我可以在case语句中使用子查询吗？

Question

Can I use a subquery inside a case statement? Is this possible?
Here's my code:

SELECT SUM(icd.discrepancy * (
    CASE WHEN $priceVal = 1 
    THEN SELECT i.dPrice FROM item_tb i 
    WHERE i.ID = icd.itemID
    ELSE SELECT p.dPrice FROM price_tb p 
    WHERE p.brID = '". $brID ."' AND p.itemID = icd.itemID
    END
)) as total
FROM invcountd_tb icd
WHERE icd.coID = '". $_SESSION['coID'] ."'
AND icd.invCounthID = '". $ID ."'

Answer 1

Yes you can but you have to enclose the sub query in parentheses ee:

CASE WHEN $priceVal = 1 THEN (SELECT i.dPrice FROM item_tb i WHERE i.ID = icd.itemID)                           
 ELSE (SELECT p.dPrice FROM price_tb p WHERE p.brID = '". $brID ."' AND p.itemID = icd.itemID)                           
 END

However you can still get errors if there are more than one row returned

Answer 2

Yes, but your subqueries need to be in parentheses:

SELECT SUM(icd.discrepancy * (
                   CASE WHEN $priceVal = 1 THEN 
                            (SELECT i.dPrice FROM item_tb i WHERE i.ID = icd.itemID)
                        ELSE 
                            (SELECT p.dPrice FROM price_tb p WHERE p.brID = '". $brID ."' AND p.itemID = icd.itemID)
                        END
                    )) as total
FROM
    invcountd_tb icd
WHERE
    icd.coID = '". $_SESSION['coID'] ."'
AND
    icd.invCounthID = '". $ID ."'

I would however rewrite this as follows:

SELECT  SUM(icd.discrepancy * COALESCE(i.dPrice, p.dPrice))
FROM    invcountd_tb icd
        LEFT JOIN item_tb i 
            ON i.ID = icd.itemID
            AND $priceVal = 1
        LEFT JOIN price_tb p 
            ON p.brID = '". $brID ."' 
            AND p.itemID = icd.itemID
            AND $priceVal <> 1
WHERE   icd.coID = '". $_SESSION['coID'] ."'
AND     icd.invCounthID = '". $ID ."'

Your join conditions will do the case statement for you (ie .only return i.dPrice if $priceVal = 1 , and only return p.dPrice if $priceVal <> 1 . This avoids expensive subqueries which are poorly optimised in MySQL.

Simple Example on SQL Fiddle

Answer 3

To answer your question, Yes, you can have a subquery inside a Case statement.

Moreover, in your query above, I think you have another issue, which is that you are performing an aggregation on an expression containing a subquery, which gives an error.