mysql数据库基础(2）索引、主键、复合主键、外键

时间：2018-10-18 01:17:07 阅读：215 评论：0 收藏：0 [点我收藏+]

标签：rom day 条件 name 最大的 odi ror 删除表 sleep

增、删、改、查

加“”表示为普通字符

mysql> insert into t9 values("NULL",21,"game,flim");

+++++++++++++++++++++++++++++++

添加新字段

mysql> alter table t4 add age int(2) default 19; //默认在最后面

mysql> alter table t4 add mail char(50) after name; //加在name后面

mysql> alter table t4 add stu_num char(10) first; //加在最前面

修改字段类型，类型要与字段存储的数据匹配,不能有冲突.

mysql> alter table t4 modify name varchar(3);

ERROR 1406 (22001): Data too long for column ‘name‘ at row 1

mysql> alter table t4 modify name varchar(15);

Query OK, 1 row affected (0.44 sec)

修改字段位置

mysql> alter table t4 modify age int(2) default 19

after name;

删除字段

mysql> alter table t5 drop start;

mysql> select * from t5;

mysql> desc t5;

修改字段名

alter table t4 change stu_num stu_id char(10);

修改表名

alter table t9 rename stuinfo;

应用

mysql> create table t7(meeting datetime,party timestamp);

不自动赋值会自动赋值当前时间

mysql> insert into t7 values(now(),now());

Query OK, 1 row affected (0.07 sec)

mysql> select * from t7;

+---------------------+---------------------+

| meeting | party |

+---------------------+---------------------+

| 2018-10-16 09:31:50 | 2018-10-16 09:31:50 |

+---------------------+---------------------+

mysql> insert into t7(meeting) values(now());

mysql> select * from t7;

+---------------------+---------------------+

| meeting | party |

+---------------------+---------------------+

| 2018-10-16 09:31:50 | 2018-10-16 09:31:50 |

| 2018-10-16 09:33:12 | 2018-10-16 09:33:12 |

+---------------------+---------------------+

mysql> select * from t7;

+---------------------+---------------------+

| meeting | party |

+---------------------+---------------------+

| 2018-10-16 09:31:50 | 2018-10-16 09:31:50 |

| 2018-10-16 09:33:12 | 2018-10-16 09:33:12 |

| 2018-10-17 08:25:00 | 2018-10-16 09:34:01 |

| NULL | 2018-10-17 08:25:00 |

+---------------------+---------------------+

mysql> desc teadb.t5;

+----------+----------+------+-----+---------+-------+

+----------+----------+------+-----+---------+-------+

+----------+----------+------+-----+---------+-------+

mysql> insert into t5(name,start) values("tom",50);

mysql> select * from t5;

+------+------------+-------+----------+---------------------+

+------+------------+-------+----------+---------------------+

| bob | 1993-07-02 | 1993 | 08:30:00 | 2018-10-15 20:30:00 |

+------+------------+-------+----------+---------------------+

//year默认为4位数，1-69--->20 70-99--->19

mysql> create table t88(name char(3),level int(3) zerofill); //显示宽度，0补足位

Query OK, 0 rows affected (0.25 sec)

mysql> insert into t88 values("tom",1131);

mysql> insert into t88 values("tom",113);

mysql> insert into t88 values("tom",11);

mysql> insert into t88 values("tom",3);

mysql> select * from t88;

+------+-------+

| name | level |

+------+-------+

| tom | 1131 |

| tom | 113 |

| tom | 011 |

| tom | 003 |

+------+-------+

mysql> create table t9(name char(10) not null,age tinyint(2) default 19,likes set("eat","sleep","film","game") not null default "game,eat");

//no null:不能为空值 ,默认可以为空 //detault:默认值

mysql> insert into t9(name) values("bob");

Query OK, 1 row affected (0.02 sec)

mysql> select * from t9;

+---------+---------+----------------+

| name | age | likes |

+---------+---------+----------------+

| bob | 19 | game,eat |

+---------+---------+----------------+

mysql> desc t9;

