请教一个SQL如何写

Question

表字段包括：用户id,年，月，日

CREATE TABLE `a` (
  `user_id` int(11) NOT NULL,
  `day` int(10) NOT NULL,
  `month` int(10) DEFAULT NULL,
  `year` int(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

请查询在2020年每个月都有记录的用户id?

Answer 1

SELECT 
    user_id
FROM
    (SELECT 
        user_id
    FROM
        a
    WHERE
        year = 2020
    GROUP BY user_id , month) b 
GROUP BY user_id
HAVING COUNT(user_id) = 12;