Question

I have a query that fetches the list of user IDs and their corresponding user names on a board but from another table also gets a column that has a value (a name) on the row corresponding to the user ID if said user has changed their name. Using an outer join I got the three nicely displayed as in the following example of a few of the results:

member_id    name             dname_current
1            Blablabla1       blablabla2
2            Bla4444          
3            RevZ             
5            Herpaderp42      
6            Lalalala         
7            Kaboom           
14           testtesttest21   Formula21
15           Alex             Ethan
16           Bob              Radio3

The SQL query to get the three columns is as follows:

SELECT
  data_members.member_id, 
  data_members.name,
  data_dnames_change.dname_current
FROM data_members LEFT OUTER JOIN data_dnames_change
    ON data_members.member_id = data_dnames_change.dname_member_id
GROUP BY data_members.member_id

Is there a way to display this so that it merges the values which exist in the ‘dname_current’ column of that other table into the ‘name’ column, replacing any value that’s already in the corresponding row of that column?

Answer 1

COALESCE() returns the first non-null value, so you can do the following to prefer dbname_current over data_members.name unless it is NULL:

SELECT
  data_members.member_id, 
  COALESCE(data_dnames_change.dname_current, data_members.name) AS name
FROM data_members LEFT OUTER JOIN data_dnames_change
    ON data_members.member_id = data_dnames_change.dname_member_id
GROUP BY data_members.member_id

Should return:

member_id    name
1            blablabla2
2            Bla4444
3            RevZ
5            Herpaderp42
6            Lalalala
7            Kaboom
14           Formula21
15           Ethan
16           Radio3