Ø 基本常用查詢
--select
select * from student;
--all 查詢所有
select all sex from student;
--distinct 過濾重復
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 並且
select * from student where id > 2 and sex = 1;
--or 或者
select * from student where id = 2 or sex = 1;
--between ... and ... 相當於並且
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 不為空
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，並且是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 所有分組
按照年齡分組，是所有的年齡
select count(*), age from student group by all age;
--having 分組過濾條件
按照年齡分組，過濾年齡為空的數據，並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語句或其他子查詢中的查詢。任何允許使用表達式的地方都可以使用子查詢。子查詢也稱為內部查詢或內部選擇，而包含子查詢的語句也成為外部查詢或外部選擇。
# from (select … table)示例
將一個table的查詢結果當做一個新表進行查詢
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，其中外部查詢可以包含以下語句：
1、 包含常規選擇列表組件的常規select查詢
2、 包含一個或多個表或視圖名稱的常規from語句
3、 可選的where子句
4、 可選的group by子句
5、 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，這一列的結果將會作為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為的學生信息
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查詢需要內部查詢和外部查詢進行一個關聯的條件，如果沒有這個條件將是查詢到的所有信息。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
1、 distinct去掉重復數據
select distinct sex from student;
select count(sex), count(distinct sex) from student;
2、 compute和compute by匯總查詢
對年齡大於的進行匯總
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匯總年齡信息
select id, sex, age from student
where age > 20 order by sex, age compute sum(age) by sex;
按照年齡分組匯總
select age from student
where age > 20 order by age, id compute sum(age);
按照年齡分組，年齡匯總，id找最大值
select id, age from student
where age > 20 order by age compute sum(age), max(id);
compute進行匯總前面是查詢的結果，後面一條結果集就是匯總的信息。compute子句中可以添加多個匯總表達式，可以添加的信息如下：
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適合做先分組後匯總的業務。compute by後面的列一定要是order by中出現的列。
3、 cube匯總
cube匯總和compute效果類似，但語法較簡潔，而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語句完成分組匯總
Ø 排序函數
排序在很多地方需要用到，需要對查詢結果進行排序並且給出序號。比如：
1、 對某張表進行排序，序號需要遞增不重復的
2、 對學生的成績進行排序，得出名次，名次可以並列，但名次的序號是連續遞增的
3、 在某些排序的情況下，需要跳空序號，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，但是存在並列並且跳空
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，但是存在並列不跳空
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
可以完成對分組的數據進行增加排序，partition by可以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，然後按照等分排序。ntile中的參數代表分成多少等分。
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 集合運算
操作兩組查詢結果，進行交集、並集、減集運算
1、 union和union all進行並集運算
--union 並集、不重復
select id, name from student where name like 'ja%'
union
select id, name from student where id = 4;
--並集、重復
select * from student where name like 'ja%'
union all
select * from student;
2、 intersect進行交集運算
--交集（相同部分）
select * from student where name like 'ja%'
intersect
select * from student;
3、 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，有時候中間表需要重復使用，這些子查詢被重復查詢調用，不但效率低，而且可讀性低，不利於理解。那麼公式表表達式可以解決這個問題。
我們可以將公式表表達式（CET）視為臨時結果集，在select、insert、update、delete或是create view語句的執行范圍內進行定義。
--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 連接查詢
1、 簡化連接查詢
--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;
2、 left join左連接
--左連接
select s.id, s.name, c.id, c.name from student s left join classes c on s.cid = c.id;
3、 right join右連接
--右連接
select s.id, s.name, c.id, c.name from student s right join classes c on s.cid = c.id;
4、 inner join內連接
--內連接
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;
5、 cross join交叉連接
--交叉聯接查詢，結果是一個笛卡兒乘積
select s.id, s.name, c.id, c.name from student s cross join classes c
--where s.cid = c.id;
6、 自連接（同一張表進行連接查詢）
--自連接
select distinct s.* from student s, student s1 where s.id <> s1.id and s.sex = s1.sex;
Ø 函數
1、 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,
