Question

我正在努力以更有效和更优雅的方式实现这一目标

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

我显然可以INNER JOIN两个表ON Postcode，但是当PostcodeData.postcode中不存在MemberDetail.postcode时，我遇到了问题。在这种情况下，我仍然希望选择那些成员但是对于City，State和Areacode具有NULL结果。这是在当前查询中实现的，但效率非常低。任何想法，将不胜感激。干杯！

Answer 1

只需使用LEFT JOIN：

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

Answer 2

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

然后，外连接将返回FROM子句中提到的至少一个表或视图中的所有行，只要这些行满足任何WHERE或HAVING搜索条件。

Answer 3

当然，外部联接是答案吗？类似的东西：

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SQL中的select语句中提取多个值

3 个答案: