一个表挨着，同时需要用借助于括号