Question

How to express in a MySQL query: “get all new elements but at least N elements”.

I’ll try to explain better. I want to know how to make a query if I have a product list [product, price, date, new] and I want to show the user all new elements, but if there are less than N elements (e.g. 5) show also the last items. If could think in a query like:

SELECT * FROM `products` WHERE new='1' ORDER BY date DESC 

but this just returns all new elements and this:

SELECT * FROM `products` WHERE new='0' ORDER BY date DESC LIMIT 5

just returns the minimum elements required.

Some examples:

• If the table contains 4 new items, the query should return 4 new items and the latest non-new item
• If the table contains 7 new items, the query should return just the 7 new items
• If the table contains 1 new item, the query should return the new item and 4 non-new items.

Answer 1

Use UNION to combine two result sets.

(
    SELECT *
    FROM `products`
    WHERE new = '1'
)
UNION
(
    SELECT *
    FROM `products`
    ORDER BY new DESC, date DESC
    LIMIT 5
) T1
ORDER BY new DESC, date DESC

• The first part of the UNION returns all the new items.
• The second part of the UNION returns the first five new items, or if there are fewer than 5 items, all the new items and then first 5-n old items.
• The UNION removes any duplicates in the result set.

Note that this assumes that there is a unique key defined on any column or combination of columns in your table (for example, if your table has a primary key this should work).