请问这两句SQL语句有什么区别?
select Info,(select Area.[Name] from Area a where c.areaid=a.id ) as 地区 from customer c
select Info,Area.Name from Customer left join Area on (AreaId=Area.id)
select Info,(select Area.[Name] from Area a where c.areaid=a.id ) as 地区 from customer c
select Info,Area.Name from Customer left join Area on (AreaId=Area.id)
2.第一句是子查询,第二句是左联接
3.当两个表的id或areaid都不是主键的时候,则第一句中如果Areaid与Id是1:N的关系时,语句会有错,如果是1:1的关系,则表扫描的效率第一句稍好
4.当两表的id或areaid是主键的时候,没多大区别