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;
+----------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------+------+-----+---------+-------+
| name | char(10) | YES | | NULL | |
| birthday | date | YES | | NULL | |
| start | year(4) | YES | | NULL | |
| uptime | time | YES | | NULL | |
| party | datetime | YES | | NULL | |
+----------+----------+------+-----+---------+-------+
mysql> insert into t5(name,start) values("tom",50);
mysql> select * from t5;
+------+------------+-------+----------+---------------------+
| name | birthday | start | uptime | party |
+------+------------+-------+----------+---------------------+
| bob | 1993-07-02 | 1993 | 08:30:00 | 2018-10-15 20:30:00 |
| tom | NULL | 2050 | NULL | NULL |
+------+------------+-------+----------+---------------------+
//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;
+-------+-------------------------------------------------------+-------+-----+------------+--------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------------------------------------------------+------+------+------------+-------+
| name | char(10) | NO | | NULL | |
| age | tinyint(2) | YES | | 19 | |
| likes | set('eat','sleep','film','game') | NO | | game | |
+-------+-------------------------------------------------------+------+-----+-----------+---------+
//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;
+--------+------+------+------+---------+---------+
| stu_id | name | age | mail | homedir | nanjing |
+--------+------+------+------+---------+---------+
| 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;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name | char(10) | YES | MUL | NULL | |
| class | char(7) | YES | MUL | NULL | |
| 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;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name | char(10) | YES | | NULL | |
| class | char(7) | YES | MUL | NULL | |
| 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；
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name | char(10) | YES | MUL | NULL | |
| class | char(7) | YES | MUL | NULL | |
| 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);
+---------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+----------+------+-----+---------+-------+
| name | char(10) | YES | | NULL | |
| stu_num | char(9) | NO | PRI | NULL | |
+---------+----------+------+-----+---------+-------+
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;
+--------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+----------------+
| stu_id | int(11) | NO | PRI | NULL | auto_increment |
| name | char(5) | YES | | NULL | |
| 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;
+--------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| stu_id | int(2) | NO | PRI | NULL | | //原来有auto_increment
| name | char(5) | YES | | NULL | |
| age | tinyint(2) unsigned | YES | | NULL | |
+--------+---------------------+------+-----+---------+-------+
mysql> alter table t25 drop primary key; //珊主键
mysql> desc t25;
+--------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| stu_id | int(2) | NO | | NULL | |
| name | char(5) | YES | | 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;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| 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）