如何在多列上使用MySql SUM（）

Question

I am struggling with MySql SUM() function . For example , I have a table called my_table with columns('id','price_1','price_2') . I know this table is not following 1st normalization . But I need to just to demonstrate what I am in need . I want to select like this .

SELECT SUM({price_1 if price_2 is null}) GROUP BY `id`

How can I achieve this?

Answer 1

Use COALESCE :

SELECT SUM(COALESCE(price_2,price_1)) as TotalPrice
FROM my_table 
GROUP BY `id`

COALESCE will take price_1 if price_2 is NULL . ie, COALESCE will return the first parameter which is not NULL .

Sample result in SQL Fiddle

If you want to find the sum of price_1 when all the price_2 are NULL . Then:

SELECT COALESCE(SUM(price_2),SUM(price_1)) as TotalPrice
FROM my_table 
GROUP BY `id`

Sample result in SQL Fiddle