MySQL，如何在 SELECT 查询中引用子查询字段

Question

我正在尝试使用以下查询

计算 MySQL 中的账户余额

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

但是我得到这个错误：

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

如何在 select 中引用子查询别名？

Answer 1

用另一个执行计算的子查询包装它：

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

虽然这会启动您的查询运行，但您最好使用外部联接：

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 约定，应该有效

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

我想你想这样做：

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;

您需要子查询，因为您不能在定义它们的同一 SELECT 中引用派生列。您需要聚合函数将所有 debits/credits 合并为每个帐户的单个值。

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