Access中Left()和Left $()函数之间的区别 [英] Difference between Left() and Left$() function in Access
问题描述
我正在尝试调试损坏的查询.查询使用 Left $([blahblah]，4)而不是Left([blahblah]，4).
I am trying to debug a broken query. The query uses Left$([blahblah],4) instead of Left([blahblah],4).
在其中的Left()和Left $()函数之间有什么区别 Microsoft Access?
What is the difference between the Left() and Left$() functions in Microsoft Access?
推荐答案
结尾的$是String数据类型的类型声明字符 VBA.
The trailing $ is a type declaration character for the String data type in VBA.
从Left $()返回的结果是一个字符串，而Left()返回一个 变体.
The result returned from Left$() is a string, whereas Left() returns a Variant.
如果有可能出现Null值，则必须使用Left()，而不是Left $()， 因为Variant可以为Null，但String不能为Null.为了证明这一点:
You must use Left(), not Left$() if there is any chance of Null values, since the Variant can be Null but the String cannot. To demonstrate that:
-
按Ctrl + G打开Immedate窗口.
Press Ctrl+G to open the Immedate window.
输入: ?左(空，1) 答案是空.
Enter: ? Left(Null,1) The answer is Null.
现在输入: ?左$(Null，1) 这将生成错误94.由于结果应为Null，因此字符串为 不能为Null，则会收到错误消息:无效使用Null".
Now enter: ? Left$(Null,1) This generates Error 94. Since the result should be Null, and the String cannot be Null, you receive the error, "Invalid use of Null".
如果要处理字符串值，则在VBA代码中，Left $()会稍微变小 效率更高，因为它避免了与 变体.但是，如果可能有任何个可能涉及空值的情况，请使用 Left()，否则用诸如Nz()之类的东西显式处理Null.
If you are dealing with string values, in VBA code, Left$() will be slightly more efficient, as it avoids the overhead/inefficieny associated with the Variant. However, if there is any chance that Nulls may be involved, use Left(), or else explicitly handle the Null with something such as Nz().
left $()如果返回NULL，将返回一个字符串或生成错误
left$() will return a string or generate an error if passed NULL
left()如果返回NULL，将返回一个字符串或NULL
left() will return a string or NULL if passed NULL
有关Null的更多信息: http://allenbrowne.com/casu-11.html
More information on Nulls: http://allenbrowne.com/casu-11.html
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