Question

I have a table called concept-relation that contains 3 columns

(relationID, firstConceptID, secondConceptID)

I have a table called concept contains 2 columns

(ID, name)

I want to get the name for the firstConceptID and secondConceptID when the relationID = 22.

This is the query that I came up with.

select * from (
      select name as source from concept where concept.ID in (
      select firstConceptID from `concept-relation` where relationID = 22
       ) 
) as e,
(
      select name as des from concept where concept.ID in (
      select secondConceptID from `concept-relation` where relationID = 22
     )
)as e

It works well, but I want to know what is the best practice for carrying out such queries?

Answer 1

Need a self-join to make it cleaner usually considered more of a best practice as it avoids the sub selects / “IN”s

SELECT C1.name, C2.name
FROM Concept C1
INNER JOIN concept_Relation CR 
  ON CR.FirstConceptID = C1.ID
INNER JOIN Concept C2 
  ON CR.SecondConceptID = C2.ID
WHERE CR.RelationID = 22