Question

How can i join tables using column value?

I have three tables as listed below:

• messages_table

  -----------------------------------------------------------------------------
  msg_id  | msg_sub   | msg_to    | to_user_type  | msg_from  | from_user_type
  -----------------------------------------------------------------------------
  001     | test      | 88        | manager       | 42        | admin
  002     | test2     | 88        | manager       | 94        | manager
  

• admin_table

  -------------------------—
  admin_id    | admin_name
  -------------------------—
  001         | Super Admin
  

• manager_table

  ---------------------------
  manager_id  | manager_name
  ---------------------------
  88          | Mandela
  94          | Kristen
  

How can i get the desired output as shown below with SQL query. I.e.
Join tables with respect to column values when the following criteria is met:

1. If user_type = admin then it should join with admin_table.

2. If user_type = manager then it should join with manager_table.

Desired output:

-----------------------------------------------------
msg_id  | msg_sub   | msg_to_name   | msg_from_name
-----------------------------------------------------
001     | test      | Mandela       | Super Admin
002     | test2     | Mandela       | Kristen

I.e. Get the join sql query based on column value.

---

EDIT:

I want to fetch the datafrom sql query not form the serverside coding.

I tried this query from here, i.e. Winfred’s Idea ( Answered )

However, I could not understand it.

msg_by_usertype is the column based, where the value manager then it should select manager_table and if it is admin the to admin_table

Answer 1

As far as I understood your question, you can try this:

    SELECT msg_id,  
     msg_body,  
     usersBy.userName AS msg_by, 
     usersTo.userName AS msg_to,  
     msg_by_usertype
    FROM messages
    INNER JOIN 
    (SELECT admin_id As id, admin_name as userName
      FROM admin_table         
     UNION         
     SELECT manager_id As id, manager_name as userName
      FROM manager_table ) usersTo ON msg_to = usersTo.id  

     INNER JOIN 
    (SELECT admin_id As id, admin_name as userName
      FROM admin_table        
     UNION         
     SELECT manager_id As id, manager_name as userName
      FROM manager_table ) usersBy ON msg_by = usersBy.id  

Here is an SQL Fiddle to see how it works. (It only works if you cant have an admin who has the same id like a manager. Id should be unique in both tables.)