MYSQL执行如下语句报错：

UPDATE
  t_jygz_teachtask
SET
  completed_hours = completed_hours + 1
WHERE id =
  (SELECT
      id
    FROM
      t_jygz_teachtask
    WHERE class_bh = '4')

报错信息如下：

错误代码： 1093
You can't specify target table 't_jygz_teachtask' for update in FROM clause

意思是不能在同一语句中更新select出的同一张表元组的属性值

解决方法：将select出的结果通过中间表再select一遍即可。

UPDATE
  t_jygz_teachtask
SET
  completed_hours = completed_hours + 1
WHERE id =
  (SELECT
    id
  FROM
    (SELECT
      id
    FROM
      t_jygz_teachtask
    WHERE class_bh = '4') temp)