MySQL: combining 2 queries (one empty) using UNION ALL results in error “1048 - Column cannot be NULL”

Question

I need to return two different results in a single query. When I run them independently , the first returns no rows (that's fine) and the second returns some rows (also fine). When I UNION ALL them, I get 1048 - Column "Date" cannot be null .

I need resulting rows of Date, PW, errors which I will feed a graph to show me what's going on in the system at the points in time specified by Date . In both tables, Date is of the format DateTime and must never be NULL.

SELECT `Date`, COUNT(`ID`) AS `PW`, 0 AS `errors`
FROM `systemlogins`
WHERE `Result` = 'PasswordFailure' AND `Date` >= DATE_SUB(NOW(), INTERVAL 1 DAY)

UNION ALL

SELECT `Date`,  0 AS `PW`, COUNT(`ID`) AS `errors`
FROM `systemerrors`
WHERE `Date` >= DATE_SUB(NOW(), INTERVAL 1 DAY)

GROUP BY ( 4 * HOUR( `Date` ) + FLOOR( MINUTE( `Date` )/15)) --i.e. full 1/4s of hour
ORDER BY ( 4 * HOUR( `Date` ) + FLOOR( MINUTE( `Date` )/15))

I have read that MySQL might ignore tables' NOT NULL conditions in UNIONs, causing that error. I have indeed removed the "NOT NULL" restriction on the tables and, tada, it works. Now, those restrictions have been put there for a reason and I would like to keep them while running the aforementioned query - is there any way?

Edit : Order is the villain - removing it returns a correct result, albeit with one empty row where Date is NULL . For my purposes, I need to order the results by Date somehow.

Answer 1

Why are you selecting the Date column? Since you are using a aggregate function COUNT , but there is no GROUP BY clause in any of the selects, seems to me that you do not care about the Date column.

Try adding a GROUP BY clause, or removing the Date column from the select.