SQL Query与多个CASE语句使用与THEN相同的字段

Question

知道为什么我的UOMC.ToUnitOfMeasure的case语句返回NULL？如果我反转并在from字段之前显示To字段，那么将起作用但是不会。

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Answer 1

您有两个引用（TO和FROM），因此您必须引用您的目录UNITOFMEASURE两次：

SELECT
  ...
  UOM_FROM.SYMBOL as 'FROM UNIT',
  UOM_TO.SYMBOL as 'TO UNIT'
FROM
...
INNER JOIN UNITOFMEASURECONVERSION AS UOMC ON UOMC.PRODUCT =  ECORESPRODUCT.RECID
INNER JOIN UNITOFMEASURE AS UOM_FROM ON UOM_FROM.RECID = UOMC.FROMUNITOFMEASURE
INNER JOIN UNITOFMEASURE AS UOM_TO ON UOM_TO.RECID = UOMC.TOUNITOFMEASURE

Answer 2

这最终成为正确的语法。缺少匹配TO单元和FROM所需的额外连接。     SELECT ECORESPRODUCT.DISPLAYPRODUCTNUMBER, ECORESPRODUCT.SEARCHNAME, INVENTTABLE.ITEMID, UOMC.NUMERATOR, UOMC.DENOMINATOR, CASE WHEN UOMC.FROMUNITOFMEASURE = UOM_FROM.RECID THEN UOM_FROM.SYMBOL END AS 'FROM UNIT', CASE WHEN UOMC.TOUNITOFMEASURE = UOM_TO.RECID THEN UOM_TO.SYMBOL END AS 'TO UNIT' FROM ECORESPRODUCT JOIN INVENTTABLE ON INVENTTABLE.RECID = ECORESPRODUCT.RECID INNER JOIN UNITOFMEASURECONVERSION AS UOMC ON UOMC.PRODUCT = ECORESPRODUCT.RECID INNER JOIN UNITOFMEASURE AS UOM_FROM ON UOM_FROM.RECID = UOMC.FROMUNITOFMEASURE INNER JOIN UNITOFMEASURE AS UOM_TO ON UOM_TO.RECID = UOMC.TOUNITOFMEASURE WHERE INVENTTABLE.NAMEALIAS <> 'Discontinued' AND INVENTTABLE.COSTGROUPID = 'FG'