How to get the intersection of two columns - mysql

Question

For instance I have table A and table B
a.data = {1,2,3,4,5,6}
b.data = {4,5,7}
If you want to lookup one value in a.data or b.data you can use FIND_IN_SET(3, b.data).
But I want to know if at least all the values of b.data are in a.data, or else if I can find
at least the intersection between b.data and a.data. So in this case {4,5}.
WHERE INTERSECT(a.data, b.data) ... something like that. How should I do this in MySQL?
update
The b.data {4,5,7} is the column data of one 1 record, so joining a.data on b.data won't work.
table A
=======
ID DATA
1 {1,2,3,4,5,6}
2 {7,9,12}
table B
=======
ID DATA
1 {4,5,7}
2 {9,10,11,12}

Answer 1

You can take interection of tables using INNER JOIN
have a look at Visual explaination of joins
SELECT fn_intersect_string(a.data, b.data) AS result FROM table_name;
also you can write a user defined function as:
CREATE FUNCTION fn_intersect_string(arg_str1 VARCHAR(255), arg_str2 VARCHAR(255))
RETURNS VARCHAR(255)
BEGIN
SET arg_str1 = CONCAT(arg_str1, ",");
SET #var_result = "";
WHILE(INSTR(arg_str1, ",") > 0)
DO
SET #var_val = SUBSTRING_INDEX(arg_str1, ",", 1);
SET arg_str1 = SUBSTRING(arg_str1, INSTR(arg_str1, ",") + 1);
IF(FIND_IN_SET(#var_val, arg_str2) > 0)
THEN
SET #var_result = CONCAT(#var_result, #var_val, ",");
END IF;
END WHILE;
RETURN TRIM(BOTH "," FROM #var_result);
END;

Answer 2

You get the intersection from an inner join:
SELECT a.data FROM a, b WHERE a.data = b.data
To decide whether b is a subset of a, you can do
SELECT b.data FROM b LEFT JOIN a ON a.data = b.data WHERE a.data IS NULL
This will compute the difference: all values from b which are not contained in a. If it is empty, then b is a subset of a.
You can use both of these approaches as subqueries inside a larger query.

Answer 3

If your column is of type SET, then it is stored as a number internally, and will auto-convert to that number where appropriate. The operations you describe correspond to bit-wise logical operations on those numbers. For example, the intersection can be computed using the bit-wise and of the values from two columns.
a.data & b.data AS intersection,
(a.data & b.data) <> 0 AS aAndBIntersect,
(a.data & b.data) == b.data AS bIsSubsetOfA
This requires that the type of both columns is the same, so that the same strings correspond to the same bits. To turn the result back into a string, you'd could use ELT, but with all the combination that's likely to get ugly. As an alternative, you could save the result in a temporary table with the same data type, storing it as a number and later retrieving it as a string.