各种SQL查询技巧汇总

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;

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
);

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

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;

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%’;

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;

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