4.1 创建数据表
在创建完数据库之后，接下来的工作就是创建数据表。所谓创建数据表指的是在已经创建好了的数据库中建立新表。创建数据表的过程是规定数据列的属性的过程，同时也是实施数据完整性（包括实体完整性、引用完整性和域完整性等）约束的过程。本节将介绍创建数据表的语法形式、如何添加主键约束、外键约束、非空约束等。
4.1.1 创建表的语法形式
数据表属于数据库，在创建数据表之前，应该使用语句“USE <数据库名>”指定操作是在哪个数据库中进行，如果没有选择数据库，会抛出“No database selected”的错误。
创建数据表的语句为 CREATE TABLE，语法规则如下：
CREATE TABLE <表名>
(
字段名1 数据类型 [列级别约束条件] [默认值],
字段名2 数据类型 [列级别约束条件] [默认值],
……
[表级别约束条件]
);
使用CREATE TABLE创建表时，必须指定以下信息：
⑴ 要创建的表的名称，不区分大小写，不能使用SQL语言中的关键字，如DROP，ALTER，INSERT等。
⑵ 数据表中每一个列（字段）的名称和数据类型，如果创建多个列，要用逗号隔开。
【例4.1】创建员工表tb_emp1，结构如【表4.1】
表4.1 tb_emp1 表结构
|
字段名称
|
数据类型
|
备注
|
|
id
|
INT(11)
|
员工编号
|
|
name
|
VARCHAR(25)
|
员工名称
|
|
deptId
|
INT(11)
|
所在部门编号
|
|
salary
|
FLOAT
|
工资
|
首先选择创建表的数据库，SQL语句如下：
USE test;
创建tb_emp1表，SQL语句为：
CREATE TABLE tb_emp1
(
id INT(11),
name VARCHAR(25),
deptId INT(11),
salary FLOAT
);
语句执行后，便创建了一个名称为tb_emp1的数据表，使用SHOW TABLES；语句查看数据表是否创建成功，SQL语句如下：
mysql> SHOW TABLES;
+-----------------------+
| Tables_in_ test |
+----------------------+
| tb_emp1 |
+----------------------+
1 row in set (0.00 sec)
可以看到test数据库中已经有了数据表tb_tmp1，数据表创建成功。
4.1.2 使用主键约束
主键，又称主码，是表中一列或多列的组合。主键约束（Primary Key constraint）要求主键列的数据唯一，并且不允许为空。主键能够惟一地标识表中的一条记录，可以结合外键来定义不同数据表之间的关系，并且可以加快数据库查询的速度。主键和记录之间的关系如同身份证和人之间的关系，它们之间是一一对应的。主键分为两种类型：单字段主键，多字段联合主键。
1. 单字段主键
主键由一个字段组成，SQL语句格式分以下两种情况。
⑴ 在定义列的同时指定主键，语法规则如下：
字段名 数据类型 PRIMARY KEY
【例4.2】定义数据表tb_emp 2，其主键为id，SQL语句如下：
CREATE TABLE tb_emp2
(
id INT(11) PRIMARY KEY,
name VARCHAR(25),
deptId INT(11),
salary FLOAT
);
⑵ 在定义完所有列之后指定主键。
[CONSTRAINT <约束名>] PRIMARY KEY [字段名]
【例4.3】定义数据表tb_emp 3，其主键为id，SQL语句如下：
CREATE TABLE tb_emp3
(
id INT(11),
name VARCHAR(25),
deptId INT(11),
salary FLOAT,
PRIMARY KEY(id)
);
上述两个例子执行后的结果是一样的，都会在id字段字段上设置主键约束。
2. 多字段联合主键
主键由多个字段联合组成，语法规则如下：
PRIMARY KEY [字段1, 字段2,. . ., 字段n]
【例4.4】定义数据表tb_emp4，假设表中间没有主键id，为了唯一确定一个员工，可以把name、deptId联合起来做为主键，SQL语句如下：
CREATE TABLE tb_emp4
(
name VARCHAR(25),
deptId INT(11),
salary FLOAT,
PRIMARY KEY(name,deptId)
);
语句执行后，便创建了一个名称为tb_emp4的数据表，name字段和deptId字段组合在一起成为tb_emp4的多字段联合主键。
4.1.3 使用外键约束
外键用来在两个表的数据之间建立链接，它可以是一列或者多列。一个表可以有一个或多个外键。外键对应的是参照完整性，一个表的外键可以为空值，若不为空值，则每一个外键值必须等于另一个表中主键的某个值。
外键：首先它是表中的一个字段，它可以不是本表的主键，但对应另外一个表的主键。外键主要作用是保证数据引用的完整性，定义外键后，不允许删除在另一个表中具有关联关系的行。例如，部门表tb_dept的主键是id，在员工表tb_emp5中有一个键deptId与这个id关联。
主表（父表）：对于两个具有关联关系的表而言，相关联字段中主键所在的那个表即是主表。
从表（子表）：对于两个具有关联关系的表而言，相关联字段中外键所在的那个表即是从表。
创建外键的语法规则如下：
[CONSTRAINT <外键名>] FOREIGN KEY 字段名1 [ ,字段名2,…]
REFERENCES <主表名> 主键列1 [ ,主键列2,…]
外键名为定义的外键约束的名称，一个表中不能有相同名称的外键；字段名表示从表的需要添加外键约束的字段列；主表名，即被从表外键所依赖的表的名称；主键列表示主表中定义的主键字段，或者字段组合。
【例4.5】定义数据表tb_emp5，并在tb_emp5表上创建外键约束。
表4.2 tb_dept1 表结构
<tbody>
<tr class="firstRow">
<td style="height:15.3pt; vertical-align:top; width:140.0pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:center"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><strong><span style="font-family:宋体">字段名称</span></strong></span></span></p>
</td>
<td style="height:15.3pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:center"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><strong><span style="font-family:宋体">数据类型</span></strong></span></span></p>
</td>
<td style="height:15.3pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:center"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><strong><span style="font-family:宋体">备注</span></strong></span></span></p>
</td>
</tr>
<tr>
<td style="height:16.05pt; vertical-align:top; width:140.0pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">Id</span></span></p>
</td>
<td style="height:16.05pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">INT(11)</span></span></p>
</td>
<td style="height:16.05pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><span style="font-family:宋体">部门编号</span></span></span></p>
</td>
</tr>
<tr>
