数据库增强，增删改查

嵌套查询步骤

• 以修改用户为** 的 省份为上海

• 步骤拆解

1. 确定用户
   SELECT cst_id
   FROM t_user_user
   WHERE user_name='wzj1234'
2. 确定后续过滤条件
   UPDATE t_cst_customer SET province ='上海' WHERE cst_id = ()

• 组合
  UPDATE t_cst_customer SET province ='上海' WHERE cst_id = (
  SELECT cst_id
  FROM t_user_user
  WHERE user_name='wzj1234'
  )

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