在case sql中使用join

Question

我正在试着做一个查询，但它不正确。

我想做的是应用join以防

我的问题是

SELECT LEFT(Student_First_Name,LEN(Student_First_Name)-LEN(Name_Lookup_Table.Dirty_Name)),Name_Lookup_Table.Dirty_Name, Name_Lookup_Table.Standard_Name

case
when Transformed_All_Student.Student_First_Name like '% '+Name_Lookup_Table.Dirty_Name
then
from Transformed_All_Student left join Name_Lookup_Table 
on Transformed_All_Student.Student_First_Name like '% '+Name_Lookup_Table.Dirty_Name

when Transformed_All_Student.Student_First_Name like '% '+Name_Lookup_Table.Dirty_Name+'%'
then from Transformed_All_Student left join Name_Lookup_Table on Transformed_All_Student.Student_First_Name like '% '+Name_Lookup_Table.Dirty_Name+'%'

when Transformed_All_Student.Student_First_Name like Name_Lookup_Table.Dirty_Name+'% '
then from Transformed_All_Student left join Name_Lookup_Table on Transformed_All_Student.Student_First_Name like Name_Lookup_Table.Dirty_Name+'% '

有人能帮上忙吗？

Answer 1

我不认为你能把它放在什么时候。一种可能的解决方案是使用UNION。您可能可以优化以下内容：

    SELECT LEFT(Student_First_Name
        , LEN(Student_First_Name)-LEN(nlt.Dirty_Name))
        , nlt.Dirty_Name
        , nlt.Standard_Name

    FROM Transformed_All_Student tas
            LEFT JOIN Name_Lookup_Table nlt
                        ON tas.Student_First_Name like '% '+nlt.Dirty_Name
    WHERE tas.Student_First_Name like '% '+nlt.Dirty_Name

    UNION

    SELECT LEFT(Student_First_Name
        , LEN(Student_First_Name)-LEN(nlt.Dirty_Name))
        , nlt.Dirty_Name
        , nlt.Standard_Name

    FROM Transformed_All_Student tas 
            LEFT JOIN Name_Lookup_Table nlt 
                ON tas.Student_First_Name like '% '+nlt.Dirty_Name+'%'

    WHERE tas.Student_First_Name like '% '+nlt.Dirty_Name+'%'

    UNION

    SELECT LEFT(Student_First_Name
        , LEN(Student_First_Name)-LEN(nlt.Dirty_Name))
        , nlt.Dirty_Name
        , nlt.Standard_Name
    FROM Transformed_All_Student tas 
            LEFT JOIN Name_Lookup_Table nlt 
                    ON tas.Student_First_Name like nlt.Dirty_Name+'% '

    WHERE tas.Student_First_Name like nlt.Dirty_Name+'% '

Answer 2

既然您正在尝试这样做，我假设性能不是主要考虑因素。另外，我假设这是为MS SQL，因为没有另外指定。一般形式如下：

SELECT 1
FROM foo f
JOIN bar b
   ON CASE WHEN f.col1 = 'X' then 'Y' ELSE  END = b.col1

获得相同结果的另一种可能方法是使用子查询：

SELECT 1
FROM (
    SELECT *, CASE WHEN f.col1 = 'X' then 'Y' ELSE  END JoinCol
    FROM foo
) f
JOIN bar b
    ON f.JoinCol = b.col1

可能还有其他几种方法。如果您能给我们一个涉及的表和您想要在输出中的列的快速定义，那将是最好的。

希望这能有所帮助。