Searching for data in SQL - mysql

Question

Please take a look at the following table:
I am building a search engine which returns card_id values, based on search of category_id and value_id values.
To better explain the search mechanism, imagine that we are trying to find a car (card_id) by supplying information what part (value_id) the car should has in every category (category_id).
In example, we may want to find a car (card_id), where category "Fuel Type" (category_id) has a value "Diesel" (value_id), and category "Gearbox" (category_id) has a value "Manual" (value_id).
My problem is that my knowledge is not sufficient to build a query, which will returns card_ids which contains more than one pair of category_id and value_id.
For example, if I want to search a car with diesel engine, I could build a query like this:
SELECT card_id FROM cars WHERE category_id=1 AND value_id=2
where category_id = 1 is a category "Fuel Type" and value_id = 2 is "Diesel".
My question is, how can I build a query, which will look for more category-value pairs? For example, I want to look for diesel cars with manual gearbox.
Any help will be very appreciated. Thank you in advance.

Answer 1

You can do this using aggregation and a having clause:
SELECT card_id
FROM cars
GROUP BY card_id
HAVING SUM(category_id = 1 AND value_id = 2) > 0 AND
SUM(category_id = 3 and value_id = 43) > 0;
Each condition in the having clause counts the number of rows that match a given condition. You can add as many conditions as you like. The first, for instance, says that there is at least one row where the category is 1 and the value is 2.
SQL Fiddle

Answer 2

Another approach is to create a user defined function that takes a table of attribute/value pairs and returns a table of matching cars. This has the advantage of allowing an arbitrary number of attribute/value pairs without resorting to dynamic SQL.
--Declare a "sample" table for proof of concept, replace this with your real data table
DECLARE #T TABLE(PID int, Attr Int, Val int)
--Populate the data table
INSERT INTO #T(PID , Attr , Val) VALUES (1,1,1), (1,3,5),(1,7,9),(2,1,2),(2,3,5),(2,7,9),(3,1,1),(3,3,5), (3,7,9)
--Declare this as a User Defined Table Type, the function would take this as an input
DECLARE #C TABLE(Attr Int, Val int)
--This would be populated by the code that calls the function
INSERT INTO #C (Attr , Val) VALUES (1,1),(7,9)
--The function (or stored procedure) body begins here
--Get a list of IDs for which there is not a requested attribute that doesn't have a matching value for that ID
SELECT DISTINCT PID
FROM #T as T
WHERE NOT EXISTS (SELECT C.ATTR FROM #C as C
WHERE NOT EXISTS (SELECT * FROM #T as I
WHERE I.Attr = C.Attr and I.Val = C.Val and I.PID = T.PID ))