假设有表score记录学生id(student_id)、课程id(course_id)和课程分数(score)要求查询表中每门课程成绩最好的前2名学生的id、相应课程id和分数。则可以用如下sql语句select * from score score1 where( select count(1) from score score2 where score1.course_id ...

文章 2017-11-14 来自：开发者社区

SQL优化一例：GROUP BY的语句

SQL语句： select t.type, count(t.id) as todo_count from mc_job_form t where t.state = '2' and t.customs_code = :a group by t.type 表结构: ID VARCHAR2(24) N   id IN_OUT_FLAG CHA...

文章 2014-01-08 来自：开发者社区

合计函数 (比如 SUM) 常常需要添加 GROUP BY 语句。 GROUP BY 语句 GROUP BY 语句用于结合合计函数，根据一个或多个列对结果集进行分组。 SQL GROUP BY 语法 SELECT column_name, aggregate_function(column_name) FROM table_name WHERE column_name operator va...

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