查询两张表中不相同的数据

SELECT 
  c.userId,
  c.userName 
FROM
  (SELECT 
    * 
  FROM
    a 
  UNION
  SELECT 
    * 
  FROM
    b) c 
WHERE c.userId NOT IN 
  (SELECT 
    b.`userId` 
  FROM
    a 
    JOIN b 
      ON a.`userId` = b.`userId`) ;