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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...

發布時間：2024/10/12 数据库 24 豆豆

生活随笔收集整理的這篇文章主要介紹了引擎: 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... 小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

Microsoft Windows [版本 10.0.17134.472]
(c) 2018 Microsoft Corporation。保留所有權利。

C:\Users\Administrator>SQL;
'SQL' 不是內部或外部命令，也不是可運行的程序
或批處理文件。

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' 不是內部或外部命令，也不是可運行的程序
或批處理文件。

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 ?數據庫存取數據方式建表時使用，

show engines;

完整的表結構

create ?table 表名（字段名類型 ?（寬度）約束條件）engine =innodb ? charset=utf8；

innodb

myisam 效率高不支持外鍵

blackhole 黑洞數據放入就丟失

memory ?重啟數據刪除

sql_mode

數據庫模式

no_engine_substitution 非安全性默認

strict_trans_tables ?安全模式

show variables like '%sql_mode%' 查看數據庫模式 %模糊匹配

set global ?sql_mode=‘strict_trance_tables'’ 設置安全模式，需要重新連接 quit；

數據類型

整形 ?浮點型 ? 字符型 ?枚舉 ?集合時間

整形

tinyint ?1

smallint ?2

mediumint ?3字節

int ? ?4

bigint ? 8

非安全模式下超出限制也可以存，會丟失一部分；安全模式超出報錯

有zerofill 不足用0填充

浮點型

float 4字節(255,30)

double 8字節(255,30)

decimal M(65,30)

會丟失精度

字符型

char 定長

vchar 不定長

超出設置位數非安全模式下超出限制也可以存，會丟失一部分；安全模式超出報錯

數據接近時用定長省空間，反之不定長省空間

set global sql_mode="strict_trans_tables,PAD_CHAR_TO_FULL_LENGTH

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

之后不定長的空間省

char: 一定按規定的寬度存放數據, 以規定寬度讀取數據, 通常更占空間

varchar: 首先根據數據長度計算所需寬度, 并在數據開始以數據頭方式將寬度信息保存起來, 是一個計算耗時過程, 取先讀取寬度信息,以寬度信息為依準讀取數據, 通常節省空間

時間類型

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 這種多加數據丟失，不夠報錯

時間戳設置null 的話默認為元時間，

datetime：時間范圍，不依賴當前時區，8字節，可以為null

timestamp：時間范圍，依賴當前時區，4字節，有默認值CURRENT_TIMESTAMP

枚舉與集合

enum：單選

set：多選

枚舉單選集合多選

在枚舉后定義約束not null ?default ‘zf'’枚舉必須有值不能為null 如果為（）為default 的值

約束鍵

primary 主鍵唯一標識不設置默認第一個唯一不為空未標識自動創建隱藏字段

foreign key：外鍵

unique：唯一性數據, 該條字段的值需要保證唯一,不能重復

auto_increment：自增，只能加給key字段輔助修飾

not null：不為空

default：默認值?

unsigned：無符號

zerofill：0填充

如果聯合兩個字段,兩個字段全相同才相同,否則為不同

1.表默認都有主鍵, 且只能擁有一個主鍵字段(單列主鍵 | 聯合主鍵)

# 2.沒有設置主鍵的表, 數據庫系統會自上而下將第一個規定為unique not null字段自動提升為primary key主鍵

# 3.如果整個表都沒有unique not null字段且沒有primary key字段, 系統會默認創建一個隱藏字段作為主鍵

# 4.通常必須手動指定表的主鍵, 一般用id字段, 且id字段一般類型為int, 因為int類型可以auto_increment

create table t21(id int primary key auto_increment); # 自增要結合key,不賦值插入,數據會自動自增, 且自增的結果一直被記錄保留

聯合主鍵分開顯示pri 聯合唯一 key欄合并

轉載于:https://www.cnblogs.com/wrqysrt/p/10241010.html

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