数据库教程:高级SQL基础查询

基础查询 — 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; — coun

基础查询

  -- 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;    -- limit是mysql的语法  select * from table limit m,n    其中m是指记录开始的index,从0开始,表示第一条记录    n是指从第m+1条开始,取n条。    -- 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;

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    );

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

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;

– 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

– union 并集、不重复

  select id, name from student where name like 'ja%'    union select id, name from student where id = 4;

– union all 并集、重复

  select * from student where name like 'ja%'    union all select * from student;

– 交集相同

  select * from student where name like 'ja%'    intersect select * from student;

– 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。

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;

表的联合查询

  -- 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;        -- 左连接 (左为基表)    select s.id, s.name, c.id, c.name from student s left join classes c on s.cid = c.id;        -- 右连接 (右为基表)    select s.id, s.name, c.id, c.name from student s right join classes c on s.cid = c.id;        -- 内连接 (等值联合)    select s.id, s.name, c.id, c.name from student s inner join classes c on s.cid = c.id;        -- 交叉联接查询,结果是一个笛卡儿乘积    select s.id, s.name, c.id, c.name from student s ,classes c        -- 自连接    select distinct s.* from student s, student s1 where s.id <> s1.id and s.sex = s1.sex;

函数

– 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;

– 日期时间函数

select dateAdd(day, 3, getDate());

select dateAdd(year, 3, getDate());

select dateAdd(hour, 3, getDate());

select dateDiff(day, ‘2011-06-20’, getDate());

select dateDiff(second, ‘2011-06-22 11:00:00’, getDate());

select dateDiff(hour, ‘2011-06-22 10:00:00’, getDate());

select dateName(month, getDate());

select dateName(minute, getDate());

select dateName(weekday, getDate());

select datePart(month, getDate());

select datePart(weekday, getDate());

select datePart(second, getDate());

select day(getDate());

select day(‘2011-06-30’);

select month(getDate());

select month(‘2011-11-10’);

select year(getDate());

select year(‘2010-11-10’);

select getDate();

select getUTCDate();

– 数学函数

select pi();

select rand(100), rand(50), rand();

select round(rand(), 3), round(rand(100), 5);

select round(123.456, 2), round(254.124, -2);

select round(123.4567, 1, 2);

– 元数据

select col_name(object_id(‘student’), 1); // 返回列名

select col_name(object_id(‘student’), 2);

– 该列数据类型长度

select col_length(‘student’, col_name(object_id(‘student’), 2));

– 该列数据类型长度

select col_length(‘student’, col_name(object_id(‘student’), 1));

– 返回类型名称、类型id

select type_name(type_id(‘varchar’)), type_id(‘varchar’);

– 返回列类型长度

select columnProperty(object_id(‘student’), ‘name’, ‘PRECISION’);

– 返回列所在索引位置

select columnProperty(object_id(‘student’), ‘sex’, ‘ColumnId’);

– 字符串函数

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