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