SQL join on multiple levels - sql

Question

I'm struggle with a SQL join for parent client records and just posted a query, but I realised after posting it that my example was slightly wrong (but still a useful post) so i've created a new one, that's more accurate :)
If i have the following database structure:
Table Regions
|Region_no | Region_Level | owning_region_no |
| 1 | 1 | |
| 2 | 2 | 1 |
| 3 | 2 | 1 |
| 4 | 3 | 2 |
| 5 | 3 | 2 |
| 6 | 3 | 3 |
Table Postcodes
| Postcode | Region_no |
| PO32 3AE | 4 |
| PO32 3AA | 5 |
| PO32 3AF | 6 |
Table UnitsMappings
| Unit_No | region_no |
| 1 | 1 |
| 1 | 2 |
| 2 | 2 |
| 2 | 1 |
| 3 | 3 |
| 3 | 3 |
| 4 | 6 |
| 4 | 5 |
Table Units
| Unit_no | Unit_Name |
| 1 | South |
| 2 | SouthEast |
| 3 | Central |
| 4 | SouthWest |
[Updated sample answer]
What I really want, is the following:
| Unit_name | Postcode |
| South | PO32 3AE |
| South | PO32 3AA |
| South | PO32 3AF |
| SouthEast | PO32 3AE |
| SouthEast | PO32 3AA |
| SouthEast | PO32 3AF |
Even though South has only regions 1 and 2 mapped to it, the level 3's are mapped to the level 2's and then 1's (and the postcodes are mapped to the level 3's)
Now the difficulty is, that i just want the postcodes (from the postcode tables) which are associated to the level 3 regions levels in regions. So there may be a region level 1 associated to a unit_no, but i need all of the postcodes at the bottom that are mapped to the level 3.

Answer 1

There is no valid unit name in you test data for the region in region level 3.
The region with region level 3 are 4, 5 and 6.
There is no unit associated with region 4, the unit associated with region 5 and 6 is the unit 4, but in the table unit_name there is no unit 4.
If you have the data then this query will get the data
SELECT pc.postcode, u.unit_name
FROM regions reg
INNER JOIN Postcodes pc ON reg.region_no = pc.region_no
INNER JOIN UnitsMappings um ON reg.region_no = um.region_no
INNER JOIN Units u ON um.unit_no = u.unit_no
WHERE reg.region_level = 3
changing the join on Units from INNER to LEFT will get you the postcodes with NULL as unit_name with the test data.

Answer 2

based on your details here's my mapping ,
unit => units_mapping
units_mapping => (Postcodes or regions) // assuming using all table we use postcodes
Postcodes => regions
regions (main)
select reg.region_no, Units.name , pc.postcodes
from regions reg,
Postcodes pc,
UnitsMappings UM,
Units units
where reg.region_no = pc.region_no and
pc.region_no = UM.region_no and
UM.Unit_No = Units.Unit_no
order by reg.region_no asc
if there is null in one table, the record won't appear ...
to handle use inner join
not yet tested ..... just assume :)

Answer 3

Try this,
Select u.Unit_name,pc.Postcode from Units as u
inner join UnitsMappings um on u.Unit_no = um.Unit_no
inner join Postcodes as pc on pc.Region_no = um.region_no