Compute column value with CASE syntax - mysql

Question

I am pretty confused and absolutely not sure if this is the right way.
In the example below I am trying to check if the promotion type is 1 ( percentage e.g. 10% ) or 2 ( hard price e.g 10 EUR ) and compute the price after it and that only if main_product_id IS NOT NULL. Otherwise the price stays the same.
SELECT p.price
FROM product as p
LEFT JOIN promotion_product as pp ON p.id=pp.main_product_id
LEFT JOIN promotion as pr ON pp.promo_id=pr.id
(
CASE
WHEN pp.main_product_id IS NOT NULL THEN
CASE
WHEN pr.type=1 THEN p.price = p.price - (p.price * pr.value/100)
WHEN pr.type=2 THEN p.price = p.price - pr.value
END
END
)
What I get as error is:
#1305 - FUNCTION pr.id does not exist
This is pretty clear I know. But how to compute the new price and is it possible with CASE syntax ?
product table :
+----------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| category_id | int(11) | YES | MUL | NULL | |
| brand_id | int(11) | YES | MUL | NULL | |
| sort | int(11) | NO | | 999 | |
| enable | tinyint(1) | NO | MUL | 2 | |
| product_number | varchar(255) | YES | MUL | NULL | |
| price | float | YES | | NULL | |
| quantity | float | YES | | NULL | |
| rating | tinyint(4) | NO | | 0 | |
+----------------+--------------+------+-----+---------+----------------+
promotion table:
+------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| start_date | timestamp | YES | | NULL | |
| end_date | timestamp | YES | | NULL | |
| type | tinyint(4) | NO | | NULL | |
| value | float | NO | | NULL | |
| enable | tinyint(4) | NO | | 2 | |
+------------+------------+------+-----+---------+----------------+
promotion_product table:
+-----------------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| promo_id | int(11) | NO | MUL | NULL | |
| product_id | int(11) | YES | MUL | NULL | |
| main_product_id | int(11) | YES | MUL | NULL | |
+-----------------+---------+------+-----+---------+----------------+

Answer 1

CASE...WHEN...THEN expressions are for use in SELECT clauses. You have yours wrapped in parentheses at the end of your query. Because your query looks like pr.id(yadda yadda) MySQL thinks pr.id should be a function. It isn't, so MySQL throws your error.
Try something like this:
SELECT p.price,
CASE
WHEN pr.type=1 THEN p.price = p.price - (p.price * pr.value/100)
WHEN pr.type=2 THEN p.price = p.price - pr.value
ELSE p.price
END promoted_price
FROM product as p
LEFT JOIN promotion_product as pp ON p.id=pp.main_product_id
LEFT JOIN promotion as pr ON pp.promo_id=pr.id
I refactored your case expressions so it's not nested. The ELSE clause deals with the default cases where pr.type isn't 1 or 2, and where the ON clauses of your left joins don't match anything.

Answer 2

This is the way I will approach your goal:
SELECT
IF(pp.main_product_id IS NOT NULL,
CASE
WHEN pr.type = 1 THEN p.price - (p.price * pr.value / 100)
WHEN pr.type = 2 THEN p.price - pr.value
ELSE p.price -- Added a default case.
END,
p.price) AS finalPrice
FROM
product AS p
LEFT JOIN
promotion_product AS pp ON p.id = pp.main_product_id
LEFT JOIN
promotion AS pr ON pp.promo_id = pr.id
Note also, that you have start_date and end_date on your promotion table that you are currently ignoring.