如果值存在，则从其他表中选择

select r.id,     IF(c.name != '',c.name,r.name) as name,     r.age  FROM Records r  LEFT JOIN Clients c ON c.id = r.id  GROUP BY c.id

使用上面的查询。

编辑：

  SELECt t.id, t.name, t.age FROM (    (        SELECt r.id,        CASE WHEN c.name <> '' THEN c.name ELSE r.name END  as name,        r.age         FROM Records r         LEFT JOIN Clients c ON c.id = r.id    )    UNIOn     (        SELECt c.id, c.name, null as age FROM Clients c where c.id NOT IN (select id from Records)    )) as t ORDER BY t.id

使用此查询。