使用 LEFT JOIN 和 UNION 组合多个表答案

【问题标题】：Combine Multiple Tables using LEFT JOIN and UNION使用 LEFT JOIN 和 UNION 组合多个表
【发布时间】：2016-07-16 19:03:05
【问题描述】：

我有 5 个不同的表格 来获取数据。但首先我使用 3 个表进行测试。使用 LEFT JOIN 和 UNION。到目前为止，这是我一直在使用的代码。

SELECT sts.TransferDate as 'Transaction Date', sts.TransNumber as 'Document Number', orderset_details.Quantity as 'Quantity', '' as 'Po Number', '' as 'Sales Invoice', orderset_details.Cost as 'Cost', orderset_details.SerialNumber as 'Serial Number', orderset_details.LotNumber as 'Lot Number', location.Location as 'Location' 
FROM orderset_details 
LEFT JOIN sts ON sts.OrdersetId = orderset_details.OrdersetId
LEFT JOIN location ON location.LocationId = sts.LocationIdTo  
WHERE orderset_details.ItemId = '3071' 
UNION 
SELECT drs.OrderDate as 'Transaction Date', drs.TransNumber as 'Document Number', orderset_details.Quantity, drs.PONumber as 'Po Number', '' as 'Sales Invoice', orderset_details.Cost as 'Cost', orderset_details.SerialNumber as 'Serial Number', orderset_details.LotNumber as 'Lot Number', suppliers.Supplier as 'Location' 
FROM orderset_details 
LEFT JOIN drs ON drs.OrdersetId = orderset_details.OrdersetId 
LEFT JOIN suppliers ON suppliers.SupplierId = drs.SupplierId 
WHERE orderset_details.ItemId = '3071'
UNION 
SELECT stockadjustment.TransactionDate as 'Transaction Date', stockadjustment.TransactionId as 'Document Number', orderset_details.Quantity as 'Quantity', '' as 'Po Number', '' as 'Sales Invoice', orderset_details.Cost as 'Cost', orderset_details.SerialNumber as 'Serial Number', orderset_details.LotNumber as 'Lot Number', location.Location as 'Location' 
FROM orderset_details 
LEFT JOIN stockadjustment ON stockadjustment.OrdersetId = orderset_details.OrdersetId 
LEFT JOIN location ON location.LocationId = stockadjustment.LocationId 
WHERE orderset_details.ItemId = '3071' 
Group By orderset_details.ItemId

这是输出。

输出应该只有 1 并且是最后一行。我的问题是，当我的 1 或 2 个查询没有任何意义时，它会重复一些不正确的列。

如何只显示与特定 ItemId 相等的数据？我的查询有什么问题？

【问题讨论】：

标签： mysql join

【解决方案1】：

在您的查询中，您应该只在第一个选择中使用别名，并且您没有聚合功能，因此您不需要 group by .. 因为 unio 只返回不同的结果

SELECT 
    sts.TransferDate as 'Transaction Date'
  , sts.TransNumber as 'Document Number'
  , orderset_details.Quantity as 'Quantity'
  , '' as 'Po Number'
  , '' as 'Sales Invoice'
  , orderset_details.Cost as 'Cost'
  , orderset_details.SerialNumber as 'Serial Number'
  , orderset_details.LotNumber as 'Lot Number'
  , location.Location as 'Location' 
FROM orderset_details 
LEFT JOIN sts ON sts.OrdersetId = orderset_details.OrdersetId
LEFT JOIN location ON location.LocationId = sts.LocationIdTo  
WHERE orderset_details.ItemId = '3071' 
UNION 
SELECT 
      drs.OrderDate 
    , drs.TransNumber 
    , orderset_details.Quantity
    , drs.PONumber 
    , ''
    , orderset_details.Cost 
    , orderset_details.SerialNumber 
    , orderset_details.LotNumber 
    , suppliers.Supplier 
FROM orderset_details 
LEFT JOIN drs ON drs.OrdersetId = orderset_details.OrdersetId 
LEFT JOIN suppliers ON suppliers.SupplierId = drs.SupplierId 
WHERE orderset_details.ItemId = '3071'
UNION 
SELECT 
      stockadjustment.TransactionDate 
    , stockadjustment.TransactionId 
    , orderset_details.Quantity 
    , '' 
    , '' 
    , orderset_details.Cost 
    , orderset_details.SerialNumber 
    , orderset_details.LotNumber 
    , location.Location     
FROM orderset_details 
LEFT JOIN stockadjustment ON stockadjustment.OrdersetId = orderset_details.OrdersetId 
LEFT JOIN location ON location.LocationId = stockadjustment.LocationId 
WHERE orderset_details.ItemId = '3071'

【讨论】：

它仍然是相同的输出:(
一一检查选择..并检查不同的结果是否正确..以及为什么需要3个选择的联合来仅选择一行？