SQL'COUNT'函数不工作

本文介绍了SQL'COUNT'函数不工作的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 我想要一个SQL查询返回我的总行数。以下是我的查询。 SELECT COUNT（*）AS tot_std FROM`student_main` AS sm，`student_subjects` AS stds，`sub_subjects` AS ss WHERE stds.`student_id`= sm.`studen。 t_id` AND stds.`subs_id` = ss.`subs_id` AND stds.`subs_id` = 1 AND sm.`std_postcode` LIKE'％HA02AN％' GROUP BY stds.`student_id` 查询连接三个表： 1） student_main 这是主表并存储学生信息。 2） sub_subjects 这是存储主题/课程信息的主表 3） student_subjects 此表由student_id和subs_id作为外键组成，并显示学生正在学习的主题。 以下是 student_subjects 表。 现在当我运行查询时应该在输出中显示3，因为总共3个学生正在学习id为1的主题，而是查询给我这样的输出： 删除 GROUP BY 。并学习正确的显式 JOIN 语法： SELECT COUNT tot_std FROM`student_main` sm JOIN `student_subjects` stds ON stds.`student_id`= sm.`studen。 t_id` JOIN `sub_subjects` ss ON stds.`subs_id` = ss.`subs_id` WHERE stds.`subs_id` = 1 AND sm.`std_postcode` LIKE'％HA02AN％ '; 简单的规则：从不在 FROM 子句。 *总是使用 ON 子句明确 JOIN 语法。 I want an SQL Query to return me the total number of rows. Following is my query.SELECT COUNT(*) AS tot_std FROM `student_main` AS sm, `student_subjects` AS stds, `sub_subjects` AS ssWHERE stds.`student_id` = sm.`studen. t_id` AND stds.`subs_id` = ss.`subs_id` AND stds.`subs_id` = 1 AND sm.`std_postcode` LIKE '%HA02AN%'GROUP BY stds.`student_id`The query joins three tables:1) student_main which is the main table and stores student info.2) sub_subjects which is the main table to store info about subjects/courses3) student_subjects this table consists of both student_id and subs_id as foreign keys and shows the subjects which the student is taking.Following is the screen shot of the student_subjects table. Now when I run the Query, it is supposed to display 3 in the output because a total of 3 students are studying the subject with id 1. but instead the query gives me an output like this:Please Help. 解决方案 Remove the GROUP BY. And learn proper explicit JOIN syntax:SELECT COUNT(*) AS tot_std FROM `student_main` sm JOIN `student_subjects` stds ON stds.`student_id` = sm.`studen. t_id` JOIN `sub_subjects` ss ON stds.`subs_id` = ss.`subs_id` WHERE stds.`subs_id` = 1 AND sm.`std_postcode` LIKE '%HA02AN%';Simple rule: Never use commas in the FROM clause. *Always use explicit JOIN syntax with an ON clause. 这篇关于SQL'COUNT'函数不工作的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！