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This article brings you relevant knowledge about key values in mysql, including key value types, introduction to indexes, index advantages and disadvantages, and issues related to the use of mysql key values ,I hope everyone has to help.
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According to the data storage requirements, select the key value
index
##fulltext Full-text index
—— By creating a unique index, you can ensure the uniqueness of each row of data in the database table
——It can speed up the data query
Disadvantages of indexmysql> create table 库名.表名( 字段列表, index(字段名), index(字段名) );Create an information table and specify the name and age in the fields as indexes (the values in the fields name and age must be sorted)
mysql> create table db2.t2( -> name char(10), -> age int, -> class char(7), -> email char(30), -> index(name),index(age) -> ); mysql> desc db2.t2;
—— desc 库名.表名; //查看key那一列
—— show index from 表名 \G; mysql> show index from db2.t2 \G //因为有两个字段作为索引,所以下面有两列,\G以列的形式展现输出的结果
mysql> drop index age on db2.t2; //删除索引age mysql> desc db2.t2;
##mysql> show index from db2.t2 \G //Only index name remains, index age has been deleted
Store data into table t2 in database db2:
mysql> insert into db2.t2 values("bob",19,"B180601","stu1@163.com"); mysql> insert into db2.t2 values("tom",19,"B180602","stu2@163.com"); mysql> insert into db2.t2 values("lucy",19,"B180603","stu3@163.com"); mysql> insert into db2.t2 values("jack",19,"B180604","stu4@163.com"); mysql> select * from db2.t2; //表中的数据显示是按照插入表记录的顺序排进行排序的
In the existing table Create index
—— create index index name on table name (field name);
mysql> use db2; mysql> show tables;
mysql> desc db2.stuinfo ;
mysql> create index name on db2.stuinfo(name); //索引名一般情况和字段名一样,索引名也可以用别的名字 mysql> desc db2.stuinfo;
mysql> show index from db2.stuinfo \G; //查看索引的详细信息
使用规则
—— 字段值不允许重复,且不允许赋NULL值
—— 一个表中只能有一个primary key字段
—— 多个字段都作为主键,称为复合主键,必须一起创建
—— 主键字段的标志是PRI
—— 主键通常与 auto_increment 连用
—— 通常把表中唯一标志记录的字段设置为主键
[记录编号字段]
建表时创建主键
—— primary key(字段名)
mysql> create table t8( name char(5) primary key, id int );
格式一:
mysql> create table db2.t3(name char(10) primary key,age int); //设置name字段为主键,但设置完主键后,自动将约束条件是否为空设置为NO(即不允许为空) mysql> desc db2.t3;
格式二:
mysql> create table db2.t4(name char(10),age int,primary key(name)); //也可以先将字段创建出来,最后在指定哪个字段为做主键 mysql> desc db2.t4;
向表t3中插入数据
mysql> insert into db2.t3 values("bob",19); mysql> select * from t3;
mysql> insert into db2.t3 values("bob",21); //错误原因,由于字段name是主键,所以字段name中的值不允许重复,表中name字段里面已经有个值为bob了,所以在向字段name赋值时就不能再赋bob值了 ERROR 1062 (23000): Duplicate entry 'bob' for key 'PRIMARY' mysql> insert into db2.t3 values(null,21); //报错原因,由于字段name是主键,所以字段name不允许赋NULL值 ERROR 1048 (23000): Column 'name' cannot be null
在已有表里创建主键
—— 格式
mysql> alter table 库名.表名 add primary key(字段名); mysql> desc db2.t2;
mysql> select * from db2.t2;
//我们要将字段name设置为主键,所以我们要检查原表中name字段的值,确保没有空(NULL)值和重复的值,如上图所示,表t2中name字段没有空值和重复的值,也可以通过下面的方法 mysql> select name from db2.t2 where name is null; //查看name字段中的哪些值为空值,输出结果显示没有 Empty set (0.01 sec) mysql> alter table db2.t2 add primary key(name); //设置字段name为主键 mysql> desc db2.t2;
删除主键
—— alter table 表名 drop primary key;
注:移除主键前,如果有自增属性,必须先去掉
创建复合主键
—— alter table 表名 add primary key(字段名列表);
注:多个字段都作为主键时,称为复合主键,作为主键的这几个字段里的值,在插入记录时不能同时重复。比如字段class有两个值都为classA,字段name的两个值不能都为tom,即字段name中不能有一样的名字,可以为tom和lucy,但不能都为tom
创建复合主键,将class字段和name字段都设置为主键
mysql> create table db2.t5( -> class char(7), -> name char(10), -> money enum("no","yes"), -> primary key(class,name) -> ); mysql> desc db2.t5;
