マルチカラムなunique key

gistfile1.sql

CREATE TABLE articles (
  blog_id int,
  article_id int,
  unique key (blog_id, article_id)
) engine = mroonga default charset = utf8;

INSERT INTO articles VALUES (1, 100);
UPDATE articles SET blog_id = 2;

SELECT * FROM articles;
-- +---------+------------+
-- | blog_id | article_id |
-- +---------+------------+
-- |       2 |        100 |
-- +---------+------------+
-- 1 row in set (0.00 sec)

SELECT * FROM articles WHERE blog_id = 1;
-- Empty set (0.00 sec)

SELECT * FROM articles WHERE blog_id = 2;
-- +---------+------------+
-- | blog_id | article_id |
-- +---------+------------+
-- |       2 |        100 |
-- +---------+------------+
-- 1 row in set (0.00 sec)

SELECT * FROM articles WHERE blog_id = 2 ORDER BY article_id;
-- +---------+------------+
-- | blog_id | article_id |
-- +---------+------------+
-- |       2 |        100 |
-- +---------+------------+
-- 1 row in set (0.00 sec)