mysql常用语句 4 + mysql的约束(非空，唯一，主键，外键)

1.更新语句
update dept1 set loc = \'wz\',dname = \'xueshenghui\' where deptno = 10;

mysql> update dept1 set loc = \'wz\',dname = \'xueshenghui\' where deptno = 10;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from dept1;
+--------+-------------+---------+
| DEPTNO | DNAME | LOC |
+--------+-------------+---------+
| 10 | xueshenghui | wz |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+-------------+---------+
4 rows in set (0.00 sec)

更新语句格式：update 表名 set 字段=\'\',字段=\'\' where 字段=\'\';

2.更新所有记录，不加条件即可

update dept1 set dname = \'bangongshi\', loc = \'cqust\' ;
mysql> update dept1 set dname = \'bangongshi\', loc = \'cqust\' ;
Query OK, 4 rows affected (0.01 sec)
Rows matched: 4 Changed: 4 Warnings: 0

mysql> select * from dept1;
+--------+------------+-------+
| DEPTNO | DNAME | LOC |
+--------+------------+-------+
| 10 | bangongshi | cqust |
| 20 | bangongshi | cqust |
| 30 | bangongshi | cqust |
| 40 | bangongshi | cqust |
+--------+------------+-------+
4 rows in set (0.00 sec)

3.删除数据

delete from dept1 where deptno = 10;
mysql> delete from dept1 where deptno = 10;//这里如果不加条件会删除表中的所有数据。
和drop分开区别。drop是删除表，delete是删除数据。
Query OK, 1 row affected (0.01 sec)

mysql> select * from dept1;
+--------+------------+-------+
| DEPTNO | DNAME | LOC |
+--------+------------+-------+
| 20 | bangongshi | cqust |
| 30 | bangongshi | cqust |
| 40 | bangongshi | cqust |
+--------+------------+-------+
3 rows in set (0.00 sec)

5.约束：对字段进行合理的限制。
not null非空，unique 唯一，primary key 主键,foreign key外键。

6.非空约束
drop table if exists t_user;
create table t_user(

id int(3) not null default 100,
name varchar(7) default \'hch\'

);
insert into t_user (id,name) values(1,\'h1\');
insert into t_user (id,name) values(null,\'h2\');

//这里不能为空
mysql> insert into t_user (id,name) values(null,\'h2\');
ERROR 1048 (23000): Column \'id\' cannot be null

7.唯一约束，存在列级约束，表级约束
drop table if exists t_user;
create table t_user(

id int(3) ,
name varchar(7),
unique(id,name)

);
insert into t_user (id,name) values(1,\'h1\');
insert into t_user (id,name) values(1,\'h2\');

//这里添加了唯一约束，
mysql> insert into t_user (id,name) values(1,\'h2\');
ERROR 1062 (23000): Duplicate entry \'1\' for key \'id\'
mysql>

mysql> create table t_user(
->
-> id int(3) unique ,
-> name varchar(7) unique//这里两个字段都添加约束，要求都不一样才可以成功。
-> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t_user (id,name) values(1,\'h1\');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t_user (id,name) values(2,\'h2\');
Query OK, 1 row affected (0.00 sec)

mysql> create table t_user(
->
-> id int(3) ,
-> name varchar(7),
-> unique(id,name)//表级约束，插入数据要求两个联合起来唯一
-> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t_user (id,name) values(1,\'h1\');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t_user (id,name) values(1,\'h2\');
Query OK, 1 row affected (0.00 sec)

8.主键(primary key)
按照数量分：单一，复合主键(不推荐)
按照性质：自然，业务主键(不推荐)
在实际开发中不要把业务中的某一个属性作为主键。
主键约束只能有一个。
作用：是一条数据的唯一标识
主键值
主键约束
主键字段

drop table if exists t_user;
create table t_user(

id int(3) primary key auto_increment ,
name varchar(7)

);
insert into t_user (name) values(\'h1\');
insert into t_user (name) values(\'h2\');
insert into t_user (name) values(\'h3\');
insert into t_user (name) values(\'h4\');
insert into t_user (name) values(\'h5\');
insert into t_user (name) values(\'h6\');

mysql> create table t_user(
->
-> id int(3) primary key ,
-> name varchar(7)
->
-> );
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t_user (id,name) values(1,\'h1\');
Query OK, 1 row affected (0.00 sec)
//主键不能重复，而且不能为空
mysql> insert into t_user (id,name) values(1,\'h2\');
ERROR 1062 (23000): Duplicate entry \'1\' for key \'PRIMARY\'

mysql> create table t_user(
->
-> id int(3) primary key auto_increment ,//主键自增，这里主键只是一个标识作用
-> name varchar(7)
->
-> );
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t_user (name) values(\'h1\');
Query OK, 1 row affected (0.01 sec)

mysql> insert into t_user (name) values(\'h2\');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t_user (name) values(\'h3\');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t_user (name) values(\'h4\');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t_user (name) values(\'h5\');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t_user (name) values(\'h6\');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t_user;
+----+------+
| id | name |
+----+------+
| 1 | h1 |
| 2 | h2 |
| 3 | h3 |
| 4 | h4 |
| 5 | h5 |
| 6 | h6 |
+----+------+
6 rows in set (0.00 sec)

9.外键(foreign key)
外键约束表现出来两张表存在父子关系。
在建表的时候
例如：t_teacher(父表)
t_id t_name
1 laoyu
2 laodu
3 niexin
4 ruanqidong

t_student(子表)
stu_id stu_name t_name(添加外键约束)
1 h1 laoyu//这里的数据必须是父表中的数据。以此获得联系
2 h2 laoyu
3 h3 laoyu
4 h4 laodu

在创建表的时候，先创建父表，再创建子表
添加数据，先添加父表，再添加子表
删除数据，先删除子表中的数据，再删除父表中的数据
删表，先删除子表，再删除父表
drop table if exists t_teacher2;
create table t_teacher2(
t_id int(2) primary key,
t_name1 varchar(10)
);
drop table if exists t_student2;
create table t_student2(
stu_id int(2) ,
stu_name varchar(10),
t_name2 int (10),
foreign key (t_name2) references t_teacher2(t_id)//这里的字段一般是主键，至少具有唯一性。
外键值可以为null

);

来源：https://www.cnblogs.com/journeyhch/p/15563870.html
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