mysql - 执行前子查询基本错误

Question

我不知道这个查询有什么问题，在多次尝试后我无法运行。请指出这有什么问题我该如何解决。谢谢。

   SELECT 
     cd.dr
    FROM
    (
     (SELECT SUM(cc.credit_amount) AS cr FROM cust_credit cc) cc
      UNION  
     (SELECT  SUM(cd.debit_amount) AS dr FROM cust_debit cd ) cd  
     )

错误是。

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'cd ) LIMIT 0, 25' at line 7

Answer 1

您不能为 UNION 中的子查询指定别名，也不能使用别名引用子查询的结果。并且 UNION 中的列别名总是来自第一个子查询中的别名，因此您不能引用 dr。

你可以做的是:

SELECT amount
FROM (
    SELECT 'cc' AS type, SUM(credit_amount) as amount FROM cust_credit
    UNION ALL
    SELECT 'cd' AS type, SUM(debit_amount) AS amount FROM cust_debit
) x
WHERE type = 'cd'

或者不使用 UNION，您可以将查询放在 SELECT 列表中。

SELECT dr AS amount
FROM (
    SELECT (SELECT SUM(credit_amount) FROM cust_credit) AS cr,
           (SELECT SUM(debit_amount) FROM cust_debit) AS dr
) x