Mysql If condition on INNER JOIN

Question

The following request gives me a list of products, the stores that sells them and the "parent" store , which I need for other purposes:

SELECT  `allproducts`.`id_store` ,  `uniques` . * 
FROM  `uniques` 
INNER JOIN  `allproducts` ON  `allproducts`.`SKU` =  `uniques`.`SKU` 

Result is as follows :

id_store SKU     id_parent name      sale_sdate  sale_edate
1085     100768  1088      productA  5/18/2014   6/14/2014
1088     100768  1088      productA  5/18/2014   6/14/2014
1089     100768  1088      productA  5/18/2014   6/14/2014

You can see that on the second line, the id_store and id_parent are the same (parent store sells the product too). In this case, I should replace the id_parent with a 0 like so :

id_store SKU     id_parent name      sale_sdate  sale_edate
1085     100768  1088      productA  5/18/2014   6/14/2014
1088     100768  0         productA  5/18/2014   6/14/2014
1089     100768  1088      productA  5/18/2014   6/14/2014

I am having a really hard time integrating the IF condition in this big request (I can post here all the things I tried but it might make things very confusing). Could anyone help?

Answer 1

Use CASE. And remove the WHERE clause if there is no other conditions.

SELECT  `allproducts`.`id_store`, `uniques`.SKU,
        CASE WHEN `allproducts`.`id_store`=`uniques`.`id_parent` THEN 0 ELSE `uniques`.`id_parent` END as id_parent,  
        `uniques`.`name`,`uniques`.`sale_sdate`,`uniques`.`sale_edate`
FROM  `uniques` 
INNER JOIN  `allproducts` ON  `allproducts`.`SKU` =  `uniques`.`SKU` 

Explanation:

If id_store is equal to id_parent, it will select 0. Otherwise, it will select id_parent itself.

EDIT:

I would like to rewrite your query with alias names for better readability.

SELECT  ap.`id_store`, u.SKU,
        CASE WHEN ap.`id_store`=u.`id_parent` THEN 0 ELSE u.`id_parent` END as id_parent,  
        u.`name`,u.`sale_sdate`,u.`sale_edate`
FROM  `uniques` u
INNER JOIN  `allproducts` ap ON  ap.`SKU` =  u.`SKU` 

Answer 2

Use the if like this:

SELECT  a.`id_store`, a.`SKU`,
IF (a.`id_store` = u.`id_store`, 0, u.id_store`) as `id_parent`,
u.`name`, u.`sale_sdate`, `u.sale_edate`
FROM  `uniques` u
INNER JOIN  `allproducts` a ON  u.`SKU` =  a.`SKU` 

The WHERE clause can be removed since you specify the fields to join on in your INNER JOIN clause.