MySQL Removing duplicate columns on Left Join, 3 tables

Question

I have three tables, each have a foreign key. When I perform a join, I get duplicate columns.

Given

mysql> describe Family;
+---------------+----         


        

Answer 1

You are not getting duplicate columns, what you are really getting is the Supervisor_ID column from table Family (that is Family.Supervisor_ID) and Supervisor_ID from table Supervisor (that is Supervisor.Supervisor_ID) but to your result set, you will see both as Supervisor_ID, and that is why you think they are duplicated. The same will happen with Center_iD.

The solution is to specify the fields that you need from each table, and decide if you need to get the Supervisor_ID and Center_ID and which table to get it from.

Answer 2

By default MySQL will return all columns for all tables if you use *. You will need to explicitly enter column names in your query to retrieve them the way you want. Use the query as follows:

SELECT A.HEAD_name, A.Family_Size, A.Gender, A.ID_Number, A.DOB,
    B.Supervisor_ID, B.Supervisor_Name, B.Supervisor_Number,
    C.Center_ID, C.Location
FROM Family A
JOIN SUPERVISOR B on ( A.Supervisor_ID = B.Supervisor_ID)
JOIN CENTER C on (B.Center_ID = C.Center_ID);

Answer 3

The problem can be solved by "USING" keyword.

SELECT * from Family
   JOIN SUPERVISOR on ( Family.Supervisor_ID = SUPERVISOR.Supervisor_ID)
   JOIN CENTER on (SUPERVISOR.Center_ID = CENTER.Center_ID); 

In your case the query will become

SELECT * FROM FAMILY 
  JOIN (SUPERVISOR JOIN CENTER USING(Center_ID)) USING(Supervisor_ID);

The point is Simple, If you have two Tables A(a,b) and B(b,c) then after joining to produce the result in the form of (a,b,c)

Select *
    from A JOIN B USING(b);

will give the Result-Set with three columns(a,b,c)

NOTE : Since I don't know whether we can use multiple params in Using, therefore I made it as subquery.

Answer 4

According to oreilly,

Look for an explain plan.
You can do a _select x from y inner join.... where w=z ... this is just an example