mysql语句合并优化

问题：给定多条查询同一表不同分组结果的sql语句，能否将其合并成一条sql语句执行？

select *, count(*) as count from t_search where mark = 'a' group by title order by count desc limit 0, 20select *, count(*) as count from t_search where mark = 'b' group by title order by count desc limit 0, 20select *, count(*) as count from t_search where mark = 'c' group by title order by count desc limit 0, 20...

总共有24条sql语句，每个查询的mark字段从a到z，每条语句查询20条结果。

解答：

方法1：mysql 8.0+

使用with子句和窗口函数：

with ranked_data as (    select *,           count(*) over (partition by title, mark) as count,           row_number() over (partition by mark order by count(*) desc) as row_num    from t_search    where mark between 'a' and 'z'    group by title, mark)select *from ranked_datawhere row_num <= 20order by mark, count desc;

方法2：mysql 8.0以下

使用变量和子查询：

SELECT *FROM (    SELECT *,           @rank := IF(@prev_mark = mark, @rank + 1, 1) AS rank,           @prev_mark := mark,           COUNT(*) AS count    FROM t_search    JOIN (SELECT @rank := 0, @prev_mark := '') AS vars    WHERE mark BETWEEN 'a' AND 'z'    GROUP BY title, mark    ORDER BY mark, count DESC) AS ranked_dataWHERE rank <= 20ORDER BY mark, count DESC;