使用CASE语法计算列值

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 eg 10% ) or 2 ( hard price eg 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

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.

Answer 2

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.