Question

I have a mysql table named “category”. the basic structure looks like this-

------     --------     ------
cat_id     cat_name     parent
------     --------     ------
1          test1        NULL
2          test2        NULL
3          test3        2
4          test4        1
5          test5        3

now i want all the category data with parent category name (not only id) in a single query. is that possible? i could do it by using a second query (getting child’s parent name) in a while loop and merging data as a whole. but is it possible to do this with a single query?

Answer 1

Join the table with itself, using the parent column to link to the cat_id of the parent.

SELECT c1.cat_id as childID, c1.cat_name ChildName, c2.cat_name as ParentName
from category c1
LEFT OUTER JOIN category c2
ON c1.parent = c2.cat_id

• Be careful: since some elements have no parents (NULL), I put a LEFT
OUTER JOIN so those rows are displayed as well. If you don’t want
  that, use a JOIN instead of LEFT OUTER JOIN.
• You can also show the lines, but display something else (empty or a
  text or …) instead of the NULL by using COALESCE.
• You can consider the result as one (big) new table, so you can add WHERE clauses as you usually do, for example filtering on the parent name: WHERE c2.cat_name = 'test2'