MySQL的，我怎麼能引用一個子查詢字段在SELECT查詢

Question

我嘗試使用下面的查詢

SELECT accountNumber, 
(
    SELECT amount 
    FROM accountDebits 
    WHERE accountNumber = accounts.accountNumber 
) debits, 
(
    SELECT amount 
    FROM accountCredits 
    WHERE accountNumber = accounts.accountNumber 
) credits, 
credits - debits as balance 
FROM accounts 

計算MySQL中的賬戶餘額，但我得到這個錯誤：

Error Code: 1054. Unknown column 'credits' in 'field list'

如何我可以引用select中的子查詢別名嗎？

Answer 1

與另一個子查詢，做計算把它包：

SELECT accountNumber, debits, credits, credits - debits as balance 
FROM (
    SELECT accountNumber, 
    (
    SELECT amount 
    FROM accountDebits 
    WHERE accountNumber = accounts.accountNumber 
) debits, 
    (
    SELECT amount 
    FROM accountCredits 
    WHERE accountNumber = accounts.accountNumber 
) credits 
    FROM accounts 
) x 

雖然這得到您的查詢運行起來，你會與外最好加入：

SELECT accountNumber, debits, credits, credits - debits as balance 
FROM (
    SELECT 
     a.accountNumber, 
     coalesce(sum(d.amount), 0) debits, 
     coalesce(sum(c.amount), 0) credits, 
    FROM accounts a 
    LEFT JOIN accountDebits d 
    ON d.accountNumber = a.accountNumber 
    LEFT JOIN accountCredits c 
    ON c.accountNumber = a.accountNumber 
) x 

Answer 2

使用table.field約定，應該工作

SELECT accountNumber, 
(
    SELECT amount 
    FROM accountDebits 
    WHERE accountNumber = accounts.accountNumber 
) debits, 
(
    SELECT amount 
    FROM accountCredits 
    WHERE accountNumber = accounts.accountNumber 
) credits, 
credits.amount - debits.amount as balance 
FROM accounts 

Answer 3

我認爲你想要這樣做：

SELECT a.*, 
     (COALESCE(credits, 0) - COALESCE(debits, 0)) as balance 
FROM (SELECT accountNumber, 
      (SELECT SUM(ad.amount) 
       FROM accountDebits ad 
       WHERE ad.accountNumber = a.accountNumber 
      ) as debits, 
      (SELECT SUM(ac.amount) 
       FROM accountCredits ac 
       WHERE ac.accountNumber = a.accountNumber 
      ) credits 
     FROM accounts a 
    ) a; 

您需要子查詢，因爲您無法在定義它們的相同SELECT中引用派生列。您需要彙總功能將所有借記/貸項合併到每個帳戶的單個值中。