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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(yè)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(zhǎng)
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(shuō)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(shuō)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(jiàn)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(yè)
基本語(yǔ)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(kāi)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(lái)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(yǔ)文成績(jī)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(lái)財(cái) | 82 | 84 | 67 | | 7 | 張小明 | 75 | 65 | 30 | +------+--------+---------+---------+------+ 2 rows in set (0.00 sec)
擴(kuò)展，分頁(yè):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(guò)程技巧大全》、《MySQL數(shù)據(jù)庫(kù)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(duì)大家MySQL數(shù)據(jù)庫(kù)計(jì)有所幫助。
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