select * from a,b与select * from a inner join b 的取数方式和执行效率有什么

来源：学生作业帮编辑：作业帮分类：英语作业时间：2024/05/11 13:59:45

select * from a,b与select * from a inner join b 的取数方式和执行效率有什么不同,

(1)连接
select * from table1,table2
等价于
select * from table1 cross join table2
select * from table1,table2 where table1.row=table2.row
(2)自连接
select * from emploly e1 ,emploly e2
select e1.name,e2.name from employ e1,employ e2
where e1.name=e2.name
(3)内连接(inner join)
select stuname as '姓名',classname as '班级' from student inner join class on student.stuid=class.stuid
inner join '表名' on 条件 --连接多个表
它等价于:
select stuname as '姓名',classname as '班级'
from student,class
where student.stuid=class.stuid
(4)外连接：（outer join）
允许限制一张表中的行,而不限制另外一张表中的行.
注意:外连接不一定非要有外键约束
1: left outer join --不能用left out join
左表中的记录全部会出现在结果集中,匹配不上的显示NULL
2: right outer join
右表中的记录全部会出现在结果集中,匹配不上的显示NULL
3: full outer join|full join --不能用full out join
返回两个表中的匹配和不匹配的所有记录.

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