Difference between Left() and Left$() function in

问题:

I am trying to debug a broken query. The query uses Left$([blahblah],4) instead of Left([blahblah],4).

What is the difference between the Left() and Left$() functions in Microsoft Access?

回答1:

The trailing $ is a type declaration character for the String data type in VBA.

The result returned from Left$() is a string, whereas Left() returns a Variant.

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:

1. Press Ctrl+G to open the Immedate window.

2. Enter: ? Left(Null,1) The answer is Null.

3. 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".

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$() will return a string or generate an error if passed NULL

left() will return a string or NULL if passed NULL

More information on Nulls: http://allenbrowne.com/casu-11.html

回答2:

The only difference is that Left accepts Null while Left$ does not.

If passed a string, both return a String. VarType: vbString

If passed Null, Left returns a Null value. VarType: vbNull

They both runs at the same speed, about 12 mio. iterations pr. second, thus of no importance. Actually, if many samples are recorded, Left in average is a fraction faster than Left$.

So, to put it short: Use Left$ if you wish to raise an error on a parameter value of Null, otherwise save your typing.