VBA SQL Where IS NULL - sql

Question

Im trying to create a VBA sql string with the where clause using inputs from an access form. I need the code to still run properly even if one of the user inputs is null/unselected. The code below runs correctly when all user inputs have been selected but when one input is null/unselected the query ends up pulling back all the records in the table.
Dim SQL As String
SQL = "SELECT qry_1.field1, qry_1.field2, qry_1.field3 INTO recordset FROM qry_1" & _
" WHERE ((qry_1.Field1) IS NULL OR (qry_1.Field1) = '" & [Forms]![frm_Parameters]![cmbx_1] & "') " &_
" AND ((qry_1.Field2) IS NULL OR (qry_1.Field2) = '" & [Forms]![frm_Parameters]![cmbx_2] & "' ) " & _
" AND ((qry_1.Field3) IS NULL OR (qry_1.Field3) = '" & [Forms]![frm_Parameters]![cmbx_3] & "') ;"
DoCmd.RunSQL SQL

Answer 1

It is better to escape single apostrophes in the text for two reasons
You might get an invalid SQL statement if you don't.
A nasty user could can inject evil code (see: SQL injection).
Therefore I use this function
Public Function SqlStr(ByVal s As String) As String
'Input: s="" Returns: NULL
'Input: s="abc" Returns: 'abc'
'Input: s="x'y" Returns: 'x''y'
If s = "" Then
SqlStr = "NULL"
Else
SqlStr = "'" & Replace(s, "'", "''") & "'"
End If
End Function
To account for empty entries, create the condition like this:
Dim cond As String, sql As String
If Nz([Forms]![frm_Parameters]![cmbx_1]) <> "" Then
cond = "qry_1.field1 = " & SqlStr(Nz([Forms]![frm_Parameters]![cmbx_1]))
End If
If Nz([Forms]![frm_Parameters]![cmbx_2]) <> "" Then
If cond <> "" Then cond = cond & " AND "
cond = cond & "qry_1.field2 = " & SqlStr(Nz([Forms]![frm_Parameters]![cmbx_2]))
End If
If Nz([Forms]![frm_Parameters]![cmbx_3]) <> "" Then
If cond <> "" Then cond = cond & " AND "
cond = cond & "qry_1.field3 = " & SqlStr(Nz([Forms]![frm_Parameters]![cmbx_3]))
End If
sql = "SELECT qry_1.field1, qry_1.field2, qry_1.field3 INTO recordset FROM qry_1"
If cond <> "" Then
sql = sql & " WHERE " & cond
End If
This assumes that you want to test a condition only when the corresponding field is not empty.
If you have date constants, format them the Access-SQL way as #2018-08-21# with
Format$(date, "\#yyyy-MM-dd\#")

Answer 2

DoCmd.RunSQL is for action queries (UPDATE, INSERT, DELETE).
To answer the question, here is what I would do:
Nz(Field1,Nz([Forms]![frm_Parameters]![cmbx_1]))=Nz([Forms]![frm_Parameters]![cmbx_1])
That will create equality if either Field1 or [Forms]![frm_Parameters]![cmbx_1] is NULL without breaking the rest of the query.
To use that in VBA:
"Nz(Field1,'" & Nz([Forms]![frm_Parameters]![cmbx_1]) & "')='" & Nz([Forms]![frm_Parameters]![cmbx_1]) & "'"
Also see comment in Olivier Jacot-Descombes answer about taking single quotes in your strings into account.