如何在MySQL中获取此表？

Question

我有以下架构：

我想生成一个包含3列的表格 - 标签的名称，线程的名称，答案数。

示例：

Foo bar 5
Foo something 6
Foo2 somethingElse 3

更重要的是，有一个表“priority_threads”（带有线程的1：1）。我想在这个表中只显示那些优先级线程。

我该怎么做？我完全不知道如何开始。我唯一做的就是：

SELECT tag.name, thread.title, COUNT(answer.id_answer)
FROM tag, thread, answer
WHERE thread.id_tag = tag.id_tag 
AND answer.id_thread = thread.id_thread
AND thread.id_thread = priority_threads.id_thread
GROUP BY tag.name, thread.title
ORDER BY tag.name;

然而，使用此查询，答案的count列中的所有值都相同 - 来自Answer表的count（*）...

Answer 1

试试这个：

SELECT tag.name, thread.title, COUNT(answer.id_answer)
FROM tag
JOIN thread ON
tag.id_tag=thread.id_tag
JOIN answer ON
    thread.id_thread = answer.id_thread
JOIN priority_threads
ON thread.id_thread = priority_threads.id_thread
GROUP BY tag.name, thread.title
ORDER BY tag.name;

以下是查询的SQLfiddle链接以进行测试;

示例代码：

 CREATE TABLE Tag
    (
 id_tag int auto_increment primary key, 
 name varchar(20)
 );

INSERT INTO Tag
(name)
VALUES
('Foo'), 
('Foo2');

CREATE TABLE Thread
    (
 id_thread int auto_increment primary key, 
 id_tag int, 
 title varchar(20)
 );

INSERT INTO Thread
(id_tag, title)
VALUES
(1,'Bar'), 
(1,'Something'),
(2,'SomethingElse');

CREATE TABLE Answer
    (
 id_answer int auto_increment primary key, 
 id_thread int,
 text varchar(200)
 );

INSERT INTO Answer
(id_thread, text)
VALUES
(1,'jlkjalkjl'), 
(1,'ioioixhakjjkj'),
(1, 'jjalkjijkajk'),
(1, 'jjalkjijkajk'),
(1, 'jjalkjijkajk'),
(2, 'jjalkjijkajk'),
(2, 'jjalkjijkajk'),
(2, 'jjalkjijkajk'),
(2, 'qqweeweraata'),
(2, 'jjalkjijkajk'),
(2, 'jjalkjijkajk'),
(3, 'popoapopop'),
(3, 'zkkasjkljz'),
(3, 'jjalkjijkajk')
;

CREATE TABLE priority_threads
    (
 id_priority_threads int auto_increment primary key, 
 id_thread int, 
 priority int
 );

INSERT INTO priority_threads
(id_thread, priority)
VALUES
(1,1), 
(3,2);

Answer 2

这样：

    SELECT tag.name, thread.title, COUNT(answer.id_answer)
FROM tag, thread, answer
WHERE thread.id_tag = tag.id_tag 
AND answer.id_thread = thread.id_thread
GROUP BY tag.name, thread.title
ORDER BY tag.name;

可能如下所示：

SELECT tag.name, thread.title, COUNT(answer.id_answer)
FROM tag
JOIN thread ON
tag.id_tag=thread.id_tag
JOIN answer ON
    thread.id_thread = answer.id_thread
GROUP BY tag.name, thread.title
ORDER BY tag.name;

结果相同，前者是最受欢迎的。好的，但我认为这里的真正问题是你发布的代码中的这一行：

AND thread.id_thread = priority_threads.id_thread

您正在引用另一个表priority_threads，它不在您的查询中。添加联接或删除它，你应该很高兴。

-james