如何将多条查询同一表不同分组结果的SQL语句合并成一条语句执行？

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;

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