select * from user_info where user_name in (select user_name

来源：学生作业帮编辑：作业帮分类：数学作业时间：2024/05/02 20:13:35

select * from user_info where user_name in (select user_name from user)怎么改写提高效率
谢谢了.

用关系代数就行了,关系代数应该学过的吧,那个符号不会打,就用文字表示吧.“投影符号”*(“选择符号”user_name（user_info“连接符号”user)),用双引号引出来的就是关系代数中这三种运算相应的符号,替换上去就行了~

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