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;
+---------------+-------------+------+-----+---------+-------+
| Field         | Type        | Null | Key | Default | Extra |
+---------------+-------------+------+-----+---------+-------+
| HEAD_name     | varchar(45) | NO   | PRI |         |       |
| Family_Size   | int(11)     | NO   |     |         |       |
| Gender        | char(1)     | NO   |     |         |       |
| ID_Number     | int(11)     | NO   |     |         |       |
| DOB           | date        | NO   |     |         |       |
| Supervisor_ID | int(11)     | NO   | MUL |         |       |
+---------------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

mysql> describe SUPERVISOR;
+-------------------+---------------+------+-----+---------+-------+
| Field             | Type          | Null | Key | Default | Extra |
+-------------------+---------------+------+-----+---------+-------+
| Supervisor_ID     | int(11)       | NO   | PRI |         |       |
| Supervisor_Name   | varchar(45)   | NO   |     |         |       |
| Supervisor_Number | decimal(10,0) | NO   |     |         |       |
| Center_ID         | int(11)       | NO   | MUL |         |       |
+-------------------+---------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> describe CENTER;
+-----------+-------------+------+-----+---------+-------+
| Field     | Type        | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| Center_ID | int(11)     | NO   | PRI |         |       |
| Location  | varchar(45) | NO   |     |         |       |
+-----------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

My query statement:

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

My objective is to get one row of all the columns from the join without duplicate columns. So what is the SQL statement syntax that I should use?

Solution 1

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);

Solution 2

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.

Solution 3

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.

Comments

[paddy]

If you are feeling lazy, you can take all columns from the largest table (in this case A) and then add in any remaining ones you want from the others: A.*, B.Supervisor_Name, B.Supervisor_Number, B.*.

[Tanzeel Kazi]

@paddy Sure that works! But if you want to optimize your query never use *. It adds an extra lookup for the table column names. In a small query it might not matter but it can add up for multiple queries in the long run.

[paddy]

Cool, that's good to know. I never do, unless I'm just doing throw-away queries.