高级查询在数据库中用得是最频繁的,也是应用最广泛、最普遍的。
Ø 基本常用查询
-- all 查询所有,几乎从来不用 all 关键字,因为是默认关键字
select all sex from student;
-- distinct 过滤重复 (常用语检查一列数据是否有异常值)
select distinct sex from student;
-- count 统计
select count(distinct sex) from student;
-- top 取前N条记录
select top 3 * from student;
-- column 列运算
select (age + id) col from student;
select s.name + '-' + c.name from classes c, student s where s.cid = c.id;
-- having 分组过滤条件
-- 按照年龄分组,过滤年龄为空的数据,并且统计分组的条数和现实年龄信息
select count(*), age from student group by age having age is not null;
-- 按照年龄和cid组合分组,过滤条件是cid大于1的记录
select count(*), cid, sex from student group by cid, sex having cid > 1;
-- 按照年龄分组,过滤条件是分组后的记录条数大于等于2
select count(*), age from student group by age having count(age) >= 2;
-- 按照cid和性别组合分组,过滤条件是cid大于1,cid的最大值大于2
select count(*), cid, sex from student group by cid, sex having cid > 1 and max(cid) > 2;
Ø 嵌套子查询
子查询是一个嵌套在select、insert、update 或 delete 语句或其他子查询中的查询。任何允许使用表达式的地方都可以使用子查询。子查询也称为内部查询或内部选择,而包含子查询的语句也成为外部查询或外部选择。
-- 将一个 table 的查询结果当做一个新表进行查询
select * from (
select id, name from student where sex = 1
) t where t.id > 2;
上面括号中的语句,就是子查询语句(内部查询)。在外面的是外部查询,其中外部查询可以包含以下语句:
1、 包含常规选择列表组件的常规select查询
2、 包含一个或多个表或视图名称的常规from语句
3、 可选的where子句
4、 可选的group by子句
5、 可选的having子句
-- 查询班级信息,统计班级学生人生
select *, (select count(*) from student where cid = classes.id) as num
from classes order by num;
-- 查询班级id大于小于X的这些班级的学生信息
select * from student where cid in (
select id from classes where id > 2 and id < 4
);
-- 查询不是X班的学生信息
select * from student where cid not in (
select id from classes where name = '2班'
)
-- all,any,some
select * from student where cid = 5 and age > all (
select age from student where cid = 3
);
select * from student where cid = 5 and age > any (
select age from student where cid = 3
);
select * from student where cid = 5 and age > some (
select age from student where cid = 3
);
Ø 聚合查询
1、 distinct去掉重复数据
select distinct sex from student;
select count(sex), count(distinct sex) from student;
2、 compute和compute by汇总查询
-- 对年龄大于20的进行汇总
select age from student where age > 20 order by age compute sum(age) by age;
-- 对年龄大于20的按照性别进行分组汇总年龄信息
select id, sex, age from student where age > 20 order by sex, age compute sum(age) by sex;
-- 按照年龄分组汇总
select age from student where age > 20 order by age, id compute sum(age);
-- 按照年龄分组,年龄汇总,id找最大值
select id, age from student where age > 20 order by age compute sum(age), max(id);
compute 进行汇总,前面是查询的结果,后面一条结果集就是汇总的信息。compute 子句中可以添加多个汇总表达式,可以添加的信息如下:
a、 可选by关键字。它是每一列计算指定的行聚合
b、 行聚合函数名称。包括sum、avg、min、max、count等
c、 要对其执行聚合函数的列
compute by适合做先分组后汇总的业务。compute by 后面的列一定要是 order by 中出现的列。
3、 cube 汇总
cube汇总和compute效果类似,但语法较简洁,而且返回的是一个结果集。
select count(*), sex from student group by sex with cube;
select count(*), age, sum(age) from student where age is not null group by age with cube;
cube要结合group by语句完成分组汇总
Ø 排序函数
排序在很多地方需要用到,需要对查询结果进行排序并且给出序号。比如:
1、 对某张表进行排序,序号需要递增不重复的
2、 对学生的成绩进行排序,得出名次,名次可以并列,但名次的序号是连续递增的
3、 在某些排序的情况下,需要跳空序号,虽然是并列
-- 基本语法
-- 排序函数 over([分组语句] 排序子句[desc][asc])
-- 排序子句 order by 列名, 列名
-- 分组子句 partition by 分组列, 分组列
-- row_number 函数
-- 根据排序子句给出递增连续序号,按照名称排序的顺序递增
select s.id, s.name, cid, c.name, row_number() over(order by c.name) as number
from student s, classes c where cid = c.id;
-- rank 函数函数
-- 根据排序子句给出递增的序号,但是存在并列并且跳空
-- 顺序递增
select id, name, rank() over(order by cid) as rank from student;
-- dense_rank 函数
-- 根据排序子句给出递增的序号,但是存在并列不跳空
select s.id, s.name, cid, c.name, dense_rank() over(order by c.name) as dense
from student s, classes c where cid = c.id;
-- partition by 分组子句
-- 可以完成对分组的数据进行增加排序,partition by可以与以上三个函数联合使用。
select s.id, s.name, cid, c.name, row_number() over(partition by c.name order by s.id) as rank
from student s, classes c where cid = c.id;
select s.id, s.name, cid, c.name, rank() over(partition by c.name order by s.id) as rank
from student s, classes c where cid = c.id;
select s.id, s.name, cid, c.name, dense_rank() over(partition by c.name order by s.id) as rank
from student s, classes c where cid = c.id;