有时需要以某一字段作为分组，筛选每一组的另一字段值最大（或最小）的记录。例如，有如下表 app，存储了 app 的 ID、名称、版本号等信息。现在要筛选出每个 app 版本最大的记录。
方法一
SELECT a.* FROM app a WHERE a.app_version = (SELECT MAX(b.app_version) FROM app b WHERE b.app_name = a.app_name) ORDER BY a.app_name;
方法二
SELECT a.* FROM app a WHERE NOT EXISTS(SELECT 1 FROM app b WHERE b.app_name = a.app_name AND b.app_version > a.app_version) ORDER BY a.app_name;
方法三
SELECT a.* FROM app a, (SELECT app_name, MAX(app_version) AS app_version FROM app GROUP BY app_name) b WHERE a.app_name = b.app_name AND a.app_version = b.app_version ORDER BY a.app_name;
方法四
SELECT a.* FROM app a INNER JOIN (SELECT app_name, MAX(app_version) AS app_version FROM app GROUP BY app_name) b ON a.app_name = b.app_name AND a.app_version = b.app_version ORDER BY a.app_name;