sql各類查詢技巧

SQL Server T-SQL高級查詢

高級查詢在數據庫中用得是最頻繁的，也是應用最普遍的。

Ø 基本經常使用查詢

--select

select * from student;

--all 查詢全部

select all sex from student;

--distinct 過濾重複

select distinct sex from student;

--count 統計

select count(*) from student;

select count(sex) from student;

select count(distinct sex) from student;

--top 取前N條記錄

select top 3 * from student;

--alias column name 列重命名

select id as 編號, name '名稱', sex 性別 from student;

--alias table name 表重命名

select id, name, s.id, s.name from student s;

--column 列運算

select (age + id) col from student;

select s.name + '-' + c.name from classes c, student s where s.cid = c.id;

--where 條件

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 而且

select * from student where id > 2 and sex = 1;

--or 或者

select * from student where id = 2 or sex = 1;

--between ... and ... 至關於而且

select * from student where id between 2 and 5;

select * from student where id not between 2 and 5;

--like 模糊查詢

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 子查詢

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 不爲空

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 分組

按照年齡進行分組統計

select count(age), age from student group by age;

按照性別進行分組統計

select count(*), sex from student group by sex;

按照年齡和性別組合分組統計，並排序

select count(*), sex from student group by sex, age order by age;

按照性別分組，而且是id大於2的記錄最後按照性別排序

select count(*), sex from student where id > 2 group by sex order by sex;

查詢id大於2的數據，並完成運算後的結果進行分組和排序

select count(*), (sex * id) new from student where id > 2 group by sex * id order by sex * id;

--group by all 全部分組

按照年齡分組，是全部的年齡

select count(*), age from student group by all age;

--having 分組過濾條件

按照年齡分組，過濾年齡爲空的數據，而且統計分組的條數和現實年齡信息

select count(*), age from student group by age having age is not null;

按照年齡和cid組合分組，過濾條件是cid大於1的記錄

select count(*), cid, sex from student group by cid, sex having cid > 1;

按照年齡分組，過濾條件是分組後的記錄條數大於等於2

select count(*), age from student group by age having count(age) >= 2;

按照cid和性別組合分組，過濾條件是cid大於1，cid的最大值大於2

select count(*), cid, sex from student group by cid, sex having cid > 1 and max(cid) > 2;

Ø 嵌套子查詢

    子查詢是一個嵌套在select、insert、update或delete語句或其餘子查詢中的查詢。任何容許使用表達式的地方均可以使用子查詢。子查詢也稱爲內部查詢或內部選擇，而包含子查詢的語句也成爲外部查詢或外部選擇。

 

# from (select … table)示例

將一個table的查詢結果當作一個新表進行查詢

select * from (

    select id, name from student where sex = 1

) t where t.id > 2;

上面括號中的語句，就是子查詢語句（內部查詢）。在外面的是外部查詢，其中外部查詢能夠包含如下語句：

     一、 包含常規選擇列表組件的常規select查詢

     二、 包含一個或多個表或視圖名稱的常規from語句

     三、 可選的where子句

     四、 可選的group by子句

     五、 可選的having子句

 

# 示例

查詢班級信息，統計班級學生人生

select *, (select count(*) from student where cid = classes.id) as num 

from classes order by num;

 

# in, not in子句查詢示例

查詢班級id大於小於的這些班級的學生信息

select * from student where cid in (

    select id from classes where id > 2 and id < 4

);

查詢不是班的學生信息

select * from student where cid not in (

    select id from classes where name = '2班'

)

in、not in 後面的子句返回的結果必須是一列，這一列的結果將會做爲查詢條件對應前面的條件。如cid對應子句的id；

 

# exists和not exists子句查詢示例

查詢存在班級id爲的學生信息

select * from student where exists (

    select * from classes where id = student.cid and id = 3

);

查詢沒有分配班級的學生信息

select * from student where not exists (

    select * from classes where id = student.cid

);

exists和not exists查詢須要內部查詢和外部查詢進行一個關聯的條件，若是沒有這個條件將是查詢到的全部信息。如：id等於student.id；

 

# some、any、all子句查詢示例

查詢班級的學生年齡大於班級的學生的年齡的信息

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 聚合查詢

一、 distinct去掉重複數據

select distinct sex from student;

select count(sex), count(distinct sex) from student;

 

二、 compute和compute by彙總查詢

對年齡大於的進行彙總

select age from student 

where age > 20 order by age compute sum(age) by age;

對年齡大於的按照性別進行分組彙總年齡信息

select id, sex, age from student 

where age > 20 order by sex, age compute sum(age) by sex;

按照年齡分組彙總

select age from student 

where age > 20 order by age, id compute sum(age);

按照年齡分組，年齡彙總，id找最大值

select id, age from student 

where age > 20 order by age compute sum(age), max(id);

compute進行彙總前面是查詢的結果，後面一條結果集就是彙總的信息。compute子句中能夠添加多個彙總表達式，能夠添加的信息以下：

     a、 可選by關鍵字。它是每一列計算指定的行聚合

     b、 行聚合函數名稱。包括sum、avg、min、max、count等

     c、 要對其執行聚合函數的列

     compute by適合作先分組後彙總的業務。compute by後面的列必定要是order by中出現的列。

 

三、 cube彙總

cube彙總和compute效果相似，但語法較簡潔，並且返回的是一個結果集。

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要結合group by語句完成分組彙總

 

Ø 排序函數

   排序在不少地方須要用到，須要對查詢結果進行排序而且給出序號。好比：

   一、 對某張表進行排序，序號須要遞增不重複的

   二、 對學生的成績進行排序，得出名次，名次能夠並列，但名次的序號是連續遞增的

   三、 在某些排序的狀況下，須要跳空序號，雖然是並列

基本語法

排序函數 over([分組語句] 排序子句[desc][asc])

排序子句 order by 列名, 列名

分組子句 partition by 分組列, 分組列

 

# row_number函數

根據排序子句給出遞增連續序號

按照名稱排序的順序遞增

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函數函數

根據排序子句給出遞增的序號，可是存在並列而且跳空

順序遞增

select id, name, rank() over(order by cid) as rank from student;

跳過相同遞增

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函數

根據排序子句給出遞增的序號，可是存在並列不跳空

不跳過，直接遞增

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分組子句

能夠完成對分組的數據進行增長排序，partition by能夠與以上三個函數聯合使用。

利用partition by按照班級名稱分組，學生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平均排序函數

將要排序的數據進行平分，而後按照等分排序。ntile中的參數表明分紅多少等分。

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 集合運算

操做兩組查詢結果，進行交集、並集、減集運算

一、 union和union all進行並集運算

--union 並集、不重複

select id, name from student where name like 'ja%'

union

select id, name from student where id = 4;

--並集、重複

select * from student where name like 'ja%'

union all

select * from student;

 

二、 intersect進行交集運算

--交集（相同部分）

select * from student where name like 'ja%'

intersect

select * from student;

 

三、 except進行減集運算

--減集（除相同部分）

select * from student where name like 'ja%'

except

select * from student where name like 'jas%';

 

Ø 公式表表達式

查詢表的時候，有時候中間表須要重複使用，這些子查詢被重複查詢調用，不但效率低，並且可讀性低，不利於理解。那麼公式表表達式能夠解決這個問題。

咱們能夠將公式表表達式（CET）視爲臨時結果集，在select、insert、update、delete或是create view語句的執行範圍內進行定義。

--表達式

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 鏈接查詢

一、 簡化鏈接查詢

--簡化聯接查詢

select s.id, s.name, c.id, c.name from student s, classes c where s.cid = c.id;

 

二、 left join左鏈接

--左鏈接

select s.id, s.name, c.id, c.name from student s left join classes c on s.cid = c.id;

 

三、 right join右鏈接

--右鏈接

select s.id, s.name, c.id, c.name from student s right join classes c on s.cid = c.id;

 

四、 inner join內鏈接

--內鏈接

select s.id, s.name, c.id, c.name from student s inner join classes c on s.cid = c.id;

--inner能夠省略

select s.id, s.name, c.id, c.name from student s join classes c on s.cid = c.id;

 

 

一、 聚合函數

max最大值、min最小值、count統計、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;

 

二、 日期時間函數

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

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

select dateAdd(hour, 3, getDate());--加小時

--返回跨兩個指定日期的日期邊界數和時間邊界數

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

--相差秒數

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

--相差小時數

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

select dateName(month, getDate());--當前月份

select dateName(minute, getDate());--當前分鐘

select dateName(weekday, getDate());--當前星期

select datePart(month, getDate());--當前月份

select datePart(weekday, getDate());--當前星期

select datePart(second, getDate());--當前秒數

select day(getDate());--返回當前日期天數

select day('2011-06-30');--返回當前日期天數

select month(getDate());--返回當前日期月份

select month('2011-11-10');

select year(getDate());--返回當前日期年份

select year('2010-11-10');

select getDate();--當前系統日期

select getUTCDate();--utc日期