使用EXISTS的SQL語法錯誤

Question

由於某種原因，當我嘗試運行此SQL時出現語法錯誤。

SELECT C.CUSTNUM 
FROM CUSTOMERS AS C 
WHERE C.CUSTNUM EXISTS (
    SELECT P.CNUM 
    FROM PURCHASES AS P 
    WHERE P.PNUM EXISTS (
     SELECT * 
     FROM PRODUCTS AS PR 
     WHERE PR.MSRP = 9.99)) 

我確定關閉了括號，但在查詢表達式中仍然出現語法錯誤。在我的代碼中可能有一些簡單的東西。有任何想法嗎？謝謝一堆。

Answer 1

試試這個

SELECT C.CUSTNUM 
FROM CUSTOMERS AS C 
WHERE EXISTS (
    SELECT P.CNUM 
    FROM PURCHASES AS P 
    WHERE EXISTS (
     SELECT * 
     FROM PRODUCTS AS PR 
     WHERE PR.MSRP = 9.99)) 

Answer 2

當您使用EXISTS指定的子查詢本身內的約束。如果子查詢爲給定條件C.CUSTNUM = P.CNUM返回至少一行，則認爲條件滿足。

我想這可以重寫一個JOIN到PRODUCTS表。

SELECT C.CUSTNUM 
FROM CUSTOMERS AS C 
WHERE EXISTS (
    SELECT 1 
    FROM PURCHASES AS P 
    JOIN PRODUCTS AS PR ON 
     ON P.PNUM = PR.PNUM -- not sure what column to join on in PRODUCTS 
    WHERE 
     C.CUSTNUM = P.CNUM -- added the correlation, I believe this is what you mean 
     PR.MSRP = 9.99 
    ) 

如果不是這種情況，只要堅持到您的修改後的查詢：

SELECT C.CUSTNUM 
FROM CUSTOMERS AS C 
WHERE EXISTS (
    SELECT 1 
    FROM PURCHASES AS P 
    WHERE 
     EXISTS (
     SELECT 1 
     FROM PRODUCTS AS PR 
     WHERE PR.MSRP = 9.99 
     ) 
     AND C.CUSTNUM = P.CNUM 

但要注意的是，外部子查詢將從PURCHASES返回所有行，如果有至少一個排PRODUCTS表無論任何列的匹配如何（您沒有指定它們之間的任何相關條件）都有MSRP = 9.99。

Answer 3

使用的例子存在：

create table tab1(id1 number); 
create table tab2(id2 number); 
insert into tab1 values (1); 
insert into tab1 values (2); 
insert into tab2 values (2); 
select * 
from tab1 
where exists (select 1 
       from tab2 
       where tab2.id2 = tab1.id1 
      ) 

結果：2

注意，子查詢相關，基於一些領域，這似乎是你需要做什麼外一個。

Answer 4

我想你想使用，而不是EXISTS

SELECT C.CUSTNUM 
FROM CUSTOMERS AS C 
WHERE C.CUSTNUM IN (
    SELECT P.CNUM 
    FROM PURCHASES AS P 
    WHERE P.PNUM IN (
     SELECT PR.PNUM /* I'm unsure of the proper column here*/ 
     FROM PRODUCTS AS PR 
     WHERE PR.MSRP = 9.99))