<td style="height:15.3pt; vertical-align:top; width:140.0pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">Name</span></span></p>
</td>
<td style="height:15.3pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">VARCHAR(22)</span></span></p>
</td>
<td style="height:15.3pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><span style="font-family:宋体">部门名称</span></span></span></p>
</td>
</tr>
<tr>
<td style="height:16.05pt; vertical-align:top; width:140.0pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">location</span></span></p>
</td>
<td style="height:16.05pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman"">VARCHAR(50)</span></span></p>
</td>
<td style="height:16.05pt; vertical-align:top; width:140.05pt">
<p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="font-family:"Times New Roman""><span style="font-family:宋体">部门位置</span></span></span></p>
</td>
</tr>
</tbody></table><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">创建一个部门表</span><span style="font-family:"Times New Roman"">tb_dept1</span><span style="font-family:宋体">，表结构如【表</span><span style="font-family:"Times New Roman"">4.2</span><span style="font-family:宋体">】，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_dept1</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id i nt(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(22) NOT NULL,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">location VARCHAR(50)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">定义数据表</span><span style="font-family:"Times New Roman"">tb_emp5</span><span style="font-family:宋体">，让它的键</span><span style="font-family:"Times New Roman"">deptId</span><span style="font-family:宋体">作为外键关联到</span><span style="font-family:"Times New Roman"">tb_dept1</span><span style="font-family:宋体">的主键</span><span style="font-family:"Times New Roman"">id</span><span style="font-family:宋体">，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句为：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_emp5 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(25),</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">deptId INT(11), </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">salary FLOAT,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CONSTRAINT fk_emp_dept1 FOREIGN KEY(deptId) REFERENCES tb_dept1(id)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">以上语句执行成功之后，在表</span><span style="font-family:"Times New Roman"">tb_emp5</span><span style="font-family:宋体">上添加了名称为</span><span style="font-family:"Times New Roman"">fk_emp_dept1</span><span style="font-family:宋体">的外键约束，外键名称为</span><span style="font-family:"Times New Roman"">deptId</span><span style="font-family:宋体">，其依赖于表</span><span style="font-family:"Times New Roman"">tb_dept1</span><span style="font-family:宋体">的主键</span><span style="font-family:"Times New Roman"">id</span><span style="font-family:宋体">。<br/><img alt="" src="http://www.jbsage.com/attachment/MTUzMjY2OTAzNDIyNA==.png" style="height:116px; width:450px"/></span></span></span><br/> </p><h3 style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:16pt"><span style="font-family:Calibri"><span style="font-size:14.0pt"><span style="font-family:"Times New Roman"">4.1.4 </span></span><span style="font-size:14.0pt"><span style="font-family:楷体_GB2312">使用非空约束</span></span></span></span></h3><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">非空约束（</span><span style="font-family:"Times New Roman"">Not Null constraint</span><span style="font-family:宋体">）指字段的值不能为空。对于使用了非空约束的字段如果用户在添加数据时，没有指定值，数据库系统会报错。