sql各种查询技巧

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