sql - 如何从子查询返回两个字段

Question

下面的查询有效，但是我想知道是否有更好的方法可以做到这一点。

有一个子查询使用两个子查询。这两个子查询是相同的，但是返回两个不同的字段。有没有一种方法可以只使用一个返回两个字段的子查询？

我检查了类似的问题（this，this和this），但我认为它们不适用于这种情况。

这是查询：

SELECT *,
       time(strftime('%s', EndTime) - strftime('%s', StartTime), 'unixepoch') AS Duration
  FROM (
           SELECT (
                      SELECT Time
                        FROM Log AS LogStart
                       WHERE LogStart.User = Log.User AND 
                             LogStart.Time <= Log.Time AND 
                             LogStart.[Action] != 'done'
                       ORDER BY LogStart.Time DESC
                       LIMIT 1
                  )
                  AS StartTime,
                  Time AS EndTime,
                  User,
                  (
                      SELECT [Action]
                        FROM Log AS LogStart
                       WHERE LogStart.User = Log.User AND 
                             LogStart.Time <= Log.Time AND 
                             LogStart.[Action] != 'done'
                       ORDER BY LogStart.Time DESC
                       LIMIT 1
                  )
                  AS [Action]
             FROM Log
            WHERE [Action] = 'done'
       )
 ORDER BY duration DESC;

这是一些测试数据：

CREATE TABLE Log (
    Time     DATETIME,
    User     CHAR,
    [Action] CHAR
);

insert into Log values('2017-01-01 10:00:00', 'Joe', 'Play');
insert into Log values('2017-01-01 10:01:00', 'Joe', 'done');
insert into Log values('2017-01-01 10:02:00', 'Joe', 'Sing');
insert into Log values('2017-01-01 10:03:00', 'Joe', 'done');
insert into Log values('2017-01-01 10:04:00', 'Ann', 'Play');
insert into Log values('2017-01-01 10:04:30', 'Bob', 'Action without corresponding "done" which must be ignored');
insert into Log values('2017-01-01 10:05:00', 'Joe', 'Play');
insert into Log values('2017-01-01 10:06:00', 'Ann', 'done');
insert into Log values('2017-01-01 10:07:00', 'Joe', 'done');
insert into Log values('2017-01-01 10:08:00', 'Ann', 'Play');
insert into Log values('2017-01-01 10:09:00', 'Ann', 'done');

Answer 1

使用自我连接...我没有SQLLite，因此语法可能不在此处，但是您应该明白...

Select e.*, time(strftime('%s', e.[Time]) - 
           strftime('%s', s.[Time]), 'unixepoch') AS Duration
From log e join log s  -- s is for the startevent; e for end event
    on s.[User] = e.[User]
       and s.[Action] != 'done'
       and e.[Action] = 'done'
       and s.[Time] = 
          (Select Max([time] from log
           where [User] = e.[User]
                   and [time] <= e.[Time]
                   and [Action] != 'done')