Need a more efficient join query

Question

It's been a long time since I last did something with database so my knowledge has beenyet rusted. I have 2 SQL tables:

data(id, attr);
dependency(child, parent);
 #**child** and **parent** are the id from table **data**

I need to query all the attr corresponding to the entries in table dependency. Following is my attempt:

SELECT 
(SELECT data.attr FROM data WHERE data.id = child) AS child_attr,
(SELECT data.attr FROM data WHERE data.id = parent) AS parent_attr 
from dependency;

It works but very slow. I'm sure there's a better way to do it with join query but cannot come up with it yet. Can someone please help?

Thanks,

Answer 1

You could join the dependency table to the data table twice to get the information you need:

SELECT d1.attr AS ChildAttr, d2.attr AS ParentAttr
FROM dependency AS dep
INNER JOIN data AS d1 ON dep.child = d1.id
INNER JOIN data AS d2 ON dep.parent = d2.id

Answer 2

try this:

SELECT t2.attr, t3.attr
FROM dependency as t1
    INNER JOIN data as t2 on t1.child = t2.id
    INNER JOIN data as t3 on t1.parent = t3.id