MySQL SUM by if statement

Question

我的MySQL表中有3列type、sum和custom_sum

SELECT `type`, SUM(`sum`) FROM `localhost-test` GROUP BY `type`

如果custom_sum为空，我需要在SUM()中使用sumustom_sum。是否可以根据mysql中的"if"语句按不同列求和？

Answer 1

如果custom_sum值为null:

SELECT 
    `u.type`,
    SUM(COALESCE(`custom_sum`,`sum`))
FROM 
    `localhost-test` 
GROUP BY 
    `type`

如果custom_sum值为empty（==''）：

 SELECT 
    `u.type`,
    SUM(CASE WHEN `custom_sum` = '' THEN `sum` ELSE `custom_sum` END)
FROM 
    `localhost-test` 
GROUP BY 
    `type`