快捷導(dǎo)航

MySQL學(xué)習(xí)筆記之?dāng)?shù)據(jù)定義表約束,分頁方法總結(jié)

更新時(shí)間：2016年09月14日 09:00:40 作者：hbiao68

這篇文章主要介紹了MySQL學(xué)習(xí)筆記之?dāng)?shù)據(jù)定義表約束,分頁方法,結(jié)合實(shí)例形式總結(jié)分析了數(shù)據(jù)定義、主鍵、外鍵、自增長、約束等概念與用法,并給出了關(guān)于分頁的實(shí)例與相關(guān)操作技巧,需要的朋友可以參考下

本文實(shí)例講述了MySQL學(xué)習(xí)筆記之?dāng)?shù)據(jù)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(diǎn)：主鍵是用于唯一標(biāo)識(shí)一條記錄的約束，一張表最多只能有一個(gè)主鍵，不能為空也不能重復(fù)

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 唯一約束

特點(diǎn)：表的某列值不能重復(fù)，可以添加重復(fù)的NULL

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'

允許插入null，并且可以多個(gè)

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表的列默認(rèn)情況下可以為null,如果不允許某列為空則可以使用not null說明

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

5. foreign key 外鍵

從理論上說先建立主表，再建立從表

雇員表：

create table dept(id int primary key , name varchar(32));

部門表：

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)

總結(jié)：

① 外鍵只能指向主表的主見列或者unique
② 外鍵的數(shù)據(jù)類型應(yīng)該與它指向的列類型一致
③ 外鍵的值：NULL 或者指向列中存在的值
④ 外鍵可以指向本表的主鍵列或者unique

mysql 不支持check

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 |
+------+

mysql 分頁

基本語法：

select * from 表明 where 條件 limit 從第幾條取,取出幾條
mysql 是從第0條開始取數(shù)據(jù)

mysql> select * from student;
+------+--------+---------+---------+------+
| id  | name  | chinese | english | math |
+------+--------+---------+---------+------+
|  1 | 張小明   |   89 |   78 |  90 |
|  2 | 李進(jìn)    |   67 |   98 |  56 |
|  3 | 王五    |   87 |   78 |  77 |
|  4 | 李一   |   88 |   98 |  90 |
|  5 | 李來財(cái)    |   82 |   84 |  67 |
|  6 | 張進(jìn)寶   |   55 |   85 |  45 |
|  7 | 張小明   |   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)

按照語文成績排序，查處第3條到第5條

mysql> select * from student order by chinese desc limit 3,2;
+------+--------+---------+---------+------+
| id  | name  | chinese | english | math |
+------+--------+---------+---------+------+
|  5 | 李來財(cái)    |   82 |   84 |  67 |
|  7 | 張小明   |   75 |   65 |  30 |
+------+--------+---------+---------+------+
2 rows in set (0.00 sec)

擴(kuò)展，分頁:pageNow , pageSize

select * from 表明 where 條件 [group by … having … order by …]limit 從第幾條取,取出幾條
select * from 表明 where 條件 [group by … having … order by …]limit (pageNow-1)*pageSize, pageSize

更多關(guān)于MySQL相關(guān)內(nèi)容感興趣的讀者可查看本站專題：《MySQL索引操作技巧匯總》、《MySQL日志操作技巧大全》、《MySQL事務(wù)操作技巧匯總》、《MySQL存儲(chǔ)過程技巧大全》、《MySQL數(shù)據(jù)庫鎖相關(guān)技巧匯總》及《MySQL常用函數(shù)大匯總》

希望本文所述對大家MySQL數(shù)據(jù)庫計(jì)有所幫助。

您可能感興趣的文章: