mysql語句合并優化
問題：給定多條查詢同一表不同分組結果的sql語句，能否將其合并成一條sql語句執行？
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 ...
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總共有24條sql語句，每個查詢的mark字段從a到z，每條語句查詢20條結果。
解答：
方法1：mysql 8.0+
使用with子句和窗口函數：
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;
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使用變量和子查詢：
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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