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, 20 select *, count(*) as count from t_search where mark = 'b' group by title order by count desc limit 0, 20 select *, 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_data
where row_num <= 20
order 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_data
WHERE rank <= 20
ORDER BY mark, count DESC; 
