不同的用户体验数据类型: 规定了数据库可以存放哪些数据约束: 限制存储数据的规则键

Microsoft Windows [版本 10.0.17134.472]
(c) 2018 Microsoft Corporation。保留所有权利。

C:\Users\Administrator>SQL;
'SQL' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

C:\Users\Administrator>SQL
Enter password: ****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 11
Server version: 5.6.42 MySQL Community Server (GPL)

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> drop database db1;
Query OK, 6 rows affected (0.41 sec)

mysql> drop database db2;
Query OK, 1 row affected (0.19 sec)

mysql> drop database db3;
ERROR 1008 (HY000): Can't drop database 'db3'; database doesn't exist
mysql> create database db1;
Query OK, 1 row affected (0.00 sec)

mysql> use db1;
Database changed
mysql> create table t1(age int)engine=memory;
Query OK, 0 rows affected (0.08 sec)

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1 |
+---------------+
1 row in set (0.04 sec)

mysql> quit;
Bye

C:\Users\Administrator>sql
Enter password: ****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.6.42 MySQL Community Server (GPL)

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> show tables;
ERROR 1046 (3D000): No database selected
mysql> use db1;
Database changed
mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1 |
+---------------+
1 row in set (0.00 sec)

mysql> show tables;
ERROR 2013 (HY000): Lost connection to MySQL server during query
mysql> create table t1(name char)engine=innodb;
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 1
Current database: db1

mysql> quit;
Bye

C:\Users\Administrator>sql
Enter password: ****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.6.42 MySQL Community Server (GPL)

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use db1;
Database changed
mysql> show create t1;
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 't1' at line 1
mysql> show create table t1;
+-------+---------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+---------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`age` int(11) DEFAULT NULL
) ENGINE=MEMORY DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------+
1 row in set (0.03 sec)

mysql> create table t2(name char(5) not null)engine=innodb charset=utf8;
Query OK, 0 rows affected (0.29 sec)

mysql> create table t3(name char(5) )engine=innodb charset=utf8;
Query OK, 0 rows affected (0.38 sec)

mysql> insert into from t1 values('zzfsn');
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 'from t1 values('zzfsn')' at line 1
mysql> insert into t1 values('zzfsn');
Query OK, 1 row affected, 1 warning (0.05 sec)

mysql> insert into t1 values('');
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> insert into t1 values(null);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t2 values('');
Query OK, 1 row affected (0.08 sec)

mysql> insert into t2 values();
Query OK, 1 row affected, 1 warning (0.07 sec)

mysql> show create table t1;
+-------+---------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+---------------------------------------------------------------------------------------+
| t1 | CREATE TABLE `t1` (
`age` int(11) DEFAULT NULL
) ENGINE=MEMORY DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t2;
+-------+------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------+
| t2 | CREATE TABLE `t2` (
`name` char(5) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------+
1 row in set (0.05 sec)

mysql> insert into t2 values();
Query OK, 1 row affected, 1 warning (0.08 sec)

mysql> insert into t2 name. values();
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. values()' at line 1
mysql> insert into t2 name.values();
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.values()' at line 1
mysql> insert into t3 name.values();
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.values()' at line 1
mysql> insert into t2(name).values();
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 '.values()' at line 1
mysql> insert into t2 (name) values();
ERROR 1136 (21S01): Column count doesn't match value count at row 1
mysql> insert into t3 (name) values();
ERROR 1136 (21S01): Column count doesn't match value count at row 1
mysql> insert into t2 (name) values(null);
ERROR 1048 (23000): Column 'name' cannot be null
mysql> insert into t3 (name) values(null);
Query OK, 1 row affected (0.04 sec)

mysql> insert into t2 (name) values('rydtfugi');
Query OK, 1 row affected, 1 warning (0.08 sec)

mysql> set global sql_mode='strict_trans_tables';
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t2 name values ('ryugiutyguh');
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 values ('ryugiutyguh')' at line 1
mysql> insert into t2 (name) values ('ryugiutyguh');
Query OK, 1 row affected, 1 warning (0.07 sec)

mysql> quit
Bye

C:\Users\Administrator>sql
Enter password: ****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.6.42 MySQL Community Server (GPL)

