加入导致笛卡尔积

Question

我在两张桌子上进行左连接，但是生产笛卡尔产品而在其他桌子上没有生产。查询的样本结果可以在下面的图片中看到。

查询当前查询是：

SELECT 
    R.Region, 
    C.CountryName, 
    D.Year, 
    I.Income, 
    D.Completion_Rate AS 'Completion Rate', 
    D.Pupil_Teacher_Ratio AS 'Pupil-Teacher Ratio'
FROM 
    (SELECT 
        C.CountryCodeC AS 'CountryCode', 
        C.YearCC AS 'Year', 
        C.Completion_Rate, 
        R.Pupil_Teacher_Ratio
    FROM 
        (SELECT 
            CountryCode AS 'CountryCodeC', 
            Data AS 'Completion_Rate', 
            YearC AS 'YearCC' 
        FROM 
            DataByYear 
        WHERE 
            SeriesCode = "SE.SEC.CMPT.LO.ZS" 
            AND YearC >= "2011%") C
        LEFT JOIN
            (SELECT 
                CountryCode AS 'CountryCodeR', 
                Data AS 'Pupil_Teacher_Ratio', 
                YearC AS 'YearCR' 
             FROM 
                DataByYear 
             WHERE 
                SeriesCode = "SE.SEC.ENRL.LO.TC.ZS" 
                AND YearC >= "2011%") R
        ON C.CountryCodeC = R.CountryCodeR) D, 
    CountryRegion R, 
    Country C, 
    CountryIncome I
WHERE 
    R.CountryCode = D.CountryCode 
    AND R.CountryCode = C.CountryCode
    AND I.CountryCode = D.CountryCode

问题是什么？我该如何解决？谢谢！

Answer 1

我认为您还希望加入Year以查询所有问题。如果您没有指定一年，那么每年都会加入所有其他年份并夸大您的价值观。所以你应该添加一个条件：

A.Year = B.Year

在整个查询过程中

。

Answer 2

尝试使用INNER JOIN代替左连接。