SQL: Split a row into multiple rows based on certain rules

Question

Here is my table in my Microsoft Access database:

Terr(Prim)    Terr(Sec)   Qty
----------    ---------   -------  
A             A           0.5   
A             B           0.5   

I want to change it to like this through SQL:

Type           Terr   Qty
-----------    ----   ----  
Original       A      0.5   
Original       A      0.5
50-50 give     A      -0.5
50-50 take     B      0.5

Rules:

1. Create two columns, [Type] and [Terr]

2. if [Terr(Prim)] = [Terr (Sec)], then keep the row and [Type] = "Original"

3. if [Terr (Prim)] <> [Terr (Sec)], then:

   • Row 1: [Type] = "Original", [Terr] = [Terr(Prim)], the other columns remain the same

   • Row 2: [Type] = "50-50 Give", [Terr] = [Terr (Prim)], [Qty] turns into negative

   • Row 3: [Type] = "50-50 take", [Terr] = [Terr (Sec)], the other columns remain the same

4. delete [Terr(Prim)] and [Terr (Sec)]

Answer 1

I think you can use a query like this:

SELECT "Original" As [Type], [TerrPrim] AS [Terr], Qty
FROM t
UNION ALL
SELECT "50-50 Give" As [Type], [TerrPrim] AS [Terr], -Qty
FROM t
WHERE [TerrPrim] <> [TerrSec]
UNION ALL
SELECT "50-50 take" As [Type], [TerrSec] AS Terr, Qty
FROM t
WHERE [TerrPrim] <> [TerrSec];