Question

我有一个需要优化的子查询。它使用

加入itemInventory 3次

左外连接

有没有办法用UNIONs做到这一点？... 3列从每个连接中获取数据，＆＃39; LastDate＆＃39;我发现很难设想一个查询来替换那个效率更高的查询。

以下是查询：

select 
ii1.vendorno, 
ii1.Quantity1, 
ii3.date1, 
ii1.fgspecialpo, 
ii1.itemid,
ii1.LastDate as [LastDate1], 
ii2.LastDate as [LastDate2], 
ii3.LastDate as [LastDate3],
ii1.qtytype as [qtytyp], ii1.quantity2 as [qty2], ii2.Quantity1 as [itemsAddlQty], 
ii3.quantity1 as [Exp Qty]  
from iteminventory ii1      
    left outer join (select * from iteminventory where qtytype=11 ) ii2 on ii1.itemid=ii2.itemid and ii1.fgspecialpo=ii2.fgspecialpo and ii1.vendorno=ii2.vendorno      
    left outer join (select * from iteminventory where qtytype=1 ) ii3 on ii1.itemid=ii3.itemid and ii1.fgspecialpo=ii3.fgspecialpo and ii1.vendorno=ii3.vendorno

Answer 1

UNION不会在这里帮助你。 UNION用于组合两个结果集，而不是您在此处拥有的结果集。您当然可以简化此操作以使用连接而不是那些子查询。我怀疑它在实际的执行计划中没有太大的区别。如果性能有问题，我们需要查看包含索引的表定义。这是你如何转换这个简单的使用连接。

SELECT ii1.vendorno
    , ii1.Quantity1
    , ii3.date1
    , ii1.fgspecialpo
    , ii1.itemid
    , ii1.LastDate AS [LastDate1]
    , ii2.LastDate AS [LastDate2]
    , ii3.LastDate AS [LastDate3]
    , ii1.qtytype AS [qtytyp]
    , ii1.quantity2 AS [qty2]
    , ii2.Quantity1 AS [itemsAddlQty]
    , ii3.quantity1 AS [Exp Qty]
FROM iteminventory ii1
LEFT OUTER JOIN iteminventory ii2 ON ii1.itemid = ii2.itemid
                                AND ii1.fgspecialpo = ii2.fgspecialpo
                                AND ii1.vendorno = ii2.vendorno
                                AND ii3.qtytype = 11
LEFT OUTER JOIN iteminventory ii3 ON ii1.itemid = ii3.itemid
                                AND ii1.fgspecialpo = ii3.fgspecialpo
                                AND ii1.vendorno = ii3.vendorno
                                AND ii3.qtytype = 1

Answer 2

如果我理解正确，我认为只需根据 qtytype 列的值显示不同列中的数据。我认为你不需要加入。

检查这是否适合您：

SELECT 
    vendorno, 
    Quantity1, 
    CASE WHEN qtytype=1 THEN date1 ELSE NULL END as date1, 
    fgspecialpo,
    itemid,
    LastDate as [LastDate1], 
    CASE WHEN qtytype=11 THEN LastDate ELSE NULL END as [LastDate2], 
    CASE WHEN qtytype=1 THEN LastDate ELSE NULL END  as [LastDate3],
    qtytype as [qtytyp], 
    quantity2 as [qty2], 
    CASE WHEN qtytype=11 THEN Quantity1 ELSE NULL END as [itemsAddlQty], 
    CASE WHEN qtytype=1 THEN quantity1 ELSE NULL END as [Exp Qty]  
FROM iteminventory

SQL：优化在同一个表上使用3个LEFT OUTER JOIN的查询

2 个答案: