sql - 两个左外连接

Question

我有两个这种形式的表：

存货：

Units   InDate OutDate

1000     11/4   12/4

2000     13/4   14/4

价格：

Date Price
11/4    5
12/4    4
13/4    6
14/4    7

我想建立下表：

Units   InDate OutDate InPrice   OutPrice

1000     11/4    12/4     5       4

2000     13/4    14/4     6       7

我想我应该使用类似的东西：

Select *
FROM Inventory
LEFT OUTER JOIN Prices ON Inventory.InDate = Prices.Date
LEFT OUTER JOIN Prices ON Inventory.OutDate = Prices.Date

但是第二个 OUTER JOIN 似乎把事情搞砸了。

我怎样才能达到这个结果？

Answer 1

Select
  Units,   
  InDate,
  OutDate,
  P1.Price as InPrice,
  P2.Price as OutPrice
FROM Inventory
LEFT OUTER JOIN Prices as P1 ON Inventory.InDate = P1.Date
LEFT OUTER JOIN Prices as P2 ON Inventory.OutDate = P2.Date

Answer 2

尝试这个。

SELECT Inventory.Units, Inventory.InDate, Inventory.OutDate, InPrices.Price AS InPrice, OutPrices.Price AS OutPrice
FROM Inventory
LEFT OUTER JOIN Prices AS InPrices ON Inventory.InDate = InPrices.Date
LEFT OUTER JOIN Prices AS OutPrices ON Inventory.OutDate = OutPrices.Date

Answer 3

您当前的查询非常接近正确。如果您在桌子上放置不同的别名，prices那么它会起作用。由于您在同一张表上加入prices两次，因此您需要使用不同的别名来区分它们：

select i.units,
  i.indate,
  i.outdate,
  inPrice.price,  
  outPrice.price
from inventory i
left join prices inPrice  -- first join with alias
  on i.indate = inPrice.date
left join prices outPrice  -- second join with alias
  on i.outdate = outPrice.date

请参阅带有演示的 SQL Fiddle