more than one row returned by a subquery used as an expression

忘是亡心i 2023-02-25 03:37 97阅读 0赞

有问题的sql

  1. delete from user where class_id in(
  2.         (select class_id from class where class_grade = '高三'),
  3.         (select class_id from class where class_grade = '高二')
  4.     );

问题所在：

  1. select class_id from class where class_grade = '高三';
  2. 查询出来的结果是多条,
  3. select class_id from class where class_grade = '高二';
  4. 查询出来的结果也是多条。

这种写法不正确。至于为什么不能这么写,以后有时间再做研究。

正确的sql
方式一：

  1. delete from user where class_id in(
  2.         select class_id 
  3.         from class 
  4.         where class_grade in('高三','高二')
  5.     );
  7. 方式二：
  9. delete from user where class_id in(
  10.         select class_id from class where class_grade = '高三'
  11.     );
  13.     delete from user where class_id in(
  14.         select class_id from class where class_grade = '高二'
  15.     );

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