Question

我遇到了选择查询的问题

对于select coid,model,km,year和AC的所有车辆，我需要MP3。

我写了这个SQL：

select distinct 
    vehicle.vehid, model, km, year
from 
    vehicle, models, extras, veh_extras
where 
    models.modid = vehicle.modid 
    and vehicle.vehid = veh_extras.vehid
    and extras.extraid = veh_extras.extraid 
    and (descr = 'AC' or descr = 'mp3')

但我觉得这是错的。 Extras.desc是用于描述额外内容的列。

schema link

Answer 1

SELECT v.vehid, m.model, v.km, v.year
FROM vehicle v 
JOIN model m ON v.modid = m.modid
WHERE EXISTS (  SELECT 'a'
                FROM extras e 
                JOIN veh_extras ve ON e.id = ve.extraid
                WHERE ve.vehid = v.vehid
                AND e.descr = 'AC'
             )
AND EXISTS (    SELECT 'a'
                FROM extras e 
                JOIN veh_extras ve ON e.id = ve.extraid
                WHERE ve.vehid = v.vehid
                AND e.descr = 'mp3'
             )

这可能不是最好的方法......但是如果你需要搜索更多的额外内容，只需添加另一个EXISTS条件

Answer 2

假设你想要同时拥有AC和MP3的车辆，那么一个选项就是多次加入veh_extras表：

select distinct v.vehid, m.model
from vehicle v
  join models m on m.modid = v.modid
  join veh_extras ve on v.vehid = ve.vehid
  join extras e on ve.extraid = e.extraid and e.descr = 'AC'
  join veh_extras ve2 on v.vehid = ve2.vehid
  join extras e2 on ve2.extraid = e2.extraid and e2.descr = 'MP3'

SQL Fiddle Demo

另一种选择是使用案例聚合：

select v.vehid, m.model
from vehicle v
  join models m on m.modid = v.modid
  join veh_extras ve on v.vehid = ve.vehid
  join extras e on ve.extraid = e.extraid 
group by v.vehid, m.model
having sum(case when e.descr = 'AC' then 1 else 0 end) > 0
  and sum(case when e.descr = 'MP3' then 1 else 0 end) > 0

More Fiddle

BTW - 我省略了一些补救栏目 - 很容易将这些列添加回来......

Answer 3

但我认为错了。

即使它起作用，也不是推荐的方式。

尽量避免使用逗号分隔的JOINS。而是试着像这样使用JOINS：

select distinct v.vehid,model,km,year
    from vehicle v inner join models m on v.modid = m.modid 
                 inner join veh_extras ve on ve.vehid = v.vehid
                 inner join extras e on e.extraid = ve.extraid          
        where  
            e.descr='AC' or e.descr='mp3'

选择包含链接表的查询

3 个答案: