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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