Left Join works with table but fails with query - sql

Question

The following left join query in MS Access 2007
SELECT
Table1.Field_A,
Table1.Field_B,
qry_Table2_Combined.Field_A,
qry_Table2_Combined.Field_B,
qry_Table2_Combined.Combined_Field
FROM Table1
LEFT JOIN qry_Table2_Combined
ON (Table1.Field_A = qry_Table2_Combined.Field_A)
AND (Table1.Field_B = qry_Table2_Combined.Field_B);
is expected by me to return this result:
+--------+---------+---------+---------+----------------+
|Field_A | Field_B | Field_A | Field_B | Combined_Field |
+--------+---------+---------+---------+----------------+
|1 | | | | |
+--------+---------+---------+---------+----------------+
|1 | | | | |
+--------+---------+---------+---------+----------------+
|2 |1 |2 |1 |John, Doe |
+--------+---------+---------+---------+----------------+
|2 |2 | | | |
+--------+---------+---------+---------+----------------+
[Table1] has 4 records, [qry_Table2_Combined] has 1 record.
But it gives me this:
+--------+---------+---------+---------+----------------+
|Field_A | Field_B | Field_A | Field_B | Combined_Field |
+--------+---------+---------+---------+----------------+
|2 |1 |2 |1 |John, Doe |
+--------+---------+---------+---------+----------------+
|2 |2 |2 | |, |
+--------+---------+---------+---------+----------------+
Really weird is that the [Combined_Field] has a comma in the second row. I use a comma to concatenate two fields in [qry_Table2_Combined].
If the left join query uses a table created from the query [qry_Table2_Combined] it works as expected.
Why does this left join query not give the same result for a query and a table? And how can i get the right results using a query in the left join?

Answer 1

Looking at you logic, it seems that you only want combined fields where field_A = "2" (SELECT '2' AS Field_A). I suspect that this is causing the problem. Would it be possible to go about a solution in a different way, for example:
SELECT
t1.Field_A,
t1.Field_B,
t2.Field_B As t2B,
[t2].[Col_1] & ", " & [t2].[Col_2] AS Combined
FROM t1 LEFT JOIN t2
ON t1.Field_B = t2.Field_B
WHERE t1.Field_A="2"
UNION ALL
SELECT
t1.Field_A,
t1.Field_B,
"None" As t2B,
"None" AS Combined
FROM t1
WHERE t1.Field_A<>"2"

Answer 2

Concatenation: change the & operators to + operators and the result should be as expected.
Missing rows: I can reproduce this issue but cannot explain it, other than to say a) it's probably a bug and b) it will probably never get fixed :(
For sanity I tested the same code in SQL Server and it works as expected.
As a general point an outer join can be simulated using union and padding the missing values e.g. pseudo code:
( A JOIN B )
UNION
( A NOT MATCH B { A.*, <pad values for B> } )
In your case and in Access SQL:
SELECT Table1.Field_A, Table1.Field_B,
qry_Table2_Combined.Field_A,
qry_Table2_Combined.Field_B,
qry_Table2_Combined.Combined_Field
FROM Table1
INNER JOIN qry_Table2_Combined
ON (Table1.Field_A = qry_Table2_Combined.Field_A)
AND (Table1.Field_B = qry_Table2_Combined.Field_B)
UNION ALL
SELECT Table1.Field_A, Table1.Field_B,
NULL AS Field_A,
NULL AS Field_B,
NULL AS Combined_Field
FROM Table1
WHERE NOT EXISTS ( SELECT *
FROM qry_Table2_Combined
WHERE (Table1.Field_A = qry_Table2_Combined.Field_A)
AND (Table1.Field_B = qry_Table2_Combined.Field_B) );
The above seems to produce the results you were expecting.
Access repro code, with concatenation fix, uncomment code for suggested workaround:
Sub EXfewfTempler()
On Error Resume Next
Kill Environ$("temp") & "\DropMe.mdb"
On Error GoTo 0
Dim cat
Set cat = CreateObject("ADOX.Catalog")
With cat
.Create _
"Provider=Microsoft.Jet.OLEDB.4.0;" & _
"Data Source=" & _
Environ$("temp") & "\DropMe.mdb"
With .ActiveConnection
Dim Sql As String
Sql = "CREATE TABLE Table1 ( Field_A VARCHAR(10), Field_B VARCHAR(10) );"
.Execute Sql
Sql = "CREATE TABLE Table2 ( Field_B VARCHAR(10), Col_1 VARCHAR(10), Col_2 VARCHAR(10));"
.Execute Sql
Sql = "CREATE VIEW qry_Table2_Combined AS SELECT '2' AS Field_A, Table2.Field_B, Table2.Col_1 + ', ' + Table2.Col_2 AS Combined_Field FROM Table2; "
.Execute Sql
Sql = "INSERT INTO Table1 VALUES (1, NULL);"
.Execute Sql
Sql = "INSERT INTO Table1 VALUES (1, NULL);"
.Execute Sql
Sql = "INSERT INTO Table1 VALUES (2, 1);"
.Execute Sql
Sql = "INSERT INTO Table1 VALUES (2, 2);"
.Execute Sql
Sql = "INSERT INTO Table2 VALUES (1, 'John', 'Doe');"
.Execute Sql
Sql = _
"SELECT " & _
"Table1.Field_A, " & _
"Table1.Field_B, " & _
"qry_Table2_Combined.Field_A, " & _
"qry_Table2_Combined.Field_B, " & _
"qry_Table2_Combined.Combined_Field " & _
"FROM Table1 " & _
"LEFT JOIN qry_Table2_Combined " & _
" ON (Table1.Field_A = qry_Table2_Combined.Field_A) " & _
"AND (Table1.Field_B = qry_Table2_Combined.Field_B);"
' Sql = _
' "SELECT Table1.Field_A, Table1.Field_B, " & _
' " qry_Table2_Combined.Field_A, " & _
' " qry_Table2_Combined.Field_B, " & _
' " qry_Table2_Combined.Combined_Field " & _
' " FROM Table1 " & _
' " INNER JOIN qry_Table2_Combined " & _
' " ON (Table1.Field_A = qry_Table2_Combined.Field_A) " & _
' " AND (Table1.Field_B = qry_Table2_Combined.Field_B) " & _
' "UNION ALL " & _
' "SELECT Table1.Field_A, Table1.Field_B, " & _
' " NULL AS Field_A, " & _
' " NULL AS Field_B, " & _
' " NULL AS Combined_Field " & _
' " FROM Table1 " & _
' " WHERE NOT EXISTS ( SELECT * " & _
' " FROM qry_Table2_Combined " & _
' " WHERE (Table1.Field_A = qry_Table2_Combined.Field_A) " & _
' " AND (Table1.Field_B = qry_Table2_Combined.Field_B) );"
Dim rs
Set rs = .Execute(Sql)
MsgBox rs.GetString(2, , vbTab & vbTab, , "<NULL>")
End With
Set .ActiveConnection = Nothing
End With
End Sub

Answer 3

isn't this a problem with MSAccess parsing. for a test change the field names in the query to Field_C and Field_D and see if you still have the same problem