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聚合函数

• 聚合函数一般配合group by分组查询使用
• 常用聚合函数
  • COUNT()：计数
  • SUM()：求和
  • AVG()：平均值
  • MAX()：最大值
  • MIN()：最小值

• 创建测试数据库表t_student

  CREATE TABLE t_student (
  	id INT INT PRIMARY KEY AUTO_INCREMENT,
  	stuName VARCHAR (60),
  	age INT,
  	sex VARCHAR (30),
  	per INT,
  	gradeName VARCHAR (60)
  ); 
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('1','张三','23','男',80,'一年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('2','张三丰','25','男',95,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('3','李四','23','男',99,'一年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('4','王五','22','男',88,'三年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('5','珍妮','21','女',67,'一年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('6','李娜','26','女',85,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('7','王峰','20','男',78,'三年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('8','梦娜','21','女',93,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('9','小黑','22','男',97,'一年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('10','追风','25','男',66,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('11','小小张三','21',NULL,53,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('12','小张三','23','男',89,'二年级');
  INSERT INTO t_student (id, stuName, age, sex, per, gradeName) VALUES('13','张三锋小','24',NULL,90,'二年级');
  

• COUNT()

  -- 统计每个年级人数
  
  select gradeName,count(stuName) from t_student group by gradeName
  

• SUM()

  -- 统计每个年级总分数,并汇总所有成绩
  
  SELECT COALESCE(gradeName,'汇总') gradeName,SUM(per) FROM t_student2 GROUP BY gradeName WITH ROLLUP
  

• AVG()

  -- 统计每个年级的平均成绩，并按平均成绩排序
  
  select gradeName avg(per) from t_student group by gradeNmae
  

• MAX()

  -- 统计每个年级的最高成绩
  
  select gradeName MAX(per) from t_student group by gradeNmae
  

• MIN()

  -- 统计每个年级的最低成绩
  
  select gradeName MIN(per) from t_student group by gradeNmae