您可以按app和server分组，并按照您所描述的方式进行操作：

SELECT app, server
       CASE
          WHEN     has_a AND     has_b AND     has_c
          THEN 'No action'
          WHEN     has_a AND NOT has_b AND NOT has_c
          THEN 'add to B'
          WHEN     has_a AND     has_b AND NOT has_c
          THEN 'add to C'
          WHEN NOT has_a AND     has_b AND     has_c
          THEN 'remove from B and C'
       END AS action
FROM (SELECT app, server,
             bool_or(source = 'A') AS has_a,
             bool_or(source = 'B') AS has_b,
             bool_or(source = 'C') AS has_c
      FROM tab
      GROUP BY app, server) AS grouped;

请注意，对于您的问题中未提及的情况，action将为NULL。