mysql> insert into db2.t5 values("B180601","bob","yes"); mysql> select * from db2.t5;
mysql> insert into db2.t5 values("B180601","bob","yes"); //报错原因,因为字段class和字段name都为主键,所以这两个字段的值不能同时重复 ERROR 1062 (23000): Duplicate entry 'B180601-bob' for key 'PRIMARY' mysql> insert into db2.t5 values("B180602","bob","yes"); //两个主键字段,只要有一个字段的值没有重复,另外的主键字段不管重复没重复都可以插入 mysql> insert into db2.t5 values("B180602","tom","yes"); mysql> select * from db2.t5;
注:此时出现一个问题,若同一个班出现名字相同的两个人,然而class字段和name字段都为主键,由于这两个主键字段的值不能同时重复,此时数据是不能插入的。解决方案就是先删除这两个字段的主键,再删除表数据,然后在创建一个stu_num(学号)字段,设置stu_num、class、name字段一起做主键。
1)删除主键:
mysql> alter table db2.t5 drop primary key(name); //错误原因,由于字段class和字段name一起作为主键,所以删除时不能只删除一个,必须同时删除 ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(name)' at line 1 mysql> desc db2.t5; //发现主键已被删除
2)创建一个新字段stu_num:
mysql> alter table db2.t5 add stu_num char(9) first; //添加一个新字段stu_num,并放在表的第一列
mysql> select * from db2.t5; //由于要设置字段stu_num为主键,所以字段stu_num的值不能为NULL,所以我们要清空表数据
3)清空表数据:
mysql> delete from db2.t5; mysql> select * from db2.t5; //输出结果显示t5表为空表 Empty set (0.00 sec)
4)在已有表里创建复合主键:
mysql> alter table db2.t5 add primary key(stu_num,class,name); mysql> desc db2.t5;
mysql> insert into db2.t5 values("B18060101","B180601","bob","yes"); mysql> insert into db2.t5 values("B18060102","B180601","bob","yes"); mysql> select * from db2.t5; //三个字段都为主键,只要其中一个主键的值不重复,那么数据就能插入成功
//通过加入学号stu_num字段为,让三个字段都为主键,解决同班同名数据插入的问题
主键与auto_increment(自增) 连用
mysql> create table t8( id int primary key auto_increment, name char(5) );
注:要设置自增的前提是,字段必须为主键才可以设置自增;设置自增的字段必须是数值类型的,字符类型是不能自增的。
示例:创建一个自增主键的表
mysql> create table db2.t6( -> stu_num int primary key auto_increment, -> name char(10), -> age int -> ); mysql> desc db2.t6;
mysql> insert into db2.t6(name,age) values("bob",19); mysql> select * from db2.t6;
mysql> insert into db2.t6(name,age) values("tom",20); mysql> insert into db2.t6(name,age) values("lucy",19); mysql> select * from db2.t6;
mysql> insert into db2.t6 values(3,"lucy",19); //错误原因,因为字段stu_num是主键,所以值是不能重复的 ERROR 1062 (23000): Duplicate entry '3' for key 'PRIMARY' mysql> insert into db2.t6 values(9,"lucy",19); //可以是除了1~3以外的别的数字,可以按顺序,也可以不按顺序 mysql> select * from db2.t6;
mysql> insert into db2.t6(name,age) values("jack",25); //如果在插入一条数据,这条数据不给stu_num赋值,那么字段stu_num的默认值,是从最大数开始自增赋值,即字段stu_num默认值为10,而不是4 mysql> select * from db2.t6;
示例:如果清空表记录,在插入数据,那么字段stu_num的值是从1开始自增,还是从10开始自增呢?
mysql> delete from db2.t6; //清空表记录 mysql> select * from db2.t6; //输出结果显示为空表 Empty set (0.00 sec) mysql> insert into db2.t6(name,age) values("bob",18); //插入一个数据 mysql> select * from db2.t6; //虽然删除了表中的全部数据,但是会记录上一次自增到哪个数字了,然后插入的数据会从上一次所记录的数字开始自增
mysql> insert into db2.t6(name,age) values(null,"tom",18); //主键不能为空,但此处我们给主键字段stu_num赋值为NULL,却不报错,原因是空就相当于与没有给字段stu_num赋值,没赋值所以就会以自增的方式进行赋值,所以此处赋值NULL不会报错
mysql> select * from db2.t6;
示例:通常把表中唯一标识记录的字段设置为主键,如记录编号字段
mysql> alter table db2.stuinfo add id int primary key auto_increment first; mysql> desc db2.stuinfo;
mysql> select * from db2.stuinfo;
外键功能
—— 插入记录时,字段值在另一个表字段值范围内选择
使用规则
—— 表存储引擎必须是innodb
—— create table 库名.表名(... ...) engine=innodb; //指定表存储的引擎
—— 插入记录的表的字段与被参照的表的字段类型要一致
—— 被参照字段的值要唯一且不能为空,所以必须要是索引类型的主键(primary key)
创建外键
—— create table 表名( 字段名列表, foreign key(字段名) references 库名.表名(字段名) //指定外键 on update cascde //同步更新 on delete cascade //同步删除 ) engine=innodb; //指定存储引擎 -5
创建一个员工表作为参考表:
mysql> create table db2.yuangong( -> id int primary key auto_increment, -> name char(20), -> sex enum("boy","girl") -> )engine=innodb; mysql> desc db2.yuangong;
mysql> insert into db2.yuangong(name,sex) values("bob","boy"); mysql> insert into db2.yuangong(name,sex) values("lucy","girl"); mysql> select * from db2.yuangong;
