sql函數大小寫返回多行

Question

要將此查詢用作子查詢，問題在於它返回許多重復行。 試圖使用COUNT（）代替存在，但是它仍然返回多個答案。 每個表只能包含一條superRef記錄。

我將在SELECT col_a中使用以下查詢，[CASE]來自MyTable

SELECT CASE
        WHEN 
         EXISTS (SELECT 1 FROM A WHERE 
            A_superRef = myTable.sysno AND A_specAttr = 'value')
        THEN 3
        WHEN     EXISTS (SELECT 1 FROM B 
            INNER JOIN С ON С_ReferenceForB = B_sysNo WHERE C_superRef = myTable.sysno AND b_type = 2)
        THEN 2
        ELSE (SELECT C_intType FROM C 
            WHERE C_superRef = myTable.sysno)
        END
    FROM A, B, C

結果：

3
3
3
3
3
3...

Answer 1

如果您這樣做了怎么辦？ 因為我猜您正在獲得隱式的完全外部聯接AXBXC，然后為該結果集中的每一行運行case語句。

SELECT CASE
    WHEN 
     EXISTS (SELECT 1 FROM A WHERE 
        A_superRef = 1000001838012)
    THEN 3
    WHEN     EXISTS (SELECT 1 FROM B 
        INNER JOIN С ON С_ReferenceForB = B_sysNo AND C_superRef = 1000001838012 )
    THEN 2
    ELSE (SELECT C_type FROM C 
        WHERE C_superRef = 1000001838012)
    END
FROM ( SELECT COUNT(*) FROM A ) --This is a hack but should work in ANSI sql.
                                --Your milage my vary with different RDBMS flavors. 

Answer 2

多虧了Thorsten Kettner ，我才需要DUAL

SELECT CASE
    WHEN 
     EXISTS (SELECT 1 FROM A WHERE 
        A_superRef = 1000001838012)
    THEN 3
    WHEN     EXISTS (SELECT 1 FROM B 
        INNER JOIN С ON С_ReferenceForB = B_sysNo AND C_superRef = 1000001838012 )
    THEN 2
    ELSE (SELECT C_type FROM C 
        WHERE C_superRef = 1000001838012)
    END
FROM DUAL