+-------+-------------------------------------------------------+-------+-----+------------+--------+

+-------+-------------------------------------------------------+------+------+------------+-------+

| age | tinyint(2) | YES | | 19 | |

+-------+-------------------------------------------------------+------+-----+-----------+---------+

//不输入会默认补内容

添加字段默认在最后

mysql> alter table t4 add nanjing int(2) default 22;

mysql> select * from t4;

+--------+------+------+------+---------+---------+

+--------+------+------+------+---------+---------+

| NULL | jack | 19 | NULL | USA | 22 |

| NULL | lll | 19 | NULL | han | 22 |

+--------+------+------+------+---------+---------+

索引

索引是指给字段排队，类似书的目录

优点：可以加快查询数据速度

缺点：减慢写入的速度，增加减少都需要重新排列。排队信息需要文件来存储，占内存

平时我们看不到排序，查询时直接会提示数据在几行。

创建表的时候添加索引： //可同时添加多个索引

mysql> create table t12( name char(10),class char(7),age int(2) , index(name),index(class));

mysql> desc t12;

+-------+----------+------+-----+---------+-------+

+-------+----------+------+-----+---------+-------+

| age | int(2) | YES | | NULL | |

+-------+----------+------+-----+---------+-------+

mysql> show index from t12\G; //查看索引

mysql> drop index name on t12; //删除索引

mysql> desc t12;

+-------+----------+------+-----+---------+-------+

+-------+----------+------+-----+---------+-------+

| age | int(2) | YES | | NULL | |

+-------+----------+------+-----+---------+-------+

mysql> create index name on t12(name); //在已有的表里添加索引，索引名与表名一致

mysql> desc t12；

+-------+----------+------+-----+---------+-------+

+-------+----------+------+-----+---------+-------+

| age | int(2) | YES | | NULL | |

+-------+----------+------+-----+---------+-------+

创建主键的格式1

mysql> create table t22(name char(10),stu_num char(9),primary key(stu_num));

创建主键的格式2（同上）

mysql> create table t23(name char(10),stu_num char(9)primary key);

+---------+----------+------+-----+---------+-------+

+---------+----------+------+-----+---------+-------+

+---------+----------+------+-----+---------+-------+

mysql> select * from t22;

+------+-----------+

| name | stu_num |

+------+-----------+

| tom | nsd180705 | //设值主键字段（stu_num）值不于允许重复，不能为空

+------+-----------+

复合主键（表中多个字段一起做主键，表中多行设符合主键不能重复不能为空）

mysql> create table t24(cip char(15),sport smallint(2),status enum("deny","all"),primary key(cip,sport));

mysql> select * from t24;

+-------------+-------+--------+

| cip | sport | status |

+-------------+-------+--------+

| 192.168.4.1 | 21 | deny |

| 192.168.4.1 | 22 | deny |

| 192.168.4.2 | 22 | deny |

| 192.168.4.3 | 22 | all |

+-------------+-------+--------+

删除主键，不删除内容，还是不能为空，表将不受约束

mysql> alter table t24 drop primary key; //删除

在已有表里创建复合主建（如果表中有重复的数据不能创建，需要先改表）

mysql> alter table t24 add primary key(cip,sport); //创建

主键+自增长（自己存亦遵循约束规则//空值，重复）//自增长从最大的开始

//删除也从历史最大值开始记录

mysql> create table t25(stu_id int primary key auto_increment,name char(5),age tinyint(2) unsigned); 自加1

mysql> desc t25;

+--------+---------------------+------+-----+---------+----------------+

+--------+---------------------+------+-----+---------+----------------+

| stu_id | int(11) | NO | PRI | NULL | auto_increment |

| age | tinyint(2) unsigned | YES | | NULL | |

+--------+---------------------+------+-----+---------+----------------+

mysql> insert into t25(name,age)values("bob",19);

mysql> insert into t25(name,age)values("tom",18);

mysql> select * from t25;

