数据库&SQL基础概要

基本SQL语句：

1、说明：创建数据库
CREATE DATABASE database-name

2、说明：删除数据库
drop database dbname

3、说明：备份sql server
— 创建 备份数据的 device
USE master
EXEC sp_addumpdevice ‘disk’, ‘testBack’, ‘c:\mssql7backup\MyNwind_1.dat’
— 开始 备份
BACKUP DATABASE pubs TO testBack

4、说明：创建新表
create table tabname(col1 type1 [not null] [primary key],col2 type2 [not null],..)
根据已有的表创建新表：
A：create table tab_new like tab_old (使用旧表创建新表)
B：create table tab_new as select col1,col2… from tab_old definition only

5、说明：删除新表
drop table tabname

6、说明：增加一个列
Alter table tabname add column col type
注：列增加后将不能删除。DB2中列加上后数据类型也不能改变，唯一能改变的是增加varchar类型的长度。

7、说明：添加主键： Alter table tabname add primary key(col)
说明：删除主键： Alter table tabname drop primary key(col)

8、说明：创建索引：create [unique] index idxname on tabname(col….)
删除索引：drop index idxname
注：索引是不可更改的，想更改必须删除重新建。

9、说明：创建视图：create view viewname as select statement
删除视图：drop view viewname

10、说明：几个简单的基本的sql语句
选择：select * from table1 where 范围
插入：insert into table1(field1,field2) values(value1,value2)
删除：delete from table1 where 范围
更新：update table1 set field1=value1 where 范围
查找：select * from table1 where field1 like ’%value1%’ —like的语法很精妙，查资料!
排序：select * from table1 order by field1,field2 [desc]
总数：select count as totalcount from table1
求和：select sum(field1) as sumvalue from table1
平均：select avg(field1) as avgvalue from table1
最大：select max(field1) as maxvalue from table1
最小：select min(field1) as minvalue from table1

11、说明：几个高级查询运算词
A： UNION 运算符
UNION 运算符通过组合其他两个结果表（例如 TABLE1 和 TABLE2）并消去表中任何重复行而派生出一个结果表。当 ALL 随 UNION 一起使用时（即 UNION ALL），不消除重复行。两种情况下，派生表的每一行不是来自 TABLE1 就是来自 TABLE2。
B： EXCEPT 运算符
EXCEPT运算符通过包括所有在 TABLE1 中但不在 TABLE2 中的行并消除所有重复行而派生出一个结果表。当 ALL 随 EXCEPT 一起使用时 (EXCEPT ALL)，不消除重复行。
C： INTERSECT 运算符
INTERSECT运算符通过只包括 TABLE1 和 TABLE2 中都有的行并消除所有重复行而派生出一个结果表。当 ALL随 INTERSECT 一起使用时 (INTERSECT ALL)，不消除重复行。
注：使用运算词的几个查询结果行必须是一致的。

12、说明：使用外连接
A、left （outer） join：
左外连接（左连接）：结果集几包括连接表的匹配行，也包括左连接表的所有行。
SQL: select a.a, a.b, a.c, b.c, b.d, b.f from a LEFT OUT JOIN b ON a.a = b.c
B：right （outer） join:
右外连接(右连接)：结果集既包括连接表的匹配连接行，也包括右连接表的所有行。
C：full/cross （outer） join：
全外连接：不仅包括符号连接表的匹配行，还包括两个连接表中的所有记录。
12、分组:Group by:
一张表，一旦分组 完成后，查询后只能得到组相关的信息。
组相关的信息：（统计信息） count,sum,max,min,avg 分组的标准)
在SQLServer中分组时：不能以text,ntext,image类型的字段作为分组依据
在selecte统计函数中的字段，不能和普通的字段放在一起；

13、对数据库进行操作：
分离数据库： sp_detach_db;附加数据库：sp_attach_db 后接表明，附加需要完整的路径名

14.如何修改数据库的名称:
sp_renamedb ‘old_name’, ‘new_name’

数据库设计：

• Normalize：

Database normalization (or normalisation) is the process of organizing the columns (attributes) and tables (relations) of a relational database to minimize data redundancy.

The objective is to isolate data so that additions, deletions, and modifications of an attribute can be made in just one table and then propagated through the rest of the database using the defined foreign keys.

• First normal form (1NF)
  A relation is in first normal form if the domain of each attribute contains only atomic values, and the value of each attribute contains only a single value from that domain.

• Second normal form (2NF)
  A table that is in first normal form (1NF) must meet additional criteria if it is to qualify for second normal form. Specifically: a table is in 2NF if it is in 1NF and no non-prime attribute is dependent on any proper subset of any candidate key of the table. A non-prime attribute of a table is an attribute that is not a part of any candidate key of the table.
  Put simply, a table is in 2NF if it is in 1NF and every non-prime attribute of the table is dependent on the whole of every candidate key.

• Third normal form is a normal form used in normalizing a database design to reduce the duplication of data and ensure referential integrity by ensuring that

  1. the entity is in second normal form and
  2. all the attributes in a table are determined only by the candidate keys of that table and not by any non-prime attributes.

• Index：

A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random lookups and efficient access of ordered records.

索引的作用：

1.索引可以加快数据库的检索速度
2.索引降低了数据库插入、修改、删除等维护任务的速度
3.索引创建在表上，不能创建在视图上
4.索引既可以直接创建，也可以间接创建
5.可以在优化隐藏中，使用索引
6.使用查询处理器执行SQL语句，在一个表上，一次只能使用一个索引

• Transaction:

A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the atomicity, consistency, isolation, and durability (ACID) properties, to qualify as a transaction.

Atomicity： A transaction must be an atomic unit of work; either all of its data modifications are performed, or none of them is performed.
Consistency： When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction’s modifications to maintain all data integrity. All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.
Isolation： Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. A transaction either recognizes data in the state it was in before another concurrent transaction modified it, or it recognizes the data after the second transaction has completed, but it does not recognize an intermediate state. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.
Durability： After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.