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> insert into t2 (name) values ('ryyugiuhiougiutyguh');
ERROR 1046 (3D000): No database selected
mysql> use db1;
Database changed
mysql> insert into t2 (name) values ('ryyugiuhiougiutyguh');
ERROR 1406 (22001): Data too long for column 'name' at row 1
mysql> creat table t4;
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 'creat table t4' at line 1
mysql> create table t4(age int);
Query OK, 0 rows affected (0.28 sec)

mysql> insert into t4 values (5787);
Query OK, 1 row affected (0.07 sec)

mysql> creat table t5 (age int(4) zerofill);
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 'creat table t5 (age int(4) zerofill)' at line 1
mysql> creat table t5 (age int(4)) zerofill;
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 'creat table t5 (age int(4)) zerofill' at line 1
mysql> create table t5 (age int(4) zerofill);
Query OK, 0 rows affected (0.39 sec)

mysql> insert into t1 values (3546735467);
ERROR 1264 (22003): Out of range value for column 'age' at row 1
mysql> insert into t5 values (3546735467);
Query OK, 1 row affected (0.08 sec)

mysql> insert into t1 values (3);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t5 values (3);
Query OK, 1 row affected (0.07 sec)

mysql> select*from t5;
+------------+
| age |
+------------+
| 3546735467 |
| 0003 |
+------------+
2 rows in set (0.00 sec)

mysql> create table t6(age float);
Query OK, 0 rows affected (0.27 sec)

mysql> insert into t6 values(0.246354335555555555533333333333333);
Query OK, 1 row affected (0.08 sec)

mysql> select age from t6;
+----------+
| age |
+----------+
| 0.246354 |
+----------+
1 row in set (0.00 sec)

mysql> create table t7(age decimal);
Query OK, 0 rows affected (0.27 sec)

mysql> insert into t7 age values (1.22336564575674342223546);
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 'age values (1.22336564575674342223546)' at line 1
mysql> insert into t7 (age) values (1.22336564575674342223546);
Query OK, 1 row affected, 1 warning (0.07 sec)

mysql> select age from t7;
+------+
| age |
+------+
| 1 |
+------+
1 row in set (0.00 sec)

mysql> select*from t7;
+------+
| age |
+------+
| 1 |
+------+
1 row in set (0.00 sec)

mysql> create table t8(age decimal（65，30）);
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 'decimal（65，30）)' at line 1
mysql> create table t8(age decimal（65，30）);
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 'decimal（65，30）)' at line 1
mysql> create table t8(age decimal(65,30));
Query OK, 0 rows affected (0.25 sec)

mysql> select age from t8;
Empty set (0.00 sec)

mysql> insert into t8 values(7.46235744666666666);
Query OK, 1 row affected (0.07 sec)

mysql> select age from t8;
+----------------------------------+
| age |
+----------------------------------+
| 7.462357446666666660000000000000 |
+----------------------------------+
1 row in set (0.00 sec)

mysql> create table t9(x char(5),y varchar(5));
Query OK, 0 rows affected (0.25 sec)

mysql> insert into t9 (x,y)values('ftu','ytu');
Query OK, 1 row affected (0.07 sec)

mysql> select *from t9;
+------+------+
| x | y |
+------+------+
| ftu | ytu |
+------+------+
1 row in set (0.00 sec)

mysql> select char_length(x),char_length(y) from t9;
+----------------+----------------+
| char_length(x) | char_length(y) |
+----------------+----------------+
| 3 | 3 |
+----------------+----------------+
1 row in set (0.09 sec)

mysql> select global sql_mode='strict_tables,PAD_CHAR_TO_FULL_LENGTH';
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 '='strict_tables,PAD_CHAR_TO_FULL_LENGTH'' at line 1
mysql> set global sql_mode='strict_tables,PAD_CHAR_TO_FULL_LENGTH';
ERROR 1231 (42000): Variable 'sql_mode' can't be set to the value of 'strict_tables'
mysql> set global sql_mode='strict_trans_tables,PAD_CHAR_TO_FULL_LENGTH';
Query OK, 0 rows affected (0.00 sec)

mysql> quit;
Bye

C:\Users\Administrator>sql;
'sql' 不是内部或外部命令，也不是可运行的程序
或批处理文件。

C:\Users\Administrator>sql
Enter password: ****
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.6.42 MySQL Community Server (GPL)

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select char_length(x),char_length(y) from t9;
ERROR 1046 (3D000): No database selected
mysql> use db1;
Database changed
mysql> select char_length(x),char_length(y) from t9;
+----------------+----------------+
| char_length(x) | char_length(y) |
+----------------+----------------+
| 5 | 3 |
+----------------+----------------+
1 row in set (0.00 sec)

mysql> create table t10(name enum('zzf','sn')not null default 'zf',hobbies set('basketball','pingpang','baseball');
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 '' at line 1
mysql> create table t10(name enum('zzf','sn')not null default 'zf',hobbies set('basketball','pingpang','baseball'));
ERROR 1067 (42000): Invalid default value for 'name'
mysql> create table t10(name enum('zzf','sn')not null default 'zzf',hobbies set('basketball','pingpang','baseball'));
Query OK, 0 rows affected (0.29 sec)

