left join;right join;inner join;where
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一道题目:
大概是一个学生的主表,和一个学生成绩的子表.
用left join;right join;inner join;where 语句查询.取结果.
学生信息主表:students
成绩表:score
区别:
select * from students left join score on students.id = score.sid
// 取出students表中的所有记录,在score表中如果有对应的sid则显示score表中的对应记录,否则输出null.
select * from students right join score on students.id = score.sid
// 取出score表中的所有记录,在students表中如果有对应的id则显示students表中的对应记录,否则输出null.
select * from students inner join score on students.id = score.sid
//inner join 同 where 结果一样.只取出有对应关系的记录数.
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