How to simulate UNPIVOT in Access?

Question

UNPIVOT is available in MS SQL-Server 2005, but AFAIK not in MS Access 2010. How can it be implemented with on-board means? For example, I have a table

ID | A | B | C | Key 1 | Key 2 | Key 3
---------------------------------------
 1 | x | y | z |     3 |   199 |   452
 2 | x | y | z |    57 |   234 |   452

and want to have a table like

ID | A | B | C | Key
--------------------
 1 | x | y | z |   3
 2 | x | y | z |  57
 1 | x | y | z | 199
 2 | x | y | z | 234
 2 | x | y | z | 452

Key 452 is a special case. Currently I do the rotation in OLEDB/ATL C++. Although it is fast enough I'm still curious. What is the most efficient SQL statement for Access 2010 here?

Answer 1

This query ...

SELECT ID, A, B, C, [Key 1] AS key_field
FROM tblUnpivotSource
UNION ALL
SELECT ID, A, B, C, [Key 2] AS key_field
FROM tblUnpivotSource
UNION ALL
SELECT ID, A, B, C, [Key 3] AS key_field
FROM tblUnpivotSource;

... returns this recordset (using your sample table values as tblUnpivotSource) ...

ID A B C key_field
-- - - - ---------
 1 x y z         3
 2 x y z        57
 1 x y z       199
 2 x y z       234
 1 x y z       452
 2 x y z       452

Answer 2

Unfortunately there is no easy way to do this with access. You can do this by using a UNION to get each value

SELECT ID, A, B, C, [Key 1] As key
FROM Table
WHERE [Key 1] = 3

UNION ALL

SELECT ID, A, B, C, [Key 1] As key
FROM Table
WHERE [Key 1] = 57

UNION ALL

SELECT ID, A, B, C, [Key 2] As key
FROM Table
WHERE [Key 2] = 199

UNION ALL

SELECT ID, A, B, C, [Key 2] As key
FROM Table
WHERE [Key 2] = 234

UNION ALL

SELECT ID, A, B, C, [Key 3] As key
FROM Table
WHERE [Key 3] = 452

Answer 3

1. You can create a auxiliary table with all column names as values (can use excel copy the first row of your table to excel > paste special > transpose)

2. Create in your table a auto increment column and index this column

3. Create a new cross join query like the following

SELECT ID, A, B, C
       , AUX_TABLE.KEY_FIELD
       , DLookUp("[" & [AUX_TABLE].[KEY_FIELD] & "]","TABLE","[ID] = " & [TABLE].[ID]) AS KEY_VALUE
FROM TABLE, AUX_TABLE;

Downside would be you have to maintain AUX_TABLE to keep that working. But if this is a one-time-thing this might be the way to go.