从select语句在SQL

问题描述：

我想更有效地实现这一目标而优雅从select语句在SQL

SELECT 
    MD.*, 
    (SELECT City FROM PostcodeData WHERE MD.Postcode = Postcode) [City], 
    (SELECT State FROM PostcodeData WHERE MD.Postcode = Postcode) [State], 
    (SELECT Areacode FROM PostcodeData WHERE MD.Postcode = Postcode) [Areacode] 
FROM MemberDetails AS MD

我能明显INNER JOIN ON邮编两个表，但我有问题时，MemberDetail.postcode确实拉动多个值不存在于PostcodeData.postcode中。在这种情况下，我仍然希望选择这些成员，但对于城市，州和Areacode的结果为NULL。这在当前查询中已经实现，但效率很低。任何想法，将不胜感激。干杯!

答

只需使用一个LEFT JOIN：

SELECT 
    MD.*, 
    P.City, 
    P.State, 
    P.Areacode 
FROM MemberDetails AS MD 
    LEFT JOIN PostcodeData P ON MD.Postcode = P.Postcode

干杯@sgegges，左加入是我之后 – Gregology 2013-02-22 20:08:55

答

使用OUTER JOIN而不是INNER JOIN - 即使连接表上没有匹配的记录，它也会返回记录。

但是，外部连接返回FROM子句中提到的至少一个表或视图的所有行，只要这些行满足任何WHERE或HAVING搜索条件。

答

肯定外连接就是答案？就像：

SELECT MD.*, 
IsNull(pc.City,"No city") AS City, 
IsNull(pc.State,"No state") AS State, 
IsNull(pc.AreaCode,"No area code") AS AreaCode 
FROM 
MemberDetails AS MD 
LEFT OUTER JOIN PostCodeData AS pc ON md.PostCode=pc.PostCode

从select语句在SQL

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