SQLite在同一表上两次退出连接

Question

I have a table that has a few columns that contain IDs to one other table. Example:

T1 {id,p1,p2,p3}
T2 {id,name}

So, p1 , p2 and p3 are IDs from T2 . What I want to do is select all from T1 and have the name value from T2 as well.

This is what I am using now:

select
     T1.id,T1.p1,T1.p2,T1.p3,
     T2a.name as p1_name,T2b.name as p2_name,T2c.name as p3_name
from
     T1 left join T2 as T2a on T1.p1=T2a.id
     left join T2 as T2b on T1.p2=T2b.id
     left join T2 as T2c on T1.p3=T2c.id;

Is that how this should be done? Are there any speed issues I should be worried about?

Thank you.

Answer 1

Yes, that's the right way to do it. If you know that T2 will have all the right values then you could use inner joins instead of outer joins:

select T1.id,
       T1.p1, T21.name as p1_name,
       T1.p2, T22.name as p2_name,
       T1.p3, T23.name as p3_name
from T1
join T2 as T21 on T1.p1 = T21.id,
join T2 as T22 on T1.p2 = T22.id,
join T2 as T23 on T1.p3 = T23.id

You might want to have a look at foreign keys for the T1 columns to ensure that you do have everything you need in T2 .

The performance should be fine, that's a pretty standard query.