SQL self-referencing query. JOIN

Question

i need to query a self referencing relationship for unit prerequisites.

i know that you need to use two Joins, do i SELECT my column and then join it to itself ?

SELECT u.unit_code,
   u.name + ' is a prerequisite of ' + u.name AS unit_prerequisite

FROM units AS u

so far that is what i have, not sure where my joins have to be made? not even sure if that first part is correct.

Answer 1

You do this by joining the table to itself on the self-referencing column:

SELECT 
    u.unit_code, 
    u1.name + ' is a prerequisite of ' + u2.name AS unit_prerequisite 
FROM 
    units AS u1
    inner join units u2 on u2.RefId = u1.RefId 

Answer 2

You will need you JOIN to itself, similar to this:

SELECT u1.unit_code,
   u1.name + ' is a prerequisite of ' + u2.name AS unit_prerequisite

FROM units  u1
INNER JOIN units u2
    ON u1.id = u2.id