Efficient checking of possible duplicate entities - database

Question

I have a requirement to produce a list of possible duplicates before a user saves an entity to the database and warn them of the possible duplicates.
There are 7 criteria on which we should check the for duplicates and if at least 3 match we should flag this up to the user.
The criteria will all match on ID, so there is no fuzzy string matching needed but my problem comes from the fact that there are many possible ways (99 ways if I've done my sums corerctly) for at least 3 items to match from the list of 7 possibles.
I don't want to have to do 99 separate db queries to find my search results and nor do I want to bring the whole lot back from the db and filter on the client side. We're probably only talking of a few tens of thousands of records at present, but this will grow into the millions as the system matures.
Anyone got any thoughs of a nice efficient way to do this?
I was considering a simple OR query to get the records where at least one field matches from the db and then doing some processing on the client to filter it some more, but a few of the fields have very low cardinality and won't actually reduce the numbers by a huge amount.
Thanks
Jon

Answer 1

OR and CASE summing will work but are quite inefficient, since they don't use indexes.
You need to make UNION for indexes to be usable.
If a user enters name, phone, email and address into the database, and you want to check all records that match at least 3 of these fields, you issue:
SELECT i.*
FROM (
SELECT id, COUNT(*)
FROM (
SELECT id
FROM t_info t
WHERE name = 'Eve Chianese'
UNION ALL
SELECT id
FROM t_info t
WHERE phone = '[phone]'
UNION ALL
SELECT id
FROM t_info t
WHERE email = '42#example.com'
UNION ALL
SELECT id
FROM t_info t
WHERE address = '42 North Lane'
) q
GROUP BY
id
HAVING COUNT(*) >= 3
) dq
JOIN t_info i
ON i.id = dq.id
This will use indexes on these fields and the query will be fast.
See this article in my blog for details:
Matching 3 of 4: how to match a record which matches at least 3 of 4 possible conditions
Also see this question the article is based upon.
If you want to have a list of DISTINCT values in the existing data, you just wrap this query into a subquery:
SELECT i.*
FROM t_info i1
WHERE EXISTS
(
SELECT 1
FROM (
SELECT id
FROM t_info t
WHERE name = i1.name
UNION ALL
SELECT id
FROM t_info t
WHERE phone = i1.phone
UNION ALL
SELECT id
FROM t_info t
WHERE email = i1.email
UNION ALL
SELECT id
FROM t_info t
WHERE address = i1.address
) q
GROUP BY
id
HAVING COUNT(*) >= 3
)
Note that this DISTINCT is not transitive: if A matches B and B matches C, this does not mean that A matches C.

Answer 2

You might want something like the following:
SELECT id
FROM
(select id, CASE fld1 WHEN input1 THEN 1 ELSE 0 "rule1",
CASE fld2 when input2 THEN 1 ELSE 0 "rule2",
...,
CASE fld7 when input7 THEN 1 ELSE 0 "rule2",
FROM table)
WHERE rule1+rule2+rule3+...+rule4 >= 3
This isn't tested, but it shows a way to tackle this.

Answer 3

What DBS are you using? Some support using such constraints by using server side code.

Answer 4

Have you considered using a stored procedure with a cursor? You could then do your OR query and then step through the records one-by-one looking for matches. Using a stored procedure would allow you to do all the checking on the server.
However, I think a table scan with millions of records is always going to be slow. I think you should work out which of the 7 fields are most likely to match are make sure these are indexed.

Answer 5

I'm assuming your system is trying to match tag ids of a certain post, or something similar. This is a multi-to-multi relationship and you should have three tables to handle it. One for the post, one for tags and one for post and tags relationship.
If my assumptions are correct then the best way to handle this is:
SELECT postid, count(tagid) as common_tag_count
FROM posts_to_tags
WHERE tagid IN (tag1, tag2, tag3, ...)
GROUP BY postid
HAVING count(tagid) > 3;