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<a name="navskip"></a><td height="25" colspan="4"><img alt="7.强制数据唯一性" src="imagemaster/titlebar7.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>创建具有唯一性的记录</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共4 页）</nobr></font></td>
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<p>
可以用各种方法确保表中的行具有唯一性：
			</p>
<p>
						
<b>使用主键。</b>
在表创建期间，可以指定一列作为该表的主键：
			</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
CREATE TABLE EMPLOYEE 
  (
  EMPNO INT NOT NULL PRIMARY KEY, 
  LASTNAME VARCHAR(20) NOT NULL,
  ...
  )
</code>
</pre>
<p>
列定义上的 <code>PRIMARY KEY</code> 子句告诉 DB2 自动生成一个强制该列的唯一性的索引。此外，整个表只有一个主键，因此其它列不能包含这个子句。如果需要多个列来确保行的唯一性，则必须在表定义之后加上 <code>PRIMARY KEY</code> 子句：
			</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
CREATE TABLE EMPLOYEE 
  (
  EMPNO INT NOT NULL, 
  LASTNAME VARCHAR(20) NOT NULL,
  ...,
  PRIMARY KEY (EMPNO, LASTNAME)
  )
</code>
</pre>
<p>
						
<b>使用 <code>UNIQUE</code> 子句。</b>
也可以使用 <code>UNIQUE</code> 子句在行中生成具有唯一性的值。带有这个子句的每个列都是唯一的：
			</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
CREATE TABLE EMPLOYEE 
  (
  EMPNO INT NOT NULL UNIQUE,
  SOCINS CHAR(11) NOT NULL UNIQUE,
  ...,
  )
</code>
</pre>
<p>
在这个示例中，雇员号和社会保险号都必须是唯一的。换句话说，不会有两个人拥有相同的雇员号，也不会有两个人具有相同的社会保险号。为了强制这种唯一性，DB2 自动地生成了两个索引，每列一个。
			</p>
<p>
						
<b>使用唯一性索引。</b> 最后，还可以通过使用唯一性索引来确保唯一性。
<code>CREATE INDEX</code> 命令具有选项来指定建立索引一列（或多列）中值必须是唯一的：
			</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
CREATE UNIQUE INDEX UNIQUE_EMPLOYEE ON EMPLOYEE(EMPNO)
</code>
</pre>
<p>
必须在创建表之后立即创建索引。否则，可能会有不具备唯一性的记录被插入该表。如果是这样，则索引创建将因为有重复记录而失败。
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