How to get this table in MySQL? - mysql

Question

I have the following schema:
And I want to generate a table with 3 columns - tag's name, thread's name, answers count.
Example:
Foo bar 5
Foo something 6
Foo2 somethingElse 3
What's more, there's a table "priority_threads" (1:1 with threads). I want to display only those priority threads in this table.
How can I do this? I have absolutely no idea how to even start. The only thing I did is:
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;
However using this query all values in the answer's count column are the same - count(*) from Answer table...

Answer 1

Try this:
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;
Here is a SQLfiddle link to the query to test it out;
Sample code:
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

so:
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;
can look like this:
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;
The results are the same, the former is preferred by most. Okay, but I think the real issue here is this line here in your posted code:
AND thread.id_thread = priority_threads.id_thread
you are referencing another table, priority_threads, which is not in your query. Either add the join or remove it and you should be good to go.
-james