mercoledì 15 settembre 2010

Doing INTERSECT and MINUS in MySQL

Doing an INTERSECT
 
An INTERSECT is simply an inner join where we compare the tuples of one table with those of the other, and select those that appear in both while weeding out duplicates. So

SELECT member_id, name FROM a
INTERSECT
SELECT member_id, name FROM b


can simply be rewritten to

SELECT a.member_id, a.name
FROM a INNER JOIN b
USING (member_id, name)



Performing a MINUS
To transform the statement

SELECT member_id, name FROM a
MINUS
SELECT member_id, name FROM b


into something that MySQL can process, we can utilize subqueries (available from MySQL 4.1 onward). The easy-to-understand transformation is:

SELECT DISTINCT member_id, name
FROM a
WHERE (member_id, name) NOT IN
(SELECT member_id, name FROM table2);



Of course, to any long-time MySQL user, this is immediately obvious as the classical use-left-join-to-find-what-isn’t-in-the-other-table:

SELECT DISTINCT a.member_id, a.name
FROM a LEFT JOIN b USING (member_id, name)
WHERE b.member_id IS NULL


which tends to be a lot more efficient.

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