mysql> create table t11(my_datetime datetime, my_time time);
Query OK, 0 rows affected (0.27 sec)

mysql> insert into t11 values();
Query OK, 1 row affected (0.88 sec)

mysql> alter table t11 mondify my_time timestamp;
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 'mondify my_time timestamp' at line 1
mysql> alter table t11 mondify my_time timestamp;
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 'mondify my_time timestamp' at line 1
mysql> alter table t11 change my_time my_tim timestamp;
Query OK, 1 row affected (0.87 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> insert into t11(null,null);
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 'null,null)' at line 1
mysql> insert into t11 values(null,null);
Query OK, 1 row affected (0.08 sec)

mysql> insert into t11 values();
Query OK, 1 row affected (0.08 sec)

mysql> create table t12(ip char(16), port int,unique(ip,port));
Query OK, 0 rows affected (0.36 sec)

mysql> insert into table t12 values('192.168.12.168',4646),('192.168.12.168',3563);
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 'table t12 values('192.168.12.168',4646),('192.168.12.168',3563)' at line 1
mysql> insert into table t12 values('192.168.12.168',4646);
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 'table t12 values('192.168.12.168',4646)' at line 1
mysql> insert into table t12 (ip,port) values('192.168.12.168',4646),('192.168.12.168',3563);
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 'table t12 (ip,port) values('192.168.12.168',4646),('192.168.12.168',3563)' at line 1
mysql> insert into table t12 (ip,port) values('192.168.12.168',4646);
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 'table t12 (ip,port) values('192.168.12.168',4646)' at line 1
mysql> insert into table t12 values('192.168.12.168',4646),('192.168.12.168',3563);
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 'table t12 values('192.168.12.168',4646),('192.168.12.168',3563)' at line 1
mysql> insert into t12 values('192.168.12.168',4646),('192.168.12.168',3563);
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> select*from t12;
+------------------+------+
| ip | port |
+------------------+------+
| 192.168.12.168 | 3563 |
| 192.168.12.168 | 4646 |
+------------------+------+
2 rows in set (0.03 sec)

mysql> create table t13(ip char(16),port int,unique port);
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 ')' at line 1
mysql> create table t13(ip char(16),port int,unique(port));
Query OK, 0 rows affected (0.32 sec)

mysql> insert into t13 values('192',56),('314',56);
ERROR 1062 (23000): Duplicate entry '56' for key 'port'
mysql> insert into t13 values('192',56),('314',57);
Query OK, 2 rows affected (0.08 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> create table t14(ip char(16),port int primary key(ip,port));
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 '(ip,port))' at line 1
mysql> create table t14(ip char(16),port int, primary key(ip,port));
Query OK, 0 rows affected (0.34 sec)

mysql> select *from t13;
+------------------+------+
| ip | port |
+------------------+------+
| 192 | 56 |
| 314 | 57 |
+------------------+------+
2 rows in set (0.00 sec)

mysql> select *from t14;
Empty set (0.00 sec)

mysql> insert into t14 values('192',56),('314',56);
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> select *from t14;
+------------------+------+
| ip | port |
+------------------+------+
| 192 | 56 |
| 314 | 56 |
+------------------+------+
2 rows in set (0.00 sec)

mysql> desc t14;
+-------+----------+------+-----+------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+------------------+-------+
| ip | char(16) | NO | PRI | | |
| port | int(11) | NO | PRI | 0 | |
+-------+----------+------+-----+------------------+-------+
2 rows in set (0.02 sec)

mysql> create table t15(ip char(16),ip int,primary key(ip));
ERROR 1060 (42S21): Duplicate column name 'ip'
mysql> create table t15(ip char(16),ip int,primary key ip)
-> ;
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 ')' at line 1
mysql> create table t15 (ip char(16),ip int,primary key(port);)
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 '' at line 1
-> create table t15 (ip char(16),ip int,primary key(port));
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 ')
create table t15 (ip char(16),ip int,primary key(port))' at line 1
mysql> create table t15 (ip char(16),port int,primary key(port));
Query OK, 0 rows affected (0.29 sec)

mysql> inser into t15 values('6779',798),('6779',798);
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 'inser into t15 values('6779',798),('6779',798)' at line 1
mysql> insert into t15 values('6779',798),('6779',798);
ERROR 1062 (23000): Duplicate entry '798' for key 'PRIMARY'
mysql> insert into t15 values('6779',798),('6779',79);
Query OK, 2 rows affected (0.08 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> select * from t14;
+------------------+------+
| ip | port |
+------------------+------+
| 192 | 56 |
| 314 | 56 |
+------------------+------+
2 rows in set (0.00 sec)

mysql> desc t15;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| ip | char(16) | YES | | NULL | |
| port | int(11) | NO | PRI | 0 | |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.02 sec)

