Question

First, please note that I have little experience with SQL (in this case sqlite3) and so even the title of this question may be ill-phrased.

Suppose that I have a table of notes (n), and a table of keywords for those notes (k), and finally a table associating one or more keywords to each note (nk). What I’d like to do is to find notes that each contain two (or more) keywords.

To be a bit more specific, below (sqlite3) is how I’ve set up the database.

CREATE TABLE n(nid integer primary key autoincrement, content);
CREATE TABLE k(kid integer primary key autoincrement, content);
CREATE TABLE nk(nkid integer primary key autoincrement, nid, kid);
INSERT INTO n(content) VALUES ("note 1");
INSERT INTO n(content) VALUES ("note 2");
INSERT INTO k(content) VALUES ("keyword 1");
INSERT INTO k(content) VALUES ("keyword 2");
INSERT INTO nk(nid, kid) VALUES (1, 1);
INSERT INTO nk(nid, kid) VALUES (1, 2);
INSERT INTO nk(nid, kid) VALUES (2, 1);

With this, I can get notes tagged with a keyword ID of 1 with

select * from n LEFT JOIN nk ON nk.nid = n.nid WHERE nk.kid=1;

and my question is how I can get notes with keyword IDs 1 and 2. I’ve done some searching on the web, but I’m afraid that my knowledge is insufficient to think of good searching terms. I hope someone on this site can help and — importantly — I apologize if this is a silly question.

Answer 1

To improve on @GordonLinoff’s answer:

select * from n where nid IN 
 (select nid
  from (select * from nk where kid in (1,2)) s
  group by nid
  having count(distinct s.kid) = 2)

This will get those records that have kid matching 1 and 2 even if other kid values might be present (3,4,etc). sqlfiddle here.