问题
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.
回答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.
回答2:
Maybe something like
select i.ItemContent, t.TagName from (SELECT ItemId, ItemContent FROM item limit 10) i
INNER JOIN ItemTag it ON i.ItemId = it.ItemId --You will miss tagless items here!
INNER JOIN tag t ON t.id = it.TagId
回答3:
My first suggestion is to use a subquery to generate the list of item ID's and return items matching those item ID's. But this doesn't include the TagName in your result set. I'll submit a separate answer with another solution.
SELECT i.ItemContent
FROM item AS i
WHERE i.id IN (
SELECT it.ItemId
FROM ItemTag AS it
INNER JOIN tag AS t ON (t.id = it.TagId)
WHERE t.TagName IN ('mysql', 'database', 'tags', 'tagging')
);
This is a non-correlated subquery, so a good SQL engine should factor it out and run it only once.
回答4:
You could also use Distinct/Group By:
SELECT DISTINCT TagID, TagName FROM ((TAG T
INNER JOIN ITEM_TAG I_T ON T.TagID = I_T.TagID)
INNER JOIN ITEM I ON I_T.ItemID = I.ItemID)
GROUP BY TagID, TagName
来源：https://stackoverflow.com/questions/167067/mysql-limit-with-many-to-many-relationship