SQL case语句-根据其他两个表从第三个表中选择值

CASE expression
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ...
    ELSE result
END

SELECT 
    CASE 
        WHEN A.A1 = B.B1 THEN C.C1
        ELSE NULL
    END AS SelectedValue
FROM 
    A
JOIN 
    B ON A.A2 = B.B2
JOIN 
    C ON A.A1 = C.C2

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE result
END

SELECT 
    CASE 
        WHEN A.A1 = B.B1 AND A.A2 = B.B2 THEN C.C1
        WHEN A.A1 = B.B1 THEN C.C2
        ELSE NULL
    END AS SelectedValue
FROM 
    A
JOIN 
    B ON A.A2 = B.B2
JOIN 
    C ON A.A1 = C.C2