【Mysql】 查詢數據排序以及聚合函數

# 排序
  # order by 字段
  # asc 從小到大排序，即升序
  # desc從大到小排序，即降序
  
  # 查詢年齡在18到34歲之間的男性，按照年齡從小到大排序
  select * from students where (age between 18 and 34) and gender=1 order by age;
  select * from students where (age between 18 and 34) and gender=1 order by age asc;
  # 查詢年齡在18到34歲之間的男性，按照年齡從小到大排序
  select * from students where (age between 18 and 34) and gender=1 order by age desc;
  # 查詢年齡在18到34歲之間的女性，身高從高到矮排序
  select * from students where age between 18 and 34 and gender=2 order by height desc; 
 
  # order by 多個字段
  # 查詢年齡在18到34歲之間的女性，身高從高到矮排序，如果身高相同的情況下按照序列從小到大排序
  select * from students where (age between 18 and 34) and gender=2 order by height desc,id desc;

  # 聚合函數
  # count
  # 查詢男性有多少人
  select count(*) from students where gender=1;
  select count(*) as 男性人數 from students where gender=1;
  
  # 最大值
  # max 
  # 查詢最大年齡
  select max(age) from students;
 
  # 最大值
  # min 
  # 查詢最小年齡
  select min(age) from students;

  # 求和
  # sum
  # 查詢最大年齡
  select sum(age) from students;
  
  # 求平均值
  # avg
  # 查詢平均年齡
  select avg(age) from students;

  # 計算平均年齡
  select sum(age)/count(*) from students;

  # 四捨五入，保留兩位小數
  select round(sum(age)/count(*),2) from students;
  
  # 計算男性的平均身高,保留兩位小數
  select round(sum(height)/count(*),2) from students where gender=1;
  

  # 分組
  # 按照性別分組，查詢所有的性別
  select gender from students group by gender;
  select gender,count(*) from students group by gender; 
  # count(*) 是對前面的分組每組進行求數據
  
  # group_concat 
  select gender,group_concat(name) from students group by gender;
  select gender,group_concat(name,"_",age,"_",id) from students where gender=1 group by gender;

  # having
  # 查詢平均年齡超過30歲的性別，以及姓名
  select gender,group_concat(name)，avg(age), from students group by gender having avg(age)>30;

  # where 和 having 的區別 where在group by 前面 ，having 在group by 後面 ，where 是在對原表進行判斷，having是在對判斷分組之後的表格
 
  select gender,group_concat(name) from stundents group by gender having count(*)>2;