Sqlite - 同一个表上的多个INNER JOIN限制

Question

在我的Android应用程序中，我有一个包含6个主题ID的Sqlite数据库。现在我想要获得每个主题的一定数量的问题。这意味着：15个问题的主题ID 1,5，主题ID 2,7的主题ID 3和4和3的主题ID 5和6.

我想我需要一个multipe内部连接和限制函数，但我不太了解如何构建这样的查询。

你有什么想法吗？

Answer 1

一个简单的方法是使用union all和limit：

(select q.* from questions q where q.topicid = 1 order by random() limit 15) union all 
(select q.* from questions q where q.topicid = 2 order by random() limit 5) union all 
(select q.* from questions q where q.topicid in (3, 4) order by random() limit 7) union all 
(select q.* from questions q where q.topicid in (5, 6) order by random() limit 3) ; 

我没有意识到的SQLite似乎与子查询和union all问题。在任何情况下，这个版本似乎work：

with q1 as 
    (select q.* from questions q where q.topicid = 1 order by random() limit 15), 
    q2 as 
    (select q.* from questions q where q.topicid = 2 order by random() limit 5), 
    q34 as 
    (select q.* from questions q where q.topicid in (3, 4) order by random() limit 7), 
    q56 as 
    (select q.* from questions q where q.topicid in (5, 6) order by random() limit 3) 
select * from q1 union all 
select * from q2 union all 
select * from q34 union all 
select * from q56; 

Answer 2

要多发的查询串连行，使用compound query。

LIMIT不允许对在化合物查询的查询，所以它必须被移动到的子查询：

SELECT * FROM (SELECT * FROM Questions 
       WHERE TopicID = 1 
       ORDER BY random() LIMIT 15) 
UNION ALL 
SELECT * FROM (SELECT * FROM Questions 
       WHERE TopicID = 2 
       ORDER BY random() LIMIT 5) 
UNION ALL 
... 

Answer 3

尝试这个实施例中使用的行从两个或更多表http://www.w3schools.com/sql/sql_join.asp