If/Then in SQL Embedded in VBA - sql

Question

I've got a string like this in my Excel VBA:
strSQL = "SELECT * FROM Total WHERE (SimulationID = (" & TextBox1.Text & ") And Test1 = (" & Example & "))"
However, sometimes Test will be 'is null', which makes the query
And Example = is NULL
How can I change it to add an if/then statement or something to make it say
And Example is null
when Example has a value of "is null"?

Answer 1

I would suggest doing the NULL comparison before assembling the SQL statement strSQL.
If you check for the value of Example beforehand, you can alter your strSQL statement accordingly based on that check.
EDIT:
In response to Daniel's first and second comment below, I still would prefer the following over doing it inline:
Dim strSqlTail
strSqlTail = ""
If (Example1 = Null) Then strSqlTail = "AND Example1 IS NULL"
If (Example2 = Null) Then strSqlTail = strSqlTail & "AND Example2 IS NULL"
If (Example3 = Null) Then strSqlTail = strSqlTail & "AND Example3 IS NULL"
...
Note: The strSqlTail can be whatever SQL would make your situation work since I don't quite understand what is being queried from the sample.

Answer 2

You just create a function that puts the equal sign and space before the number if it doesn't equal "is null", and modify the string assignment statement appropriately, like so:
Public Function NullModString(Example As String) As String
If Example <> "is null" Then Example = "= " & Example
NullModString = Example
End Function
strSQL = "SELECT * FROM Total WHERE SimulationID = " & TextBox1.Text & _
"And Test1 " & NullModString(Example)
Note, that I didn't understand why the extra parentheses were in there, but it would be simple to put them back in.

Answer 3

One solution is to coalesce out the null using a Coalesce function (or if using Access, the Nz function) :
trSQL = "SELECT ..." & _
" FROM Total" & _
" WHERE SimulationID = " & TextBox1.Text & " & _
" And Coalesce(Test1,"""") = "" & Example & """
A better way would be to dynamically include or not include the entire And statement based on whether Example had a value or to substitute Test Is Null when Example did not have a value. So something akin to:
Dim testElements() As Variant
Dim vElement As Variant
Redim testElements(6,1)
testElements(0,0) = Example1
testElements(0,1) = "Test1"
testElements(1,0) = Example2
testElements(1,1) = "Test2"
...
Dim elementIndex as Integer
Dim colName As String
Dim elementValue As Variant
For elementIndex = 0 To UBound(testElements)
elementValue = testElement(elementIndex, 0)
colName = testElement(elementIndex, 1)
If Len(Nz(elementValue)) = 0 Then
trSQL = trSQL & " And " & colName & " = """ & Example1 & """
Else
trSQL = trSQL & " And " & colName & " Is Null"
End If
Next

Answer 4

First, don't embed SQL in VBA: hard to maintain, SQL injection, etc. Use a stored proc on the database side (even Access has PROCEDUREs).
Second, use COALESCE (or equivalent logic) to handle your 'empty string equates to NULL' UI logic. You don't say which SQL syntax and you haven't posted schema DLL so we're merely guessing...
SQL Server:
CREATE PROCEDURE GetTotals
#SimulationID INTEGER,
#Test1 VARCHAR(20) = NULL
AS
SELECT *
FROM Total AS T1.
WHERE T1.SimulationID = #SimulationID
AND COALESCE(T1.Test1, '') = COALESCE(#Test1, '');
Access Database Engine (a.k.a. Jet):
CREATE PROCEDURE GetTotals
(
:SimulationID INTEGER,
:Test1 VARCHAR(20) = NULL
)
AS
SELECT *
FROM Total AS T1
WHERE T1.SimulationID = :SimulationID
AND IIF(T1.Test1 IS NULL, '', T1.Test1)
= IIF(:Test1 IS NULL, '', :Test1);
Then execute the proc using your middleware (ADO, DAO, etc) with Parameter objects, using the empty string (or other 'magic' value) for NULL.