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">非空约束的语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman""><span style="font-family:宋体">字段名</span> <span style="font-family:宋体">数据类型</span> not null</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">【例</span><span style="font-family:"Times New Roman"">4.6</span><span style="font-family:宋体">】定义数据表</span><span style="font-family:"Times New Roman"">tb_emp6</span><span style="font-family:宋体">，指定员工的名称不能为空，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_emp6 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(25) NOT NULL,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">deptId INT(11), </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">salary FLOAT,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CONSTRAINT fk_emp_dept2 FOREIGN KEY (deptId) REFERENCES tb_dept1(id)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">执行后在</span><span style="font-family:"Times New Roman"">tb_emp6</span><span style="font-family:宋体">中创建了一个</span><span style="font-family:"Times New Roman"">Name</span><span style="font-family:宋体">字段，其插入值不能为空（</span><span style="font-family:"Times New Roman"">NOT NULL</span><span style="font-family:宋体">）。</span></span></span></p><h3 style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:16pt"><span style="font-family:Calibri"><span style="font-size:14.0pt"><span style="font-family:"Times New Roman"">4.1.5 </span></span><span style="font-size:14.0pt"><span style="font-family:楷体_GB2312">使用唯一性约束</span></span></span></span></h3><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">唯一性约束（</span><span style="font-family:"Times New Roman"">Unique Constraint</span><span style="font-family:宋体">）要求添加该约束的列字段的值唯一，允许为空，但只能出现一个空值。唯一约束可以确保一列或者几列不出现重复值。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">添加唯一性约束的语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">⑴</span> <span style="font-family:宋体">在定义完列之后直接指定唯一约束，语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman""><span style="font-family:宋体">字段名</span> <span style="font-family:宋体">数据类型</span> UNIQUE</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">【例</span><span style="font-family:"Times New Roman"">4.7</span><span style="font-family:宋体">】定义数据表</span><span style="font-family:"Times New Roman"">tb_dept2</span><span style="font-family:宋体">，指定部门的名称唯一，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_dept2 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(22) UNIQUE,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">location VARCHAR(50)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">⑵</span> <span style="font-family:宋体">在定义完所有列之后指定唯一约束，语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">[CONSTRAINT <<span style="font-family:宋体">约束名</span>>] UNIQUE(<<span style="font-family:宋体">字段名</span>>)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">【例</span><span style="font-family:"Times New Roman"">4.8</span><span style="font-family:宋体">】定义数据表</span><span style="font-family:"Times New Roman"">tb_dept3</span><span style="font-family:宋体">，指定部门的名称唯一，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_dept3 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(22),</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">location VARCHAR(50),</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CONSTRAINT STH UNIQUE(name)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:"Times New Roman"">UNIQUE</span><span style="font-family:宋体">和</span><span style="font-family:"Times New Roman"">PRIMARY KEY</span><span style="font-family:宋体">区别：一个表中可以有多个字段声明为</span><span style="font-family:"Times New Roman"">UNIQUE</span><span style="font-family:宋体">，但只能由一个</span><span style="font-family:"Times New Roman"">PRIMARY KEY </span><span style="font-family:宋体">声明；声明为</span><span style="font-family:"Times New Roman"">PRIMAY KEY</span><span style="font-family:宋体">的列不允许有空值，但是声明为</span><span style="font-family:"Times New Roman"">UNIQUE</span><span style="font-family:宋体">的字段允许为空值（</span><span style="font-family:"Times New Roman"">NULL</span><span style="font-family:宋体">）的存在。