將三個表連接在一起

Question

我有兩張表，能夠正確加入。我試圖引入第三個表格，即使在特定年份沒有數據，我仍然希望爲null或0值。我的問題是，當我加入該表時，我得到重複記錄或根本沒有記錄顯示。

這裏是我試過：

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 

期望的結果 位置|成就水平|數據|地理位置|數據|時間表 這裏就是我試圖結合

Answer 1

我敢肯定，你想要一個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; 

一個left outer join將保留所有的結果從m和r表匹配條件，生成NULL的g中的值不匹配。在查詢的形式中，所有行均來自g表（right outer join）。但是，由於m表中的不匹配行將具有NULL值，因此這些值將被濾除。結果是right outer join確實表現得像inner join。

Answer 2

您需要將where子句移動到您的連接中作爲連接條件。當你在where子句中陳述它，你強制查詢砸空值

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