请这样做：

select di.id,di.Client_Id,it.Name,it.Price,ca.Cat,ifnull(di.Discount,0)
from Discount di
    right join Items it on di.Item_id=it.Id
    left join Category ca on it.Cat_Id=ca.Id
order by di_Id,di.Client_id,It.Name

从折扣中获取所有行，然后获取所有项目并查找类别。如果一个项目没有折扣（空），你得到一个0

为了得到你想要的结果，
以下是查询。。。

select
     i.id item_id,
     d.id discount_id,
     i.name,
     i.price,
     c.name cat_name,
     d.discount
from items i
left join discount d on i.id = d.item_id
left join category c on i.cat_id = c.id
where d.client_id = 1 or d.client_id is null;
/*where condition added after update as OP required*/

更新
根据op的意见

select
    i.id item_id,d.id discount_id,
    i.name,i.price,c.name cat_name,d.discount
from discount d
left join items i on i.id = d.item_id
left join category c on i.cat_id = c.id
where d.client_id = 2;

这是一个简单的sql查询。

Select Id, Name, Price, Cat, Discount From Items
left join discount on Items.id=discount.Item_Id 
left join category on Items.cat_id=id

输出如下：

Id   |Name   | Price    | Cat | Discount
---- |-------|----------------|----------
 1   | Item1 |   50.00  | Cat1|   10
 2   | Item2 |   25.20  | Cat5|   0