Visual FoxPro中的空值(NULL值)问题

The "NULL" Value problem of The Visual FoxPro

空值(NULL值)是关系数据库中的一个重要概念,我们可以利用空值进行数据查询.由于空值不是任何意义上的数据,所以向数据库表或自由表中输入空值的方法与输入其他类型数据的方法不同,不论是用ED IT、CHANGE命令以及在BROWSE浏览器窗口中对表记录进行编辑和修改,还是用APPEND或INSERT命令追加或插入记录时都不能真正输入NULL值.本文给出了向表中字段输入空值的正确方法,并指出利用其他输入数据的方法输入空值时会出现的错误.

'NULL' Value is a important concept of visual FoxPro, we can use 'NULL' in looking for data. Because 'NULL' is not the data in any mean, the way of importing 'NULL' into FoxPro chart or freedom chart is different from the way of importing other kinds of data. The right way of importing 'NULL' into of the phase in the chart was given in the present paper, and point out the wrong that will appear when we use the other ways of importing data to import the 'NULL' value.