Question

I’m relatively new to mysql so bear with me.

I have a table that looks a bit like this:

ID | Name | Location  

0  | John | Los Angeles  
1  | Joe  | San Jose  
2  | Jane | New York  
3  | Sal  | Boise  
4  | Jay  | New York  
5  | Kate | San Jose  

I need a SELECT statement that gets all rows, with the exception that if Location is repeated, that row is ignored. The result should look something like this:

0  | John | Los Angeles  
1  | Joe  | San Jose  
2  | Jane | New York  
3  | Sal  | Boise  

The important thing is that my table is very, very large, with hundreds of thousands of rows. Most things I’ve tried as ended up with select statements that take literally 30+ minutes to complete!

Answer 1

This is how I would do it:

SELECT mt1.ID, mt1.Name, mt1.Location 
FROM mytable mt1
JOIN (SELECT MIN(id) AS ID 
        FROM Mytable 
        GROUP BY location) mt2
    ON mt1.id = mt2.Id

The derived table get the minimum id for each location. Then joins to the table to pull the rest of the data.

Incidentally ID is a horrible choice for naming the id field. Please think about using tablenameID instead. It is helpful for reporting not to have the same name for the id fields in differnt tables and it makes if FAR less likely that you will make an accidental join mistake and join tothe ID in the wrong table. It also makes the PK/FK relationships easier to see in my opinion.