MySQL 衍生表別名缺失：substring 查詢同樣需加別名

SQL：

SELECT mechdate, SUM(...) FROM (
    SELECT SUBSTRING(mechdate,1,6) AS mechdate, ... FROM boc_payinfo WHERE ... GROUP BY mechdate
) GROUP BY mechdate ORDER BY mechdate

錯誤：Every derived table must have its own alias

修正

SELECT mechdate, SUM(...) FROM (
    SELECT SUBSTRING(mechdate,1,6) AS mechdate, ... FROM boc_payinfo WHERE ... GROUP BY mechdate
) AS t  -- 加上別名
GROUP BY mechdate ORDER BY mechdate

說明

• FROM (subquery) 產生的臨時表在 MySQL 中**強制要求別名**。
• 別名可任意（t、tmp、sub），但不可省略。

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