</span></span></span></p><h3 style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:16pt"><span style="font-family:Calibri"><span style="font-size:14.0pt"><span style="font-family:"Times New Roman"">4.1.6 </span></span><span style="font-size:14.0pt"><span style="font-family:楷体_GB2312">使用默认约束</span></span></span></span></h3><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">默认约束（</span><span style="font-family:"Times New Roman"">Default Constraint</span><span style="font-family:宋体">）指定某列的默认值。如男性同学较多，性别就可以默认为‘男’。如果插入一条新的记录时没有为这个字段赋值，那么系统会自动为这个字段赋值为‘男’。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">默认约束的语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman""><span style="font-family:宋体">字段名</span> <span style="font-family:宋体">数据类型</span> DEFAULT <span style="font-family:宋体">默认值</span></span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">【例</span><span style="font-family:"Times New Roman"">4.9</span><span style="font-family:宋体">】定义数据表</span><span style="font-family:"Times New Roman"">tb_emp7</span><span style="font-family:宋体">，指定员工的部门编号默认为</span><span style="font-family:"Times New Roman"">1111</span><span style="font-family:宋体">，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_emp7 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(25) NOT NULL,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">deptId INT(11) DEFAULT 1111, </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">salary FLOAT,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CONSTRAINT fk_emp_dept3 FOREIGN KEY (deptId) REFERENCES tb_dept1(id)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">以上语句执行成功之后，表</span><span style="font-family:"Times New Roman"">tb_emp7</span><span style="font-family:宋体">上得字段</span><span style="font-family:"Times New Roman"">deptId</span><span style="font-family:宋体">拥有了一个默认的值</span><span style="font-family:"Times New Roman"">1111</span><span style="font-family:宋体">，新插入的记录如果没有指定部门编号，则默认的都为</span><span style="font-family:"Times New Roman"">1111</span><span style="font-family:宋体">。</span></span></span></p><h3 style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:16pt"><span style="font-family:Calibri"><span style="font-size:14.0pt"><span style="font-family:"Times New Roman"">4.1.7 </span></span><span style="font-size:14.0pt"><span style="font-family:楷体_GB2312">设置表的属性值自动增加</span></span></span></span></h3><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">在数据库应用中，经常希望在每次插入新记录时，系统就会自动生成字段的主键值。可以通过为表主键添加</span><span style="font-family:"Times New Roman"">AUTO_INCREMENT</span><span style="font-family:宋体">关键字来实现。默认的，在</span><span style="font-family:"Times New Roman"">MySQL</span><span style="font-family:宋体">中</span><span style="font-family:"Times New Roman"">AUTO_INCREMENT</span><span style="font-family:宋体">的初始值值是</span><span style="font-family:"Times New Roman"">1</span><span style="font-family:宋体">，每新增一条记录，字段值自动加</span><span style="font-family:"Times New Roman"">1</span><span style="font-family:宋体">。一个表只能有一个字段使用</span><span style="font-family:"Times New Roman"">AUTO_INCREMENT</span><span style="font-family:宋体">约束，且该字段必须为主键的一部分。</span><span style="font-family:"Times New Roman"">AUTO_INCREMENT</span><span style="font-family:宋体">约束的字段可以是任何整数类型（</span><span style="font-family:"Times New Roman"">TINYINT</span><span style="font-family:宋体">、</span><span style="font-family:"Times New Roman"">SMALLIN</span><span style="font-family:宋体">、</span><span style="font-family:"Times New Roman"">INT</span><span style="font-family:宋体">、</span><span style="font-family:"Times New Roman"">BIGINT</span><span style="font-family:宋体">等）。