Question

I’m making the following SQL call:

SELECT dpt_id, 
dpt_title, 
dpt_active, 
dpt_parent
FROM tbl_department ORDER BY dpt_title

where dpt_parent is an integer value of a parent department id (the dpt_id of another department).

I’d like the result set of the above call to include the dpt_title of that dpt_parent. My initial attempt at this was as follows:

SELECT dpt_id, 
dpt_title, 
dpt_active, 
dpt_parent, 
    (SELECT dpt_title 
     FROM tbl_department 
     WHERE tbl_department.dpt_id = dpt_parent
     ) AS parent_title
FROM tbl_department ORDER BY dpt_title

With the call in this form, all values of parent_title are NULL
I can see using the same table like this is confusing, what is the solution to this problem?

Answer 1

use a table alias:

SELECT dpt_id, 
x.dpt_title, 
x.dpt_active, 
x.dpt_parent, 
    (SELECT dpt_title 
     FROM tbl_department 
     WHERE tbl_department.dpt_id = x.dpt_parent
     ) AS parent_title
FROM tbl_department x ORDER BY dpt_title

but you’d be better off removing the sub select and just joining like:

SELECT 
     a.dpt_id
    ,a.dpt_title
    ,a.dpt_active
    ,a.dpt_parent
    ,b.dpt_title parent_title
FROM tbl_department      a
LEFT JOIN tbl_department b ON a.dpt_parent = b.dpt_id
ORDER BY a.dpt_title, b.dpt_title