通過單條 sql 語句實現(xiàn)大量重復(fù)查詢
在數(shù)據(jù)庫中，當(dāng)需要重復(fù)執(zhí)行大量幾乎相同的查詢時，使用單獨的查詢語句會造成效率低下。本文將介紹如何使用單條 sql 語句合并多個類似的查詢。
問題：根據(jù)給定示例，共有 24 條類似的 sql 語句，每條語句都針對不同的 mark 值執(zhí)行相同的查詢，從 t_search 表中按 title 分組并按計數(shù)降序排序。
解決方案：
方法 1（適用于 mysql 8.0 及更高版本）：
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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方法 2（適用于較低版本的 mysql）：
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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通過使用這些方法，您可以使用更簡潔、更高效的單條 sql 語句合并這些重復(fù)的查詢，從而提高數(shù)據(jù)庫查詢性能。
