CREATE TABLE users ( id INTEGER PRIMARY KEY, joined_on DATE NOT NULL, left_on DATE NULL ); INSERT INTO users VALUES (1, '2014-08-01', '2014-08-10'); INSERT INTO users VALUES (2, '2014-08-01', '2014-08-05'); INSERT INTO users VALUES (3, '2014-08-03', NULL); INSERT INTO users VALUES (4, '2014-08-03', '2014-08-10'); INSERT INTO users VALUES (5, '2014-08-10', NULL); 解答方法 実行方法は自由ですが、SQL Fiddleを使うとお手軽かも