+--------+------+------+

| stu_id | name | age |

+--------+------+------+

| 1 | bob | 19 |

| 2 | tom | 18 | //自加一

+--------+------+------+

mysql> alter table t88 add id int(2) zerofill primary key auto_increment first;

mysql> select * from t88;

+----+------+-------+

| id | name | level |

+----+------+-------+

| 01 | tom | 1131 |

| 02 | tom | 113 |

| 03 | tom | 011 |

| 04 | tom | 003 |

+----+------+-------+

先珊自增字段才能珊主键

mysql> alter table t25 modify stu_id int(2) not null; //先删除自增长段

mysql> desc t25;

+--------+---------------------+------+-----+---------+-------+

+--------+---------------------+------+-----+---------+-------+

| stu_id | int(2) | NO | PRI | NULL | | //原来有auto_increment

| age | tinyint(2) unsigned | YES | | NULL | |

+--------+---------------------+------+-----+---------+-------+

mysql> alter table t25 drop primary key; //珊主键

mysql> desc t25;

+--------+---------------------+------+-----+---------+-------+

+--------+---------------------+------+-----+---------+-------+

| stu_id | int(2) | NO | | NULL | |

| age | tinyint(2) unsigned | YES | | NULL | |

+--------+---------------------+------+-----+---------+-------+

外键

定义：让当前表字段的值在另一个表中字段值的范围内选择，保证数据的一致性。

使用外键的条件:

表的存储引擎必须是innodb

字段类型要一致

被参照字段必须要是索引类型的一种（primary key）

mysql> create table ygtab(yg_id int(2)primary key auto_increment,naem char(10)) engine=innodb;

mysql> insert into ygtab(naem)values("bob");

mysql> insert into ygtab(naem)values("tom");

mysql> create table gztab(gz_id int(2),pay float(7,2),foreign key(gz_id) references ygtab(yg_id)on update cascade on delete cascade)engine=innodb;

//regerences:引用 foreign key:外键

mysql> desc gztab;

+-------+------------+------+-----+---------+-------+

+-------+------------+------+-----+---------+-------+

| gz_id | int(2) | YES | MUL | NULL | |

| pay | float(7,2) | YES | | NULL | |

+-------+------------+------+-----+---------+-------+

准备案例：

mysql> insert into gztab values(1,28000);

mysql> insert into gztab values(2,38000);

mysql> select * from ygtab;

+-------+------+

| yg_id | naem |

+-------+------+

| 1 | bob |

| 2 | tom |

+-------+------+

mysql> select * from gztab;

+-------+----------+

| gz_id | pay |

+-------+----------+

| 1 | 28000.00 |

| 2 | 38000.00 |

+-------+----------+

//改参考的员工表字段，自己表应该也同部更新 //单向同步

mysql> update ygtab set yg_id=8 where yg_id=2;

mysql> select * from gztab;

+-------+----------+

| gz_id | pay |

+-------+----------+

| 1 | 28000.00 |

| 8 | 38000.00 |

+-------+----------+

2 rows in set (0.00 sec)

mysql> select * from ygtab;

+-------+------+

| yg_id | naem |

+-------+------+

| 1 | bob |

| 8 | tom |

+-------+------+

mysql> delete from ygtab where yg_id=8; //删除员工表信息

mysql> select * from gztab; //工资表信息自动删除

+-------+----------+

| gz_id | pay |

+-------+----------+

| 1 | 28000.00 |

+-------+----------+

1 row in set (0.00 sec)

//有利有弊

//有bug，gz_id 需要做主键不然可以连发工资，因为参考表有记录,被参考的表无法删除，需要先删除外键，再删除表（主键还在）

mysql数据库基础(2）索引、主键、复合主键、外键

标签：rom day 条件 name 最大的 odi ror 删除表 sleep

原文地址：https://www.cnblogs.com/hanlongyu/p/9807423.html

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