MySQL 中使用 GROUP BY 对后分组的数据进行 COUNT() 统计
SELECT id,title FROM tb_item GROUP BY id
SELECT id,title,count(*) AS counter FROM tb_item GROUP BY id
counter 实际上是每组的计数
统计分组后的数据，应该使用这样的子查询：
select count(*) from (select count(*) FROM service GROUP BY name,service) t
实例：
var replacements = {tags:"11,12,13"}var sql = `
SELECT id,tags,title,r.* FROM tb_item AS i
LEFT JOIN tb_item_tag_relation AS r
ON r.item_id = i.id
WHERE r.tag_id IN (:tags)
GROUP BY i.id
ORDER BY id DESC
LIMIT 10
`;var replacements = { tags: currentTags };var res = await db.query(sql, { type: db.QueryTypes.SELECT, raw: true, replacements });
sql = `
SELECT COUNT(id) AS count FROM(
SELECT id FROM tb_item AS i
LEFT JOIN tb_item_tag_relation AS r
ON r.item_id = i.id
WHERE r.tag_id IN (:tags)
GROUP BY i.id
) t
`;
res = await db.query(sql, { type: db.QueryTypes.SELECT, raw: true, replacements });