這篇文章主要介紹了MySQL學習筆記之資料定義表約束,分頁方法,結合實例形式總結分析了資料定義、主鍵、外鍵、自增長、約束等概念與用法,並給出了關於分頁的實例與相關操作技巧,需要的朋友可以參考下
本文實例講述了MySQL學習筆記之資料定義表約束，分頁方法。分享給大家供大家參考，具體如下：
1. primary key 主鍵
特點：主鍵是用於唯一標識一條記錄的約束，一張表最多只能有一個主鍵，不能為空也不能重複
create table user1(id int primary key,name varchar(32)); mysql> insert into user1 values(1,'hb'); Query OK, 1 row affected (0.10 sec) mysql> insert into user1 values(1,'hb'); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' mysql> insert into user1 (name) values('hb'); ERROR 1364 (HY000): Field 'id' doesn't have a default value
2. auto_increament 自增長
mysql> create table user2(id int primary key auto_increment,name varchar(34));
mysql> insert into user2 (name ) values ("name1");
Query OK, 1 row affected (0.09 sec)
mysql> insert into user2 (name ) values ("name2");
Query OK, 1 row affected (0.05 sec)
mysql> insert into user2 (name ) values ("name3");
Query OK, 1 row affected (0.13 sec)
mysql> select * from user2;
+----+-------+
| id | name |
+----+-------+
| 1 | name1 |
| 2 | name2 |
| 3 | name3 |
+----+-------+3. unique 唯一限制的特徵
create table user3(id int primary key auto_increment,name varchar(34) unique);
mysql> create table user3(id int primary key auto_increment,name varchar(34) unique);
Query OK, 0 rows affected (0.39 sec)
mysql> insert into user3 (name ) values ("name3");
Query OK, 1 row affected (0.11 sec)
mysql> insert into user3 (name ) values ("name3");
ERROR 1062 (23000): Duplicate entry 'name3' for key 'name'mysql> insert into user3 (name ) values (null); Query OK, 1 row affected (0.12 sec) mysql> insert into user3 (name ) values (null); Query OK, 1 row affected (0.12 sec) mysql> select * from user3; +----+-------+ | id | name | +----+-------+ | 3 | NULL | | 4 | NULL | | 1 | name3 | +----+-------+
4. not null
mysql表的列預設值create table user4 (id int primary key auto_increment,name varchar(32) not null);
mysql> insert into user4 (name) values(null);
ERROR 1048 (23000): Column 'name' cannot be null
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從理論上說先建立主表，再建立從表
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總結：
① 外鍵只能指向主表的主見列或unique② 外鍵的資料型別應該與它指向的列型別一致
③ 外鍵的值：NULL 或指向列中存在的值④ 外鍵可以指向本表的主鍵欄位或unique
mysql 不支援checkcreate table dept(id int primary key , name varchar(32));
sql 分頁紙 片語
create table emp( id int primary key , name varchar(32), deptid int, constraint myforeignkey foreign key(deptid) references dept(id) ); mysql> select * from dept; +----+-------+ | id | name | +----+-------+ | 1 | name1 | +----+-------+ 1 row in set (0.00 sec) mysql> insert into emp values(1,'aaa',1); Query OK, 1 row affected (0.22 sec) mysql> insert into emp values(1,'aaa',2); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' mysql> insert into emp values(1,'aaa',null); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY' mysql> insert into emp values(2,'aaa',null); Query OK, 1 row affected (0.13 sec) mysql> select * from emp; +----+------+--------+ | id | name | deptid | +----+------+--------+ | 1 | aaa | 1 | | 2 | aaa | NULL | +----+------+--------+ 2 rows in set (0.00 sec)
mysql 是從第0條開始取數據
create table user99(age int check(age>13)); mysql> create table user99(age int check(age>13)); Query OK, 0 rows affected (0.19 sec) mysql> insert into user99 values(99); Query OK, 1 row affected (0.04 sec) mysql> select * from user99; +------+ | age | +------+ | 99 | +------+
按照語文成績排序，查處第3條到第5條
select * from 表明where 條件[group by … having … order by …]limit 從第幾條取,取出幾條pagespage 頁
mysql> select * from student;
+------+--------+---------+---------+------+
| id | name | chinese | english | math |
+------+--------+---------+---------+------+
| 1 | 张小明 | 89 | 78 | 90 |
| 2 | 李进 | 67 | 98 | 56 |
| 3 | 王五 | 87 | 78 | 77 |
| 4 | 李一 | 88 | 98 | 90 |
| 5 | 李来财 | 82 | 84 | 67 |
| 6 | 张进宝 | 55 | 85 | 45 |
| 7 | 张小明 | 75 | 65 | 30 |
+------+--------+---------+---------+------+
7 rows in set (0.05 sec)
mysql> select * from student limit 2,2;
+------+------+---------+---------+------+
| id | name | chinese | english | math |
+------+------+---------+---------+------+
| 3 | 王五 | 87 | 78 | 77 |
| 4 | 李一 | 88 | 98 | 90 |
+------+------+---------+---------+------+
2 rows in set (0.00 sec)
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select * from 表明where 條件[group by … having … order by …]limit (pageNow 1)*pageSize, pageSize
