mysql distribution of combinations/values - mysql

Question

I have a mysql table which contains some random combination of numbers. For simplicity take the following table as example:
index|n1|n2|n3
1 1 2 3
2 4 10 32
3 3 10 4
4 35 1 2
5 27 1 3
etc
What I want to find out is the number of times a combination has occured in the table. For instance, how many times has the combination of 4 10 or 1 2 or 1 2 3 or 3 10 4 etc occured.
Do I have to create another table that contains all possible combinations and do comparison from there or is there another way to do this?

Answer 1

For a single combination, this is easy:
SELECT COUNT(*)
FROM my_table
WHERE n1 = 3 AND n2 = 10 AND n3 = 4
If you want to do this with multiple combinations, you could create a (temporary) table of them and join that table with you data, something like this:
CREATE TEMPORARY TABLE combinations (
id INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
n1 INTEGER, n2 INTEGER, n3 INTEGER
);
INSERT INTO combinations (n1, n2, n3) VALUES
(1, 2, NULL), (4, 10, NULL), (1, 2, 3), (3, 10, 4);
SELECT c.n1, c.n2, c.n3, COUNT(t.id) AS num
FROM combinations AS c
LEFT JOIN my_table AS t
ON (c.n1 = t.n1 OR c.n1 IS NULL)
AND (c.n2 = t.n2 OR c.n2 IS NULL)
AND (c.n3 = t.n3 OR c.n3 IS NULL)
GROUP BY c.id;
(demo on SQLize)
Note that this query as written is not very efficient due to the OR c.n? IS NULL clauses, which MySQL isn't smart enough to optimize. If all your combinations contain the same number of terms, you can leave those out, which will allow the query to make use of indexes on the data table.
Ps. With the query above, the combination (1, 2, NULL) won't match (35, 1, 2). However, (NULL, 1, 2) will, so, if you want both, a simple workaround would be to just include both patterns in your table of combinations.
If you actually have many more columns than shown in your example, and you want to match patterns that occur in any set of consecutive columns, then your really should pack your columns into a string and use a LIKE or REGEXP query. For example, if you concatenate all your data columns into a comma-separated string in a column named data, you could search it like this:
INSERT INTO combinations (pattern) VALUES
('1,2'), ('4,10'), ('1,2,3'), ('3,10,4'), ('7,8,9');
SELECT c.pattern, COUNT(t.id) AS num
FROM combinations AS c
LEFT JOIN my_table AS t
ON CONCAT(',', t.data, ',') LIKE CONCAT('%,', c.pattern, ',%')
GROUP BY c.id;
(demo on SQLize)
You could make this query somewhat faster by making the prefixes and suffixes added with CONCAT() part of the actual data in the tables, but this is still going to be a fairly inefficient query if you have a lot of data to search, because it cannot make use of indexes. If you need to do this kind of substring searching on large datasets efficiently, you may want to use something better suited for than specific purpose than MySQL.

Answer 2

You only have three columns in the table, so you are looking for combinations of 1, 2, and 3 elements.
For simplicity, I'll start with the following table:
select index, n1 as n from t union all
select index, n2 from t union all
select index, n3 from t union all
select distinct index, -1 from t union all
select distinct index, -2 from t
Let's call this "values". Now, we want to get all triples from this table for a given index. In this case, -1 and -2 represent NULL.
select (case when v1.n < 0 then NULL else v1.n end) as n1,
(case when v2.n < 0 then NULL else v2.n end) as n2,
(case when v3.n < 0 then NULL else v3.n end) as n3,
count(*) as NumOccurrences
from values v1 join
values v2
on v1.n < v2.n and v1.index = v2.index join
values v3
on v2.n < v3.n and v2.index = v3.index
This is using the join mechanism to generate the combinations.
This method finds all combinations regardless of ordering (so 1, 2, 3 is the same as 2, 3, 1). Also, this ignores duplicates, so it cannot find (1, 2, 2) if 2 is repeated twice.

Answer 3

SELECT
CONCAT(CAST(n1 AS VARCHAR(10)),'|',CAST(n2 AS VARCHAR(10)),'|',CAST(n3 AS VARCHAR(10))) AS Combination,
COUNT(CONCAT(CAST(n1 AS VARCHAR(10)),'|',CAST(n2 AS VARCHAR(10)),'|',CAST(n3 AS VARCHAR(10)))) AS Occurrences
FROM
MyTable
GROUP BY
CONCAT(CAST(n1 AS VARCHAR(10)),'|',CAST(n2 AS VARCHAR(10)),'|',CAST(n3 AS VARCHAR(10)))
This creates a single column that represents the combination of the values within the 3 columns by concatenating the values. It will count the occurrences of each.