在sql语句中注意“约束的概念":
1.实体完整性约束(主键--唯一且非空) primary key()违约处理:No action(拒绝执行)2.参照完整性约束(外键约束)foregin key() references tableName(filedName) [on delete|update casecade | no action]违约处理:级联更新或拒绝执行3.用户自定义完整性约束(not null,unique,check短语)违约处理:拒绝执行//添加列语法
//【alter table blog_article add columName type constraint】
//添加约束例子
//【alter table blog_article add CONSTRAINT foreign key(category_Name) references blog_category(category_Name) on delete cascade on update cascade】问题:如何让相册，相片，文章公用一个评论表?create database blog;create table blog_user
(user_Name char(15) not null check(user_Name !=''),user_Password char(15) not null,user_emial varchar(20) not null unique,primary key(user_Name)          )engine=innodb default charset=utf8 auto_increment=1;create table blog_category
(category_Name char(18) not null check(category_Name!=''),category_Date datetime not null,primary key(category_Name)
)engine=innod default charset=utf8 auto_increment=1;create table blog_article
(article_Id int unsigned not null  auto_increment,article_title varchar(20) not null unique,article_content longtext not null,article_date datetime not null,article_readTime int unsigned not null default 0,user_Name char(15) not null,category_Name char(18) not null,primary key(article_Id),foreign key(user_Name) references blog_user(user_Name) on delete cascade on update cascade,foreign key(category_Name) references blog_category(category_Name) on delete cascade on update cascade
)engine=innodb default charset=utf8 auto_increment=1;CREATE TABLE blog_comment (comment_Id int(10) unsigned NOT NULL AUTO_INCREMENT,comment_Content varchar(90) NOT NULL,comment_Date datetime NOT NULL,article_Id int(10) unsigned NOT NULL,user_Name char(15) NOT NULL,PRIMARY KEY (comment_Id),foreign key(article_Id) references blog_article(article_Id) on delete cascade on update cascade,foreign key(user_Name) references blog_user(user_Name) on delete cascade on update cascade
)engine=innodb default charset=utf8 auto_increment=1;create table blog_photoAlbum
(photoAlbum_Name char(20) not null check(photoAlbum_Name!=''),photoAlbum_Date datetime not null,primary key(photoAlbum_Name)
)engine=innodb default charset=utf8;create table blog_photograph
(photograph_Name varchar(20) not null check(photograph_Name!=''),photograph_Date datetime not null,photoAlbum_Name char(20)  not null,photoURL varchar(90) not null,foreign key(photoAlbum_Name) references blog_photoAlbum(photoAlbum_Name) on delete cascade on update cascade
)engine=innodb default charset=utf8;