SQL 基本关键字 函数 关联 连接

http://www.w3cschool.cn/sql_having.html  w3c中有些SQL的讲解

1 order by 排序

SELECT Company, OrderNumber FROM Orders ORDER BY Company;

2 distinct 去重

SELECT DISTINCT Company FROM Orders ;

3 limit 控制选择的条数(MYSQL)

SELECT *FROM Persons LIMIT 5;

ORACLE

SELECT * FROM Persons WHERE ROWNUM <= 5;

4 通配符

SELECT * FROM Persons WHERE LastName LIKE 'C_r_er' -- _代表一个字符

SELECT * FROM Persons WHERE City LIKE '[ALN]%'; -- 以A或L或N 开头的 %代表一个或多个字符

SELECT * FROM Persons WHERE City LIKE '[!ALN]%' -- 不以A或L或N 开头的

5 between

SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Adams' AND 'Carter';

6 union 和union all

Employees_China:

E_ID	E_Name
01	Zhang, Hua
02	Wang, Wei
03	Carter, Thomas
04	Yang, Ming

Employees_USA:

E_ID	E_Name
01	Adams, John
02	Bush, George
03	Carter, Thomas
04	Gates, Bill

SELECT E_Name FROM Employees_China

UNION

SELECT E_Name FROM Employees_USA;

结果

E_Name

Zhang, Hua

Wang, Wei

Carter, Thomas

Yang, Ming

Adams, John

Bush, George

Gates, Bill

两个表的叠加  union比union all 多了个排重操作

7 内连接 外连接

A表                B表
id      name            id      name 
1          a                1       b 
2          b                3       c
4          c
内连接就是左表和右表相同的数据,查询结果只有相等的数据：
select * from A inner join B on A.id=B.id

select * from A,B where A.id=B.id

id      name            id      name 
1          a                1       b 
外连接分为：左外连接、右外连接、全外连接
左外连接就是以左表为准，去匹配右表，左表有多少条数据，结果就是多少条数据
select * from A left join B on A.id=B.id
id      name            id      name 
1          a                1       b 
2          b                null      null
4          c                null      null
右外连接就是与左外连接反之，以右表为准，去匹配左表，右表有多少条数据，结果就是多少条数据
select * from A right join B on A.id=B.id
id      name            id      name 
1          a                1       b 
null        null               3       c
全外连接数据条数不一定，相当与是左外连接 和右外连接 的综合
select * from A full join B on A.id=B.id
id      name            id      name 
1          a                1       b 
2          b                null      null
null       null               3       c
4          c                 null       null

8 unique 唯一约束 约束还有 主键 外键 默认值 check（id>0等）不为空

CREATE TABLE Persons

(

Id_P int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

UNIQUE (Id_P)

) -- 建表时约束 MYSQL

CREATE TABLE Persons

(

Id_P int NOT NULL UNIQUE,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255)

) -- ORACLE

9 index 索引

CREATE INDEX PersonIndex ON Person (LastName DESC);

数据库索引的作用和优点缺点

10 alter 只能对表中整进行添加 修改 删除

ALTER TABLE Persons ADD Birthday date;
ALTER TABLE Persons ALTER COLUMN Birthday year;
ALTER TABLE Person DROP COLUMN Birthday;

11 MYSQL 自增 ORACLE 序列

CREATE TABLE Persons

(

P_Id int NOT NULL AUTO_INCREMENT,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

PRIMARY KEY (P_Id)

);

CREATE SEQUENCE seq_person

MINVALUE 1

START WITH 1

INCREMENT BY 1

CACHE 10;

12 视图 view 视图可以隐藏一些不想让用户看到的信息 以一种新的方式展示数据

create view v as select ename ||' ' ||deptno as data from emp;--创建视图
select * from v;

13 函数

SELECT Customer FROM Orders WHERE OrderPrice>(SELECT AVG(OrderPrice) FROM Orders);

SELECT COUNT(DISTINCT Customer) AS NumberOfCustomers FROM Orders;

SELECT FIRST(OrderPrice) AS FirstOrderPrice FROM Orders order by OrderPrice; -- 还有last

SELECT MAX(OrderPrice) AS LargestOrderPrice FROM Orders; -- min()

SELECT SUM(OrderPrice) AS OrderTotal FROM Orders;

SELECT MID(City,1,3) as SmallCity FROM Persons; -- 截取City前3个字符

14 group by 通常和sum() 合用 按group by 的字段分组 如果一个客户有两条记录 将两条记录的数值求和

SELECT Customer,SUM(OrderPrice) FROM Orders GROUP BY Customer;
SELECT Customer,OrderDate,SUM(OrderPrice) FROM Orders GROUP BY Customer,OrderDate;

15 having 和where 唯一区别就是having子句中可以有函数

SELECT Customer,SUM(OrderPrice) FROM Orders GROUP BY Customer HAVING SUM(OrderPrice)<2000;