1.   Handling nulls: Trim / Len / Nz

Last Updated 27 Feb 2019                                                                                                       Return To Top

This was originally written in response to a question by Mister Chips at Utter Access forum: Best Way To Check For 'nothing' In A Form Control

The OP wanted to know the best approach to checking if there is anything at all in a control (textbox, combo etc).

Three methods were suggested by the OP / Phil C & myself respectively

a)   Trim     e.g. If Trim(txtControl & "") = vbNullString Then
b)   Len      e.g. If Len(txtControl.Value & vbNullString) <> 0 Then
c)   Nz        e.g. If Nz(txtControl,"") < > " " Then

It was strongly suggested by several forum members that Len would be faster as
"'VBA handles numbers better then strings especially in comparing, so checking if Len(...) <> 0 is easier for VBA than comparing two strings'"

I was sceptical about this claim so decided to test all 3 methods on a local table with approx 2.6 million records.
In each case a text field was modified where it was null or a zero length string.
These were approximately 20% of the total records

I made sure nothing else was running during each test to try & ensure there were no other factors influencing the results.
I ran the tests repeatedly for each of the methods but found little variation between each set of results

Tests were repeated on several other workstations. In each case, the order was the same. Nz was marginally faster than Len with Trim being the slowest
However, the differences were relatively small.

Test Results


To my mind, this perhaps indicates that it is easy to get too bound up in optimisation worries due to the processing power of most modern computers.

I also ran the tests with the Accuracy field used in the test first unindexed & then again after indexing.
The outcome was consistently slower for the indexed field – indexing speeds up searches but significantly slows down updates

Indexed

Not Indexed

Each result is the average of 3 tests though there was minimal variation in each test

Times were measured using the system timer (updated 64 times per second approx.) and rounded off to 2 d.p. (centiseconds)

I had intended to compare all 3 approaches looping through a recordset which I knew would be much slower. The first one was indeed VERY SLOW and eventually crashed the database as the file size approached the 2 GB limit. I abandoned the remaining recordset tests!

Click to download :     Empty Fields Comparison Tests v1.2       Approx 25 MB (zipped)

NOTE This is a VERY LARGE download due to the table containing 2.6 million records

CODE:

Sub UpdateTestString()

   Dim sngStart As Single, sngEnd As Single

   DoCmd.Hourglass True

   'first reset any existing data from previous tests
   CurrentDb.Execute "UPDATE Postcodes SET Postcodes.Accuracy = Null WHERE Postcodes.Accuracy ='OK';"

   sngStart = GetCurrentSystemTime

   'run test using one of the following:
  '1. Trim
   CurrentDb.Execute "UPDATE Postcodes SET Postcodes.Accuracy = 'OK' WHERE (((Trim([Accuracy] & ''))=''));"

  '2. Len
  'CurrentDb.Execute "UPDATE Postcodes SET Postcodes.Accuracy = 'OK' WHERE (((Len([Accuracy] & ''))=0));"

   '3. Nz
  ' CurrentDb.Execute "UPDATE Postcodes SET Postcodes.Accuracy = 'OK' WHERE ((Nz(Accuracy,'') =''));"

   sngEnd = GetCurrentSystemTime

   DoCmd.Hourglass False

   MsgBox "Time taken = " & Round((sngEnd - sngStart), 2) & " seconds"

End Sub

The system time code is:

Option Compare Database
Option Explicit

Private Type SYSTEMTIME
    wYear As Integer
    wMonth As Integer
    wDayOfWeek As Integer
    wDay As Integer
    wHour As Integer
    wMinute As Integer
    wSecond As Integer
    wMilliseconds As Integer
End Type

Declare Sub GetSystemTime Lib "kernel32" (lpSystemTime As SYSTEMTIME)

Function GetCurrentSystemTime() As Single

'Use this when accurate time differences to milliseconds are required

    Dim tSystem As SYSTEMTIME

    GetSystemTime tSystem    
    GetCurrentSystemTime = (1000 * Int(Timer) + tSystem.wMilliseconds) / 1000
    'Debug.Print GetCurrentSystemTime

End Function  

As previously mentioned, system time is updated about 60 times per second - around 0.16 second intervals. So there is some potential error in the values but not as much as the time differences between the methods.

However, to me, the results indicate that in real life situations, there is minimal difference in the methods.

There are other ways of measuring time with precision
For example, you can use GetTickCount but that is also based on system time
The built in Timer function can be used to give time to centisecond accuracy though with the same limitation

You could also use the StopClock class code but this also appears to be millisecond accuracy

For a detailed comparison of the different approaches available for measuring time intervals, see my article Timer Comparison Tests



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Colin Riddington                     Mendip Data Systems                           Last Updated 27 Feb 2019



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