Thanks for the reply. Here's a short bit of SQL that shows the problem. The SELECT query at the end fails with:
DROP TABLE IF EXISTS ex_users;
CREATE TABLE ex_users (id BIGINT(19) NOT NULL,
name CHAR(20) NOT NULL,
PRIMARY KEY(id)
);
INSERT INTO ex_users(id, name) VALUES (1, 'Bill Hicks');
INSERT INTO ex_users(id, name) VALUES (2, 'PJ Harvey');
DROP TABLE IF EXISTS ex_data;
CREATE TABLE ex_data (id BIGINT(19) NOT NULL,
user_id BIGINT(19),
date TIMESTAMP(23, 10) NOT NULL,
t BIGINT(19) NOT NULL, PRIMARY KEY(id),
CONSTRAINT fk_user_id FOREIGN KEY(user_id) REFERENCES ex_users(id)
);
INSERT INTO ex_data(id, user_id, date, t) VALUES (1, 1, '2012-01-01', 10);
INSERT INTO ex_data(id, user_id, date, t) VALUES (2, 1, '2012-01-02', 20);
INSERT INTO ex_data(id, user_id, date, t) VALUES (3, 1, '2012-01-03', 30);
INSERT INTO ex_data(id, user_id, date, t) VALUES (4, 1, '2012-01-04', 40);
INSERT INTO ex_data(id, user_id, date, t) VALUES (5, 2, '2012-01-01', 11);
INSERT INTO ex_data(id, user_id, date, t) VALUES (6, 2, '2012-01-02', 21);
INSERT INTO ex_data(id, user_id, date, t) VALUES (7, 2, '2012-01-03', 31);
INSERT INTO ex_data(id, user_id, date, t) VALUES (8, 2, '2012-01-04', 41);
SELECT * FROM ex_data GROUP by date