Question

I have a table which has fields (id, parent_id, name, created_at). I want to write a query to select id, name, created_at, parent_created_at. The ‘parent_created_at’ is the ‘created_at’ field for the records matching parent (if it has one – i.e. the records parent_id >0), otherwise the ‘parent_created_at’ field should be null. Any suggestions?

Answer 1

Self Join + ANSI SQL CASE Expression

SELECT t.id
       , t.name
       , t.created_at
       , CASE WHEN t.parent_id > 0 THEN p.created_at ELSE NULL END AS parent_created_at 
FROM   Table t
JOIN   Table p
ON     t.id = p.parent_id