Access入门教程7.5怎样设置字段的属性（二）-Mysql Tutorial-php.cn

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Access入门教程7.5怎样设置字段的属性（二）

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Jun 07, 2016 pm 04:02 PM

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有效性规则是为了检查字段中的值是否有效可以在该字段的有效性规则框中输入一个表达式 Access会判断输入的值是否满足这个表达式如果满足才能输入输入违反该规则的字段值就无法将值输入到表中并会提示我们不能输入与有效性规则相悖的数值当然我们也可

有效性规则是为了检查字段中的值是否有效可以在该字段的有效性规则框中输入一个表达式Access会判断输入的值是否满足这个表达式如果满足才能输入输入违反该规则的字段值就无法将值输入到表中并会提示我们不能输入与有效性规则相悖的数值当然我们也可以单击这个属性输入文本框右面的生成按钮激活表达式生成器来生成这些表达式而有效性文本这个属性中所填写的文字则是用来当用户输入错误的值时给用户的提示信息

Access入门教程7.5怎样设置字段的属性（二）

在Access中表达式生成器就是用来生成表达式的一段特殊的程序模块通过它可以很方便地编写数据库中的各种表达式它的用法在以后的课程中会有比较详细的讲解在填写一个表的时候常常会遇到一些必须填写的重要字段像这个表中的订货数量字段就必须填写不然我们就无法知道这份订单究竟订了多少货所以要将这个字段的必填字段属性设为是而对于那些要求得不那么严格的数据就可以设定对应字段的必填字段属性为否它下面的允许空字符串属性问我们是否让这个字段里存在零长度字符串通常将它设置为否

索引属性是表中一个重要的属性当我们建立一个很大的数据库的时候就会发现通过查询在表中检索一个数据信息很慢通过分析发现如果当我们要在一个表中的查询订货单位字段内的某个值时会从整个表的开头一直查到末尾如果能将表中额值进行排序那同样的查询工作对订货单位字段检索的记录数就可以少很多速度也自然会变得更快所以很多表都需要建立索引而索引字段就是为了定义是否将这个字段定义为表中的索引字段无是不把这个字段作为索引有（有重复）和有（无重复）这两个选项都表示这个字段已经是表中的一个索引了而有（有重复）允许在表的这个字段中存在同样的值有（无重复）字段则表示在这个字段中绝对禁止相同的值对于订单信息表由于一个订货单位会多次订货也就要签订多份订单所以当我们要把这个字段作为表的索引时就需要将它的索引属性设为有（有重复）了

最后一个是UNICODE属性UNICODE是微软公司为了使一个产品在不同的国家各种语言情况下都能正常运行而编写的一种文字代码使用这种位代码时只需要一个UNICODE就可以存储一个中文文字或英文字符这样虽然很方便但实际上在计算机中本来只要位就可以存储一个英文字符所以使用这种UNICODE方式实际上是比较浪费空间的为了解决这个问题微软又想出了一个方法就是对数字或英文字符进行UNICODE压缩所以对字段的这个属性我们一般都选择有这样可以节省很多空间字段属性栏右面的提示文字可以随时提供一些帮助

结束语现在你也能使用表的设计器来设计或修改一个表了吧那就证明你对表的操作已经上了一个台阶了但是你知不知道表与表之间的数据究竟是怎样联系的？如果你还回答不上那就到下一课去看一看

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