限制LEFT JOIN组的数量

Question

使用下表：

SELECT i.*, GROUP_CONCAT(t.name) tagList
FROM items AS i
LEFT JOIN tag_rel AS tr ON (tr.item = j.id)
LEFT JOIN tags AS t ON (t.id = tr.tag)
GROUP BY i.id 
ORDER BY i.id DESC

有没有办法限制tagList的元素？而不是给我所有的标签（在某些情况下为30+），我可以将其限制为5？

Answer 1

SELECT i.*, SUBSTRING_INDEX(GROUP_CONCAT(t.name SEPARATOR ','), ',', 5) tagList
FROM items AS i
LEFT JOIN tag_rel AS tr ON (tr.item = j.id)
LEFT JOIN tags AS t ON (t.id = tr.tag)
GROUP BY i.id 
ORDER BY i.id DESC

只需用所需数字替换5