创建一个工资表设置外键,以员工表作为参考表:
mysql> create table db2.gongzi( -> id int, -> salary float(7,2), -> foreign key(id) references db2.yuangong(id) -> on update cascade -> on delete cascade -> ) engine=innodb; mysql> desc db2.gongzi;
//如图所示,外键创建成功后会自动创建索引,对数据进行排序,所以标志就是普通索引的标志,我们可以通过另一种方法进行查看 查看是不是外键的方法: mysql> show create table db2.gongzi; //查看创建表的命令
mysql> insert into db2.gongzi values(1,10000); mysql> insert into db2.gongzi values(2,20000); mysql> select * from db2.gongzi;
mysql> insert into db2.gongzi values(3,20000); //给id为3员工发工资报错,原因:给gongzi表里字段id赋值3,但由于gongzi表设置了外键,将gongzi表的id字段参考了yuangong表的id字段,所以给gongzi表id字段赋值时,这个值必须要在yuangong表的id字段里面有,才能进行赋值。
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`db2`.`gongzi`, CONSTRAINT `gongzi_ibfk_1` FOREIGN KEY (`id`) REFERENCES `yuangong` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)
同步更新
将yuangong表中id字段值为2的改为8:
mysql> select * from db2.yuangong;
mysql> update db2.yuangong set id=8 where id=2; //将yuangong表中id字段的2改为8,同步更新表gongzi表id字段为2的记录
mysql> select * from db2.yuangong;
mysql> select * from db2.gongzi; //发现yuangong表的id字段值从2改为8后,gongzi表的id字段的值也从2变为8
同步删除
将yuangong表中id字段值为8的删除掉:
mysql> delete from db2.yuangong where id=8; //将yuangong表中id字段为8删除掉,同步删除表gongzi表id字段为8的记录 mysql> select * from db2.yuangong;
mysql> select * from db2.gongzi; //发现yuangong表的id字段值为8被删除后,gongzi表的id字段值为8的也被删除
注意事项
mysql> insert into db2.yuangong(name,sex) values("jack","boy"); mysql> select * from db2.yuangong;
mysql> insert into db2.gongzi values(3,30000); mysql> select * from db2.gongzi;
mysql> insert into db2.gongzi values(3,30000);) mysql> insert into db2.gongzi values(3,30000); mysql> insert into db2.gongzi values(null,65000); mysql> select * from db2.gongzi;
注:如上所示,字段id为3的被重复赋值,且字段id也被赋空值。因为参考表yuangong表里的字段id里面的值有1和3,所以gongzi表中的字段id就可以重复赋值1或3。这样赋值在生产环境中很不合理,所以我们通过将gongzi表中的id字段设置为主键,这样就能解决重复赋值和赋值空值的问题。
1)删除gongzi表的数据
mysql> delete from db2.gongzi; //删除gongzi表中的所有记录 mysql> select * from db2.gongzi; //查询表记录为空,说明表记录已被删除 Empty set (0.00 sec)
2)将字段id设置为主键
mysql> desc db2.gongzi;
mysql> alter table db2.gongzi add primary key(id); //将gongzi表中的id字段设置为主键 mysql> desc db2.gongzi;
3)进行赋值测试
mysql> insert into db2.gongzi values(1,10000); mysql> insert into db2.gongzi values(3,30000); mysql> insert into db2.gongzi values(1,10000); //再次重复赋值,出现错误,原因是主键不允许有重复的值 ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' mysql> insert into db2.gongzi values(3,30000); //再次重复赋值,出现错误,原因是主键不允许有重复的值 ERROR 1062 (23000): Duplicate entry '3' for key 'PRIMARY' mysql> insert into db2.gongzi values(null,30000); //赋空值,出现错误,原因是主键不允许赋空值 ERROR 1048 (23000): Column 'id' cannot be null mysql> insert into db2.gongzi values(2,30000); //参考表yuangong表中的id字段没有值为2的记录,不在参考表范围内的值不能进行赋值 ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`db2`.`gongzi`, CONSTRAINT `gongzi_ibfk_1` FOREIGN KEY (`id`) REFERENCES `yuangong` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)
删除外键
—— alter table 库名.表名 drop foreign key 外键名;
mysql> show create table db2.gongzi;
//注:红线圈起来的地方就是外键的名称
mysql> alter table db2.gongzi drop foreign key gongzi_ibfk_1; //删除外键
mysql> show create table db2.gongzi; //查看创建表的命令,发现没有创建外键的记录了
此时我们就可以插入除了yuangong表id字段的值范围以外的值了
mysql> insert into db2.gongzi values(2,30000); mysql> insert into db2.gongzi values(9,30000); mysql> select * from db2.gongzi;
由于gongzi表的id字段还是主键,所以不能赋重复的值和空值
mysql> insert into db2.gongzi values(9,30000); ERROR 1062 (23000): Duplicate entry '9' for key 'PRIMARY' mysql> insert into db2.gongzi values(null,30000); ERROR 1048 (23000): Column 'id' cannot be null
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