Question

I want to redistribute some values in a table in a database I manage.

Firstname(varchar), Lastname(varchar), Date(date), Icecream_consumed(int)

Where the combination of Firstname, Lastname, and Date make the primary key for this table. Changing the structure of this table is not an option.

There are 80 total unique combinations of Firstname and LastName in the database but i want to get that number down to 5 and add some of the records with the smaller total Icecream_consumed to the ones with the highest total. So that only the top 5 FirstName and LastName combinations have all the Icecream_consumed count.

For example if I execute the query:

  SELECT Firstname, Lastname, SUM(Icecream_consumed) 
    FROM table_name 
GROUP BY Firstname, Lastname 
ORDER BY SUM(Icecream_consumed);

I want it to only have 5 results, not 80. How can I modify the records within the table to reflect this without manually updating each record? A simple update would not work as it would be inserting where existing records with the same primary key exist.

EDIT: I noticed that the column ID was kind of ambiguous. So I removed it from this example.

EDIT2: Here is an example:

Existing:

Firstname, Lastname, SUM(Icecream_consumed)

John  Doe   1500
Joe   Doe   1400
Alex  Foo   1111
John  Foo   1000
Ben   Foo    999
Sue   Cool   500
Bill  Smith  200
Ben   Smith  150

I want to change the tables so that the sum(icecream_consumed) values associated with sue, cool, bill smith, and ben smith are associated with john doe so that only 5 results are visible. However if i tried to update all records for sue cool to be john doe instead, there would be a conflict with the primary keys since they both might have values for icecream_consumed for the same dates.
The outcome might look like:

John  Doe   2000
Joe   Doe   1600
Alex  Foo   1261
John  Foo   1000
Ben   Foo    999

Answer 1

Working example on SQL Fiddle: http://sqlfiddle.com/#!2/0e8ab/1

Because MySql cannot run sub selects in update statements if the subselect is the same table as the one in the update, you need to first temporarily copy the data into another table – so DO NOT run this when data is still going into the database as it is not an atomic operation. You need to shut down any applications using the database first.

-- Create a copy of the data 
CREATE TABLE table_name_copy SELECT * FROM table_name;

-- Update the rows with the most Icecream_consumed to have the total 
UPDATE table_name t1 SET t1.Icecream_consumed = (
    SELECT SUM(t2.Icecream_consumed) 
    FROM table_name_copy t2 
    WHERE t2.Firstname = t1.FirstName AND t2.Lastname = t1.Lastname
)
WHERE id = (
    SELECT id 
    FROM table_name_copy t2 
    WHERE t2.Firstname = t1.FirstName AND t2.Lastname = t1.Lastname
    ORDER BY Icecream_consumed DESC LIMIT 1
);

-- Delete the other rows that don't have the new totalled Icecream_consumed 
DELETE FROM table_name
WHERE id != (
    SELECT t2.id 
    FROM table_name_copy t2 
    WHERE t2.Firstname = table_name.FirstName AND t2.Lastname = table_name.Lastname
    ORDER BY Icecream_consumed DESC LIMIT 1
);

-- Remove the copied table
DROP TABLE table_name_copy;