Outer Join 指令

select service.sid, sum(sale.num) from sale, service where sale.sid=service.sid group by service.sid;

• 編號6的服務點因為業績0所以不會被列出來
• outer join 可以強制列出所有的服務點，不管有無關聯到銷售表格

select service.sid, sum(sale.num) from sale right join service on sale.sid=service.sid group by service.sid;

select service.sid,sum(sale.num) from service left join sale on sale.sid=service.sid group by service.sid;