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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(常适用分组统计、合计)

       语法:
       SELECT column_name, aggregate_function(column_name)       FROM table_name       [WHERE column_name operator value]       GROUP BY column_name

       例: 查询每个班的学生数量?
       SELECT classId, count(*) FROM student
       GROUP BY classId

       例: 查询指定班的学生数量?
       SELECT classId, count(*) FROM student
       WHERE classId in(‘class1‘,‘class2‘,‘class3‘)
       GROUP BY classId

SQL Language