Rule Table (With wild cards) - sql

Question

I have searched the web for something similar to my question, but have not found anything. I am looking for a way to apply "rules" with a SQL query.
My input data schema:
a: Int (NOT NULL)
b: Int (NOT NULL)
c: Int (NOT NULL)
The rule table schema:
a: Int (NULLABLE)
b: Int (NULLABLE)
c: Int (NULLABLE)
result: Int (NOT NULL)
There could be multiple rules which could "match" with the data. NULL represents a wildcard (could be any value). For example:
Input Data
a | b | c
1 | 2 | 3
Rules table:
a | b | c | result
1 | 2 | NULL| 99
1 | NULL| NULL| 101
1 | 2 | 3 | 203
When the rules are applied, it should match with the row which has the most matches (row 3 in this case).
I have come up with a query, which appears to be working, but it is not perfect. It can be slow if the "rules" table gets significant in size, and I'm worried there are edge cases I could be missing.
SELECT input.*,
COALESCE(rule.result, -1) as 'RuleResult'
FROM dbo.input input
OUTER APPLY (
SELECT TOP 1 result
FROM dbo.RuleTable rt
WHERE (input.a = rt.a OR rt.a IS NULL)
AND (input.b = rt.b OR rt.b IS NULL)
AND (input.c = rt.c OR rt.c IS NULL)
ORDER BY rt.a DESC, rt.b DESC, rt.c DESC
) rule
The idea is: The outer apply will run the query for each row in the input. The ORDER BY clause will set the priority of the rules, and will have the columns with the least number of NULL values at the top. The top row then becomes the result. The ORDER BY clause needs to align with the business need. If there is one rule with an 'a' value, and another with a 'b' value, an input row could match with two rules which only have a single matching condition. Then one still needs to be chosen.
My question: Is this query optimal? Am I missing anything? Are there resources about this out there I have not found? Is there a better way of doing this?
Update 1: After reading replies and discussing this with other people, here are some additional thoughts (still need to test these out):
Can we reverse the join? Basically join the rule table to the data
Can we eliminate input data that we know no rules apply to? Is this possible with NULLS (wildcard) values, or would this help?
Note: I'm still working through the thought process on this, so I may not be super clear yet.

Answer 1

Maybe the ORDER BY in the OUTER APPLY could use a small change
ORDER BY (iif(rt.a is null,0,1)
+iif(rt.b is null,0,1)
+iif(rt.c is null,0,1)) desc,
rt.a desc, rt.b desc, rt.c desc
Sample Data
create table input (a int, b int, c int);
insert into input values
(1,2,3),
(1,2,null),
(1,null,null),
(4,5,6);
create table RuleTable (a int, b int, c int, result int);
insert into RuleTable values
(1,2,null, 120),
(1,null,null, 100),
(1,2,3, 123),
(4,null,null, 400),
(null,5,6, 056);
Query
SELECT input.*
, COALESCE(ruled.result, -1) as RuleResult
FROM input input
OUTER APPLY (
SELECT TOP 1 result
FROM RuleTable as rt
WHERE (input.a = rt.a OR rt.a IS NULL)
AND (input.b = rt.b OR rt.b IS NULL)
AND (input.c = rt.c OR rt.c IS NULL)
ORDER BY (iif(rt.a is null,0,1)
+iif(rt.b is null,0,1)
+iif(rt.c is null,0,1)) desc,
rt.a desc, rt.b desc, rt.c desc
) ruled
ORDER BY a desc, b desc, c desc;
Result
a
b
c
RuleResult
4
5
6
56
1
2
3
123
1
2
null
120
1
null
null
100
Test on db<>fiddle here