Sqlite - 同一個表上的多個INNER JOIN限制

Question

在我的Android應用程序中，我有一個包含6個主題ID的Sqlite數據庫。現在我想要獲得每個主題的一定數量的問題。這意味着：15個問題的主題ID 1,5，主題ID 2,7的主題ID 3和4和3的主題ID 5和6.

我想我需要一個multipe內部連接和限制函數，但我不太瞭解如何構建這樣的查詢。

你有什麼想法嗎？

Answer 1

一個簡單的方法是使用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) ; 

我沒有意識到的SQLite似乎與子查詢和union all問題。在任何情況下，這個版本似乎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

要多發的查詢串連行，使用compound query。

LIMIT不允許對在化合物查詢的查詢，所以它必須被移動到的子查詢：

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

嘗試這個實施例中使用的行從兩個或更多表http://www.w3schools.com/sql/sql_join.asp