Question

Somewhat confusing so its easier if I put down example and expected output to begin.

I have a table that could look like this: (Unit1 – Unit2 columns could span up to 30 columns in the same general format)

| ID | Name | Unit1_left | Unit2_left |
| 1  | Tom  |     50     |     NULL   |
| 2  | Tom  |    NULL    |      1     |
| 3  | Tom  |     45     |     NULL   |
| 4  | Dan  |    NULL    |     NULL   |

What I am trying to select is a table like this:

| Name | Unit1_left | Unit2_left |
| Tom  |     45     |      1     |
| Dan  |    NULL    |     NULL   |

What that is doing is grouping by name and attempting to find the last values in the 2 other columns if they exist (if not then it returns NULL).

I have looked at various other questions and they all say to use Max() however this will not work since it selects the highest value (incorrect). I have seen that in MsSQL there is a Last() function which looks vaguely like what I want it to do but its not implemented in MySQL and isn’t exactly what I need anyway.

What I am trying to ask is, does anyone know of a possible method of selecting the data like this or if I will have to use a separate programming language to do this?

Answer 1

This will produce the result set you’ve described

SELECT dname.name, 
       l1value.unit1_left, 
       l2value.unit2_left 
FROM   (SELECT DISTINCT `name` 
        FROM   table1) `DName` 
       LEFT JOIN (SELECT `name`, 
                         Max(id) id 
                  FROM   table1 
                  WHERE  unit1_left IS NOT NULL 
                  GROUP  BY `name`) l1 
              ON dname.`name` = l1.`name` 
       LEFT JOIN table1 l1value 
              ON l1.id = l1value.id 
       LEFT JOIN (SELECT `name`, 
                         Max(id) id 
                  FROM   table1 
                  WHERE  unit2_left IS NOT NULL 
                  GROUP  BY `name`) l2 
              ON dname.`name` = l2.`name` 
       LEFT JOIN table1 l2value 
              ON l2.id = l2value.id ;

DEMO

I did it by creating 2 inline views to the highest id for non-null values for both unit1_left and unit2_left (l1 and l2). Then joined it back to original table to get the values (l1value and l2value). We then join that back to a third inline view (dname) that creates the distinct names.

It’s quite messy and it might make more sense just to keep your data in a more sensible manner.