mysql 聚集函数 count 使用详解(转载)

 

本文将探讨以下问题

1.count(*) 、 count(n)、count(null)与count(fieldName)
2.distinct 与 count 连用
3.group by (多个字段) 与 count 实现分组计数
4.case when 语句与 count 连用实现按过滤计数

参考文章:
Select count(*)和Count（1）的区别和执行方式

准备工作

-- 创建表
CREATE TABLE `tb_student` (
  `id` int(11) NOT NULL,
  `stu_name` varchar(255) CHARACTER SET utf8mb4 DEFAULT NULL COMMENT \'学生姓名\',
  `tea_name` varchar(255) DEFAULT NULL COMMENT \'教师姓名\',
  `stu_class` varchar(255) DEFAULT NULL COMMENT \'所在班级名称\',
  `stu_sex` varchar(255) DEFAULT NULL COMMENT \'学生性别\',
  `stu_sex_int` tinyint(4) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- 插入数据 
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'0\', \'小明\', \'老张\', \'一班\', \'男\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'1\', \'小红\', \'老张\', \'一班\', \'女\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'2\', \'小刚\', \'老王\', \'一班\', \'男\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'3\', \'小兰\', \'老王\', \'一班\', \'女\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'4\', \'小军\', \'老张\', \'二班\', \'男\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'5\', \'小芳\', \'老张\', \'二班\', \'女\',0);
INSERT INTO `tb_student` (`id`, `stu_name`, `tea_name`, `stu_class`, `stu_sex`) VALUES (\'6\', \'小强\', \'老王\', \'二班\', \'男\',0);
INSERT INTO