How to merge two database tables when only some fields are common? - sql

Question

In MS Access I have two tables (A and B), and the task is to insert B into A. However, there are some special conditions:
All fields are of type text.
A and B have a some common fields.
The same key field is guaranteed to exist in both, and its values to be always different.
A has some fields that B does not have. The inserted records should have those fields blank.
B has some fields that A does not have. These fields must be created in A, and the existing records in A should have them blank.
There are many cases like this one, so the query should not explicitly include the field names, since it would be tedious to personalize the query for each case. However, the key field is always named the same.
Creating a new table C instead of directly replacing A is acceptable.
Example:
Table A:
key a b c
--- ------- ------- -------
k0 hello dear world
k1 bye cruel world
Table B:
key a d e
--- ------- ------- -------
k2 welcome john doe
k3 turulu ann harp
Table C (the new A):
key a b c d e
--- ------- ------- ------- ------- -------
k0 hello dear world
k1 bye cruel world
k2 welcome john doe
k3 turulu ann harp

Answer 1

Create an Access Module and use the following code. Replace the values in the test sub with your table and destination names
Option Compare Database
Option Explicit
Function SplatTablesSql(pT1 As String, pT2 As String, pDest As String)
Dim lDb As Database
Dim lTd1 As TableDef, lTd2 As TableDef
Dim lField As Field, lF2 As Field
Dim lS1 As String, lS2 As String, lSep As String
SplatTablesSql = "Select "
lS1 = "Select "
lS2 = "Select "
Set lDb = CurrentDb
Set lTd1 = lDb.TableDefs(pT1)
Set lTd2 = lDb.TableDefs(pT2)
For Each lField In lTd1.Fields
SplatTablesSql = SplatTablesSql & lSep & "x.[" & lField.Name & "]"
lS1 = lS1 & lSep & "a.[" & lField.Name & "]"
Set lF2 = Nothing
On Error Resume Next
Set lF2 = lTd2.Fields(lField.Name)
On Error GoTo 0
If lF2 Is Nothing Then
lS2 = lS2 & lSep & "Null"
Else
lS2 = lS2 & lSep & "b.[" & lField.Name & "]"
End If
lSep = ", "
Next
For Each lField In lTd2.Fields
Set lF2 = Nothing
On Error Resume Next
Set lF2 = lTd1.Fields(lField.Name)
On Error GoTo 0
If lF2 Is Nothing Then
SplatTablesSql = SplatTablesSql & lSep & "x.[" & lField.Name & "]"
lS1 = lS1 & lSep & "Null as [" & lField.Name & "]"
lS2 = lS2 & lSep & "b.[" & lField.Name & "]"
End If
lSep = ", "
Next
SplatTablesSql = SplatTablesSql & " Into [" & pDest & "] From ( " & lS1 & " From [" & pT1 & "] a Union All " & lS2 & " From [" & pT2 & "] b ) x"
End Function
Sub Test()
CurrentDb.Execute SplatTablesSql("a", "b", "c")
End Sub

Answer 2

The easiest way I can think to solve this is to use VBA to create the query definition.
I will assume that there's a column named key which is common to both tables.
I found here that you can use collections to make a dictionary-like structure. I'll use that to build the field list.
So, here we go:
public function contains(col as Collection, key as variant) as boolean
dim obj as variant
on error goto err
contains = True
obj = col(key)
exit function
err:
contains = false
end function
public sub create_this_query(tbl1 as String, tbl2 as String, keyField as String)
' tbl1 and tbl2 are the names of the tables you'll use
dim db as DAO.database, rs1 as DAO.recordset, rs2 as DAO.recordset
dim columns as Collection
dim strSQL as String
dim i as integer
dim obj as variant, colName as String
set db = currentdb()
set tbl1 = db.openrecordset(tbl1, dbopendynaset, dbreadonly)
set tbl2 = db.openrecordset(tbl2, dbopendynaset, dbreadonly)
set columns = new Collection
' Let's create the field list (ommiting the keyField)
for i = 1 to tbl1.fields.count
if not contains(columns, tbl1.fields(i).Name) _
and tbl1.fields(i).Name <> keyField then
columns.add tbl1.fields(i).Name, tbl1.fields(i).Name
end if
next i
for i = 1 to tbl2.fields.count
if not contains(columns, tbl2.fields(i).Name) _
and tbl2.fields(i).Name <> keyField then
columns.add tbl1.fields(i).Name, 1 ' The value is just a placeholder
end if
next i
' Now let's build the SQL instruction
strSQL = "select [a].[" & keyField & "]"
for colName in columns
strSQL = strSQL & ", [" & colName & "]"
next obj
strSQL = strSQL & " " & _
"from " & _
" (" & _
" select [" & keyField & "] from [" & tbl1 & "] " & _
" union " & _
" select [" & keyField & "] from [" & tbl2 & "] " & _
" ) as a " & _
"left join [" & tbl1 & "] as t1 " & _
" on a.[" & keyField & "] = t1.[" & keyField & "] " & _
"left join [" & tbl2 & "] as t2 " & _
" on a.[" & keyField & "] = t2.[" & keyField & "] "
' Finally, let's create the query object
db.createQueryDef("myNewQuery", strSQL)
end sub
Hope this helps