如何高效统计 SQL 查询中的条件出现次数？

高效计算 SQL 中的条件出现次数

本指南探讨了计算 SQL 查询中条件数据出现次数的各种方法，并按特定列分组。最佳方法取决于您的数据库系统。

PostgreSQL 9.4 及更高版本：FILTER 子句

PostgreSQL 9.4 及后续版本使用 FILTER 聚合函数中的 COUNT 子句提供了最高效且可读的解决方案：

SELECT category,
       COUNT(*) FILTER (WHERE question1 = 0) AS zero,
       COUNT(*) FILTER (WHERE question1 = 1) AS one,
       COUNT(*) FILTER (WHERE question1 = 2) AS two
FROM reviews
GROUP BY category;

PostgreSQL 的简洁替代方案

更短的语法实现相同的结果：

SELECT category,
       COUNT(CASE WHEN question1 = 0 THEN 1 END) AS zero,
       COUNT(CASE WHEN question1 = 1 THEN 1 END) AS one,
       COUNT(CASE WHEN question1 = 2 THEN 1 END) AS two
FROM reviews
GROUP BY category;

广泛数据的最佳交叉表

对于全面的交叉表查询，尤其是大量类别的查询，crosstab() 函数提供了卓越的性能：

SELECT *
FROM crosstab(
    'SELECT category, question1, COUNT(*) AS ct
     FROM reviews
     GROUP BY 1, 2
     ORDER BY 1, 2',
    'VALUES (0), (1), (2)'
) AS ct (category TEXT, zero INT, one INT, two INT);
```  This approach dynamically handles a larger number of categories more efficiently than the previous methods.

以上是如何高效统计 SQL 查询中的条件出现次数？的详细内容。更多信息请关注PHP中文网其他相关文章！