将三个表连接在一起

Question

我有两张表，能够正确加入。我试图引入第三个表格，即使在特定年份没有数据，我仍然希望为null或0值。我的问题是，当我加入该表时，我得到重复记录或根本没有记录显示。

这里是我试过：

SELECT * 
FROM [4th grade math achievement levels] m 
    INNER JOIN [4th grade reading achievement levels] r 
    ON m.Location = r.Location 
     AND m.TimeFrame = r.TimeFrame 
     AND m.[Achievement Level] = r.[Achievement Level] 
    RIGHT OUTER JOIN [4th graders who scored below proficient reading level by geographic location] g 
    ON m.Location = g.Location 
     AND m.TimeFrame = g.TimeFrame 
WHERE m.Location = 'ohio' 
    AND m.TimeFrame = 2011 

期望的结果 位置|成就水平|数据|地理位置|数据|时间表 这里就是我试图结合

Answer 1

我敢肯定，你想要一个left outer join而不是right outer join：

SELECT * 
FROM [4th grade math achievement levels] m INNER JOIN 
    [4th grade reading achievement levels] r 
    ON m.location = r.location AND 
     m.timeframe = r.timeframe AND 
     m.[achievement level] = r.[achievement level] LEFT OUTER JOIN 
     [4th graders who scored below proficient reading level by geographic location] g 
     ON m.location = g.location AND 
      m.timeframe = g.timeframe 
WHERE m.location = 'ohio' AND 
     m.timeframe = 2011; 

一个left outer join将保留所有的结果从m和r表匹配条件，生成NULL的g中的值不匹配。在查询的形式中，所有行均来自g表（right outer join）。但是，由于m表中的不匹配行将具有NULL值，因此这些值将被滤除。结果是right outer join确实表现得像inner join。

Answer 2

您需要将where子句移动到您的连接中作为连接条件。当你在where子句中陈述它，你强制查询砸空值

select * 
from [4th grade math achievement levels] m 
inner join [4th grade reading achievement levels] r 
on m.Location = r.Location 
    and m.TimeFrame = r.TimeFrame 
    and m.[Achievement Level] = r.[Achievement Level] 
    and m.Location = 'ohio' -- old where clause 
    and m.TimeFrame = 2011 
right outer join [4th graders who scored below proficient reading level by geographic location] g 
    on m.Location = g.Location 
    and m.TimeFrame = g.TimeFrame