sql各种查询技巧

2018年8月4日15:49:23 发表评论 637 views

SQL Server T-SQL高级查询

高级查询在数据库中用得是最频繁的,也是应用最广泛的。

Ø 基本常用查询

--select
select * from student;
 
--all 查询所有
select all sex from student;
 
--distinct 过滤重复
select distinct sex from student;
 
--count 统计
select count(*) from student;
select count(sex) from student;
select count(distinct sex) from student;
 
--top 取前N条记录
select top 3 * from student;
 
--alias column name 列重命名
select id as 编号, name '名称', sex 性别 from student;
 
--alias table name 表重命名
select id, name, s.id, s.name from student s;

--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;
 
--where 条件
select * from student where id = 2;
select * from student where id > 7;
select * from student where id < 3;
select * from student where id <> 3;
select * from student where id >= 3;
select * from student where id <= 5;
select * from student where id !> 3;
select * from student where id !< 5;
 
--and 并且
select * from student where id > 2 and sex = 1;
 
--or 或者
select * from student where id = 2 or sex = 1;
 
--between ... and ... 相当于并且
select * from student where id between 2 and 5;
select * from student where id not between 2 and 5;
 
--like 模糊查询
select * from student where name like '%a%';
select * from student where name like '%[a][o]%';
select * from student where name not like '%a%';
select * from student where name like 'ja%';
select * from student where name not like '%[j,n]%';
select * from student where name like '%[j,n,a]%';
select * from student where name like '%[^ja,as,on]%';
select * from student where name like '%[ja_on]%';
 
--in 子查询
select * from student where id in (1, 2);
 
--not in 不在其中
select * from student where id not in (1, 2);
 
--is null 是空
select * from student where age is null;
 
--is not null 不为空
select * from student where age is not null;
 
--order by 排序
select * from student order by name;
select * from student order by name desc;
select * from student order by name asc;
 
--group by 分组
按照年龄进行分组统计
select count(age), age from student group by age;
按照性别进行分组统计
select count(*), sex from student group by sex;
按照年龄和性别组合分组统计,并排序
select count(*), sex from student group by sex, age order by age;
按照性别分组,并且是id大于2的记录最后按照性别排序
select count(*), sex from student where id > 2 group by sex order by sex;
查询id大于2的数据,并完成运算后的结果进行分组和排序
select count(*), (sex * id) new from student where id > 2 group by sex * id order by sex * id;
 
--group by all 所有分组
按照年龄分组,是所有的年龄
select count(*), age from student group by all age;
 
--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。

 

# from (select … table)示例

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

 

# 示例

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;

 

# in, not in子句查询示例

查询班级id大于小于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
);
 
查询不是班的学生信息
select * from student where cid not in (
    select id from classes where name = '2班'
)

in、not in 后面的子句返回的结果必须是一列,这一列的结果将会作为查询条件对应前面的条件。如cid对应子句的id;

 

# exists和not exists子句查询示例

查询存在班级id为的学生信息
select * from student where exists (
    select * from classes where id = student.cid and id = 3
);
 
查询没有分配班级的学生信息
select * from student where not exists (
    select * from classes where id = student.cid
);

exists和not exists查询需要内部查询和外部查询进行一个关联的条件,如果没有这个条件将是查询到的所有信息。如:id等于student.id;

 

# some、any、all子句查询示例

查询班级的学生年龄大于班级的学生的年龄的信息
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;
 
跳过相同递增
select s.id, s.name, cid, c.name, rank() over(order by c.name) as rank 
from student s, classes c where cid = c.id;

 

# 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

不跳过,直接递增
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。

利用partition by按照班级名称分组,学生id排序
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;

 

# ntile平均排序函数

将要排序的数据进行平分,然后按照等分排序。ntile中的参数代表分成多少等分。

select s.id, s.name, cid, c.name, 
ntile(5) over(order by c.name) as ntile 
from student s, classes c where cid = c.id;

 

Ø 集合运算

操作两组查询结果,进行交集、并集、减集运算

1、 union和union all进行并集运算

--union 并集、不重复
select id, name from student where name like 'ja%'
union
select id, name from student where id = 4;
 
--并集、重复
select * from student where name like 'ja%'
union all
select * from student;

 

2、 intersect进行交集运算

--交集(相同部分)
select * from student where name like 'ja%'
intersect
select * from student;

 

3、 except进行减集运算

--减集(除相同部分)
select * from student where name like 'ja%'
except
select * from student where name like 'jas%';

 

Ø 公式表表达式

查询表的时候,有时候中间表需要重复使用,这些子查询被重复查询调用,不但效率低,而且可读性低,不利于理解。那么公式表表达式可以解决这个问题。

我们可以将公式表表达式(CET)视为临时结果集,在select、insert、update、delete或是create view语句的执行范围内进行定义。

--表达式
with statNum(id, num) as 
(
    select cid, count(*) 
    from student 
    where id > 0
    group by cid
)
select id, num from statNum order by id;
 
with statNum(id, num) as 
(
    select cid, count(*) 
    from student 
    where id > 0
    group by cid
)
select max(id), avg(num) from statNum;

 

Ø 连接查询

1、 简化连接查询

--简化联接查询
select s.id, s.name, c.id, c.name from student s, classes c where s.cid = c.id;

 

2、 left join左连接

--左连接
select s.id, s.name, c.id, c.name from student s left join classes c on s.cid = c.id;

 

3、 right join右连接

--右连接
select s.id, s.name, c.id, c.name from student s right join classes c on s.cid = c.id;

 

4、 inner join内连接

--内连接
select s.id, s.name, c.id, c.name from student s inner join classes c on s.cid = c.id;
 
--inner可以省略
select s.id, s.name, c.id, c.name from student s join classes c on s.cid = c.id;

 

 

1、 聚合函数

max最大值、min最小值、count统计、avg平均值、sum求和、var求方差

select 
    max(age) max_age, 
    min(age) min_age, 
    count(age) count_age, 
    avg(age) avg_age, 
    sum(age) sum_age, 
    var(age) var_age 
from student;

 

2、 日期时间函数

select dateAdd(day, 3, getDate());--加天
select dateAdd(year, 3, getDate());--加年
select dateAdd(hour, 3, getDate());--加小时
--返回跨两个指定日期的日期边界数和时间边界数
select dateDiff(day, '2011-06-20', getDate());
--相差秒数
select dateDiff(second, '2011-06-22 11:00:00', getDate());
--相差小时数
select dateDiff(hour, '2011-06-22 10:00:00', getDate());
select dateName(month, getDate());--当前月份
select dateName(minute, getDate());--当前分钟
select dateName(weekday, getDate());--当前星期
select datePart(month, getDate());--当前月份
select datePart(weekday, getDate());--当前星期
select datePart(second, getDate());--当前秒数
select day(getDate());--返回当前日期天数
select day('2011-06-30');--返回当前日期天数
select month(getDate());--返回当前日期月份
select month('2011-11-10');
select year(getDate());--返回当前日期年份
select year('2010-11-10');
select getDate();--当前系统日期
select getUTCDate();--utc日期

 

发表评论

:?: :razz: :sad: :evil: :!: :smile: :oops: :grin: :eek: :shock: :???: :cool: :lol: :mad: :twisted: :roll: :wink: :idea: :arrow: :neutral: :cry: :mrgreen: