Grouping results by id and create new columns - mysql

Question

I have a table which looks something like the following...
id price condition sell
21039 20.40 new 0
21039 20.41 used 1
12378 10.40 new 1
12378 5 used 0
45898 30.30 new 1
45898 12.20 used 0
(note: there will only ever be 1 new and used value for each id)
What I am trying to do is group all rows with the same id number but in the process creating new columns for each condition, which should look something like...
id new_price new_sell used_price new_sell
21039 20.40 0 20.41 1
12378 10.40 1 5 0
45898 30.30 1 12.20 0
All that I have come up with is the following query, which looks silly
SELECT id, price, condition,
IF(price > 3, 1, 0) AS sell
FROM products
GROUP BY id
How can I get the desired affect of the 2nd table.

Answer 1

This is known as a pivot table. It is done with a series of CASE statements for each column you need to produce, along with an aggregate MAX() or SUM() to eliminate NULLs and collapse it down to a single row.
SELECT
id,
SUM(CASE WHEN `condition` = 'new' THEN price ELSE 0 END) AS new_price,
SUM(CASE WHEN `condition` = 'new' THEN sell ELSE 0 END) AS new_sell,
SUM(CASE WHEN `condition` = 'used' THEN price ELSE 0 END) AS used_price,
SUM(CASE WHEN `condition` = 'used' THEN sell ELSE 0 END) AS used_sell
FROM
products
GROUP BY id
Without the SUM() and GROUP BY, you would still get 2 rows per id, with each having half its columns (not matched by condition in the CASE) as NULL. The SUM() (could also use MAX() in this case) eliminates the NULLs and produces one row since aggregate functions exclude NULL values while the GROUP BY groups the rows by id.
Here is a working sample on SQLFiddle.com
Update after comment:
To calculate sell based on the price, just replace the condition in the sell CASE statements:
SELECT
id,
SUM(CASE WHEN `condition` = 'new' THEN price ELSE 0 END) AS new_price,
SUM(CASE WHEN `condition` = 'new' AND price > 3 THEN 1 ELSE 0 END) AS new_sell,
SUM(CASE WHEN `condition` = 'used' THEN price ELSE 0 END) AS used_price,
SUM(CASE WHEN `condition` = 'used' AND price > 3 THEN 1 ELSE 0 END) AS used_sell
FROM
products
GROUP BY id
(Updated sample...)