mysql> create table t16(id int,port int primary key auto_increment);
Query OK, 0 rows affected (0.34 sec)

mysql> insert into t16 values();
Query OK, 1 row affected (0.08 sec)

mysql> insert into t16 values();
Query OK, 1 row affected (0.12 sec)

mysql> desc t16;
+-------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+----------------+
| id | int(11) | YES | | NULL | |
| port | int(11) | NO | PRI | NULL | auto_increment |
+-------+---------+------+-----+---------+----------------+
2 rows in set (0.02 sec)

mysql> select * from t16;
+------+------+
| id | port |
+------+------+
| NULL | 1 |
| NULL | 2 |
| NULL | 3 |
+------+------+
3 rows in set (0.00 sec)

mysql>

引擎数据库存取数据方式建表时使用，

show engines;

完整的表结构

create table 表名（字段名类型（宽度）约束条件）engine =innodb charset=utf8；

innodb

myisam 效率高不支持外键

blackhole 黑洞数据放入就丢失

memory 重启数据删除

sql_mode

数据库模式

no_engine_substitution 非安全性默认

strict_trans_tables 安全模式

show variables like '%sql_mode%' 查看数据库模式 %模糊匹配

set global sql_mode=‘strict_trance_tables'’ 设置安全模式，需要重新连接 quit；

数据类型

整形浮点型字符型枚举集合时间

整形

tinyint 1

smallint 2

mediumint 3字节

int 4

bigint 8

非安全模式下超出限制也可以存，会丢失一部分；安全模式超出报错

有zerofill 不足用0填充

浮点型

float 4字节(255,30)

double 8字节(255,30)

decimal M(65,30)

会丢失精度

字符型

char 定长

vchar 不定长

超出设置位数非安全模式下超出限制也可以存，会丢失一部分；安全模式超出报错

数据接近时用定长省空间，反之不定长省空间

set global sql_mode="strict_trans_tables,PAD_CHAR_TO_FULL_LENGTH

select char_length(x), char_length(y) from 表名

之后不定长的空间省

char: 一定按规定的宽度存放数据, 以规定宽度读取数据, 通常更占空间

varchar: 首先根据数据长度计算所需宽度, 并在数据开始以数据头方式将宽度信息保存起来, 是一个计算耗时过程, 取先读取宽度信息,以宽度信息为依准读取数据, 通常节省空间

时间类型

year：yyyy(1901/2155)

date：yyyy-MM-dd(1000-01-01/9999-12-31)

time：HH:mm:ss

datetime：yyyy-MM-dd HH:mm:ss(1000-01-01 00:00:00/9999-12-31 23:59:59)

timestamp：yyyy-MM-dd HH:mm:ss(1970-01-01 00:00:00/2038-01-19 11：14：08)

time 这种多加数据丢失，不够报错

时间戳设置null 的话默认为元时间，

datetime：时间范围，不依赖当前时区，8字节，可以为null

timestamp：时间范围，依赖当前时区，4字节，有默认值CURRENT_TIMESTAMP

枚举与集合

enum：单选

set：多选

枚举单选集合多选

在枚举后定义约束not null default ‘zf'’枚举必须有值不能为null 如果为（）为default 的值

约束键

primary 主键唯一标识不设置默认第一个唯一不为空未标识自动创建隐藏字段

foreign key：外键

unique：唯一性数据, 该条字段的值需要保证唯一,不能重复

auto_increment：自增，只能加给key字段辅助修饰

not null：不为空

default：默认值

unsigned：无符号

zerofill：0填充

如果联合两个字段,两个字段全相同才相同,否则为不同

1.表默认都有主键, 且只能拥有一个主键字段(单列主键 | 联合主键)

# 2.没有设置主键的表, 数据库系统会自上而下将第一个规定为unique not null字段自动提升为primary key主键

# 3.如果整个表都没有unique not null字段且没有primary key字段, 系统会默认创建一个隐藏字段作为主键

# 4.通常必须手动指定表的主键, 一般用id字段, 且id字段一般类型为int, 因为int类型可以auto_increment

create table t21(id int primary key auto_increment); # 自增要结合key,不赋值插入,数据会自动自增, 且自增的结果一直被记录保留

联合主键分开显示pri 联合唯一 key栏合并

posted on 2019-01-08 19:36 paulgeo 阅读(340) 评论(0) 编辑收藏举报

引擎: 决定数据库存取数据的方式 => 不同的特点 => 不同的用户体验 数据类型: 规定了数据库可以存放哪些数据 约束: 限制存储数据的规则 键

引擎: 决定数据库存取数据的方式 => 不同的特点 => 不同的用户体验数据类型: 规定了数据库可以存放哪些数据约束: 限制存储数据的规则键