请问大侠:
select Accident_Type.Id,Accident_Type.Contents,sum(Accident_Report.deathnumber) as death from Accident_Type left join Accident_Report on Id=AccidentTypeId group by Id,contents
和
select Accident_Type.Id,Accident_Type.Contents,sum(Accident_Report.deathnumber) as death from Accident_Type,Accident_Report where Accident_Type.ID=Accident_Report.AccidentTypeId group by Id,contents
有什么区别吗?
select Accident_Type.Id,Accident_Type.Contents,sum(Accident_Report.deathnumber) as death from Accident_Type left join Accident_Report on Id=AccidentTypeId group by Id,contents
和
select Accident_Type.Id,Accident_Type.Contents,sum(Accident_Report.deathnumber) as death from Accident_Type,Accident_Report where Accident_Type.ID=Accident_Report.AccidentTypeId group by Id,contents
有什么区别吗?
有区别
第一个是LEFT OUTER JOIN第二个是INNER JOIN
是left join
select Accident_Type.Id,Accident_Type.Contents,sum(Accident_Report.deathnumber) as death from Accident_Type,Accident_Report where Accident_Type.ID=Accident_Report.AccidentTypeId group by Id,contents
是inner join
第二条是内连接,只有满足两边条件的才会被查出来,
去查下联机帮助left join ,inner join 看下区别就晓得了