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 });