来自 stackoverflow 的内容居多。
1- MySQL VARCHAR size?
2- 数据库设计范式
3- What is InnoDB
and MyISAM
in MySQL
?
4- 为什么手写DDL?
5- Data access object (DAO) in Java
MySQL VARCHAR size?
Q:
I‘m wondering, if I have a VARCHAR of 200 characters and that I put a string of 100 characters, will it use 200 bytes or it will just use the actual size of the string?
A:
100 characters.
This is the var (variable) in varchar
: you only store what you enter (and an extra 2 bytes to store length upto 65535)
If it was char(200)
then you‘d always store 200 characters, padded with 100 spaces
See the docs: "The CHAR and VARCHAR Types" ------- by gbn
补充:
VARCHAR(M)是一种比CHAR更加灵活的数据类型,同样用于表示字符数据,但是VARCHAR可以保存可变长度的字符串。其中M代表该数据类型所允许保存的字符串的最大长度,只要长度小于该最大值的字符串都可以被保存在该数据类型中。因此,对于那些难以估计确切长度的数据对象来说,使用VARCHAR数据类型更加明智。MySQL4.1以前,VARCHAR数据类型所支持的最大长度255,5.0以上版本支持65535字节长度,utf8编码下最多支持21843个字符(不为空) ------ 来自百度百科
数据库设计范式
如何能设计出优秀的数据库呢?专家们经过多年研究终于····· 一般按照范式来进行数据库设计就得到良好的数据库,范式的目的在于:消除数据冗余和编程便利。
第一范式就是得到纯二维表, // 只有纯二维的表才能装进数据库
第二范式是消除非主键依赖关系, // “有些列并不直接依赖于主键”,就是说和主键没关系的东西应该扔到另外一张表中去。
第三范式是消除函数依赖关系。// 能算出来的就别额外拿一列装了。。。
做范式的主要手段是“拆表”,但是要它有副作用,······,所以要在数据冗余和编码容易之间寻找平衡点,到了第三范式,基本上就是平衡点了。 ------ 《 Java 就该这样学》
What is InnoDB
and MyISAM
in MySQL
?
InnoDB
and MYISAM
, are storage engines for MySQL
.
These two differ on their locking implementation: InnoDB
locks the particular row in the table, and MyISAM
locks the entire MySQL
table.
You can specify the type by giving MYISAM
OR InnoDB
while creating a table in DB. ------ by Siva
为什么手写 DDL (Data Definition Language)?
schema.sql(demo):
-- 数据库初始化脚本 -- 创建数据库 CREATE DATABASE chat; -- 使用数据库 use chat; -- 创建表 CREATE TABLE user( user_id BIGINT NOT NULL AUTO_INCREMENT COMMENT ‘用户id‘, username VARCHAR(40) NOT NULL COMMENT ‘用户名(邮箱)‘, password VARCHAR(40) NOT NULL COMMENT ‘密码‘, nickname VARCHAR(20) NOT NULL COMMENT ‘昵称‘, other VARCHAR(120) COMMENT ‘其他‘, PRIMARY KEY (user_id) )ENGINE=InnoDB AUTO_INCREMENT=1000 DEFAULT CHARSET=utf8 COMMENT=‘用户基本信息‘; CREATE TABLE friend_relation( user_id BIGINT NOT NULL COMMENT ‘用户id‘, friend_id BIGINT NOT NULL COMMENT ‘好友id‘ )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT=‘好友关系表‘; CREATE TABLE group_relation( user_id BIGINT NOT NULL COMMENT ‘用户id‘,, group_id BIGINT NOT NULL COMMENT ‘群组id‘ )ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT=‘群组关系表‘; -- 初始化数据 INSERT INTO ‘user‘(‘username‘, ‘password‘, ‘nickname‘, ‘other‘) VALUES (‘[email protected]‘, 123456, ‘老大‘, ‘这人很懒,啥也没写‘), (‘[email protected]‘, 123456, ‘老二‘, ‘平平淡淡才是真‘); -- INSERT INTO -- ‘friend_relation‘(‘user_id‘, ‘friend_id‘) -- VALUES -- 为什么手写 DDL ( Data Definition Language ) -- 记录每次上线的 DDL 修改 -- 上线 V 1.1 -- xxx -- 上线 V 1.2 -- xxx x x
MySQL 如果表存在就删掉:
DROP TABLE IF EXISTS tbl_name;
Data access object (DAO) in Java
Q:
I was going through a document and I came across a term called DAO
. I found out that it is a Data Access Object. Can someone please explain me what this actually is?
I know that it is some kind of an interface for accessing data from different types of sources, in the middle of this little research of mine I bumped into a concept called data source or data source object, and things got messed up in my mind.
I really want to know what a DAO
is programmatically in terms of where it is used. How it is used? Any links to pages that explain this concept from the very basic stuff is also appreciated. ------ by Vasanth Nag K V
A:
The Data Access Object is basically an object or an interface that provides access to an underlying database or any other persistence storage.
That definition from: http://en.wikipedia.org/wiki/Data_access_object
Check also the sequence diagram here: http://www.oracle.com/technetwork/java/dataaccessobject-138824.html
Maybe a simple example can help you understand the concept:
Let‘s say we have an entity to represent an employee:
public class Employee { private int id; private String name; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
The employee entities will be persisted into a corresponding Employee
table in a database. A simple DAO interface to handle the database operation required to manipulate an employee entity will be like:
interface EmployeeDAO { List<Employee> findAll(); List<Employee> findById(); List<Employee> findByName(); boolean insertEmployee(Employee employee); boolean updateEmployee(Employee employee); boolean deleteEmployee(Employee employee); }
Next we have to provide a concrete implementation for that interface to deal with SQL server, and another to deal with flat files, etc. ------ by Rami