Question

Given a SCHEMA for implementing tags

ITEM ItemId, ItemContent

TAG TagId, TagName

ITEM_TAG ItemId, TagId

What is the best way to limit the number of ITEMS to return when selecting with tags?

SELECT i.ItemContent, t.TagName FROM item i  INNER JOIN ItemTag it ON i.id = it.ItemId  INNER JOIN tag t ON t.id = it.TagId  

is of course the easiest way to get them all back, but using a limit clause breaks down, because you get an duplicate of all the items for each tag, which counts toward the number of rows in LIMIT.

Answer 1

My second solution uses a MySQL function GROUP_CONCAT() to combine all tags matching the item into a comma-separated string in the result set.

SELECT i.ItemContent, GROUP_CONCAT(t.TagName ORDER BY t.TagName) AS TagList FROM item AS i    INNER JOIN ItemTag AS it ON i.id = it.ItemId    INNER JOIN tag AS t ON t.id = it.TagId GROUP BY i.ItemId; 

The GROUP_CONCAT() function is a MySQL feature, it’s not part of standard SQL.