我如何使其他情况下（在有条件的情况下）mysql

Question

我有这个查询

SELECT dcp.discount_campaign_id, CASE  WHEN dcp.active = 0 
    THEN dcp.product_id ELSE (pc.category_id where c.sku_code is NOT NULL)
    END as discount_campaign_product
    FROM discount_campaign_product dcp 
    JOIN product_categories pc
    ON dcp.product_id = pc.product_id
    JOIN categories c
    ON pc.category_id = c.category_id

基本上我有3个表连接在一起，分别是Discount_campaign_product dcp，product_categories pc和category c

with dcp.product_id = pc.product_id 和pc.category_id = c.id

所以基本上我想输入大小写，如果dcp.active = 0然后插入dcp.product_id，但是如果active！= 0 然后将pc.category_id where放入pc.category_id具有c.sku_code NOT NULL

那是我编写的，但是格式错误，如何正确查询？