SQL - expand dataset into lookup table? - sql

Question

I currently have a legacy table that looks like the one below.
This is a set of rules that our business has stored over the years. the issue is the "all" and "both" values really should be separated out into rows so they can be queried more efficiently.
For example, the contract length column can only ever be between 1 and 5, the type column can only ever be "gas" or "water" and the sales channel "internal" or "external". Instead of saying all or both, another row should exist with the specific rule and the table should look like the below.
So this will have a row for every variation in the first table.
I didn't think it would be a long task to manually do myself. but I was wrong :)
Does anyone have any idea on how to achieve this quickly in SQL? I would say what I have tried so far...but I am completely stumped on this one so am wondering if it can even be done at all?

Answer 1

This could be done in a single sql statement, but for the sake of your mental health and the ability to check interim result sets before you get to the final output is probably a lot healthier and less risky.
I would approach this with a UNION query, one set of UNIONs for each column that should be split out to more granular rows.
For instance for contractlength:
SELECT Supplier, 1, Type, SalesChannel FROM yourtable WHERE contractLength in ('1', 'All')
UNION ALL
SELECT Supplier, 2, Type, SalesChannel FROM yourtable WHERE contractLength in ('2', 'All')
UNION ALL
SELECT Supplier, 3, Type, SalesChannel FROM yourtable WHERE contractLength in ('3', 'All')
UNION ALL
SELECT Supplier, 4, Type, SalesChannel FROM yourtable WHERE contractLength in ('4', 'All')
UNION ALL
SELECT Supplier, 5, Type, SalesChannel FROM yourtable WHERE contractLength in ('5', 'All')
You can write those results out to a temp table, and then build your query for type on top of it writing to a new temp table.
SELECT Supplier, contractLength, 'Gas', SalesChannel FROM previousTempTable WHERE type in ('Gas','Both')
UNION ALL
SELECT Supplier, contractLength, 'Water', SalesChannel FROM previousTempTable WHERE type in ('Gas','Both')
Rinse and repeat for SalesChannel.
There's other more elegant ways to solve this with some SELECT DISTINCT and cross joins, but your list of values for each column is limited and this solution I'm proposing feels like a quick easy way to get your data in shape. It's also easy to understand if this is auditable data or the process needs to be repeated.

Answer 2

You don't need to query your table multiple times, or use temp tables. You can do this pretty elegantly with conditional unpivots, by using CROSS APPLY
SELECT
t.Supplier,
c1.ContractLength,
c2.Type,
c3.SalesChannel
FROM YourTable t
CROSS APPLY (
SELECT t.ContractLength
WHERE t.ContractLength <> 'All'
UNION ALL
SELECT *
FROM (VALUES
(1),(2),(3),(4),(5)
) v(ContractLength)
WHERE t.ContractLength = 'All'
) c1
CROSS APPLY (
SELECT t.Type
WHERE t.Type <> 'Both'
UNION ALL
SELECT *
FROM (VALUES
('Gas'),('Water')
) v(Type)
WHERE t.Type = 'Both'
) c2
CROSS APPLY (
SELECT t.SalesChannel
WHERE t.SalesChannel <> 'Both'
UNION ALL
SELECT *
FROM (VALUES
('Internal'),('External')
) v(SalesChannel)
WHERE t.SalesChannel = 'Both'
) c3;
A somewhat less efficient, but more compact, version of the same, is to use normal joins against the VALUES clauses
SELECT
t.Supplier,
c1.ContractLength,
c2.Type,
c3.SalesChannel
FROM YourTable t
JOIN (VALUES
(1),(2),(3),(4),(5)
) c1(ContractLength)
ON c1.ContractLength = t.ContractLength OR t.ContractLength = 'All'
JOIN (VALUES
('Gas'),('Water')
) c2(Type)
ON c2.Type = t.Type OR t.Type = 'Both'
JOIN (VALUES
('Internal'),('External')
) c3(SalesChannel)
ON c3.SalesChannel = t.SalesChannel OR t.SalesChannel = 'Both';