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">设置字段值自增属性的语法规则如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman""><span style="font-family:宋体">字段名</span> <span style="font-family:宋体">数据类型</span> AUTO_INCREMENT</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">【例</span><span style="font-family:"Times New Roman"">4.10</span><span style="font-family:宋体">】定义数据表</span><span style="font-family:"Times New Roman"">tb_emp8</span><span style="font-family:宋体">，指定员工的编号自动递增，</span><span style="font-family:"Times New Roman"">SQL</span><span style="font-family:宋体">语句如下：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CREATE TABLE tb_emp8 </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">(</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">id INT(11) PRIMARY KEY AUTO_INCREMENT,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">name VARCHAR(25) NOT NULL,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">deptId INT(11), </span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">salary FLOAT,</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">CONSTRAINT fk_emp_dept5 FOREIGN KEY (deptId) REFERENCES tb_dept1(id)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Times New Roman"">);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">上述例子执行后，会创建名称为</span><span style="font-family:"Times New Roman"">tb_emp8</span><span style="font-family:宋体">的数据表。表</span><span style="font-family:"Times New Roman"">tb_emp8</span><span style="font-family:宋体">中的</span><span style="font-family:"Times New Roman"">id</span><span style="font-family:宋体">字段的值在添加新记录的时候会自动增加，在插入记录的时候，默认的自增字段</span><span style="font-family:"Times New Roman"">id</span><span style="font-family:宋体">的值从</span><span style="font-family:"Times New Roman"">1</span><span style="font-family:宋体">开始，每次添加一条新记录，该值自动加</span><span style="font-family:"Times New Roman"">1</span><span style="font-family:宋体">。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">例如，执行如下插入语句：</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">mysql> INSERT INTO <span style="font-family:"Times New Roman"">tb_emp8</span> (name,salary)</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">-> VALUES('Lucy',1000), ('Lura',1200),('Kevin',1500);</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:10.5pt"><span style="font-family:Calibri"><span style="font-family:宋体">语句执行完后，</span><span style="font-family:"Times New Roman"">tb_emp8</span><span style="font-family:宋体">表中增加</span><span style="font-family:"Times New Roman"">3</span><span style="font-family:宋体">条记录，在这里，并没有输入</span><span style="font-family:"Times New Roman"">id</span><span style="font-family:宋体">的值，但系统已经自动添加该值，使用</span><span style="font-family:"Times New Roman"">SELECT</span><span style="font-family:宋体">命令查看记录。</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">mysql> SELECT * FROM tb_emp8;</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">+----+-------+--------+--------+</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">| id | name | deptId| salary |</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">+----+-------+--------+--------+</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">| 1 | Lucy | NULL | 1000 |</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">| 2 | Lura | NULL | 1200 |</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">| 3 | Kevin | NULL | 1500 |</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">+----+-------+--------+--------+</span></span></span></p><p style="margin-left:0cm; margin-right:0cm; text-align:justify"><span style="font-size:9pt"><span style="background-color:#d9d9d9"><span style="font-family:"Courier New"">3 rows in set (0.00 sec)</span></span></span></p><p style="text-align:center"><img src="http://www.jbsage.com//attachment/20250807/e343d282daf0c36e738ad3e552aae0c7.png" alt="e343d282daf0c36e738ad3e552aae0c7.png" width="100%" height="auto"/></p>