Question

tag

tag_id      tag          post_id 
--------------------------------
1           category1    1
2           category2    1
3           etc          2
4           category3    1

post

post_id title
-------------
1       title1

How can we get title1 because its in the tag category1 && category2

something like

SELECT TITLE 
  FROM post, tag 
 WHERE tag.post_id = post.post_id 
   AND tags.tag = 'category1','category2'

Answer 1

In order to check if a title has BOTH ‘category1’ and ‘category2’, you can use the following SQL query:

SELECT title 
FROM post 
JOIN tag ON post.post_id = tag.post_id 
WHERE tag.tag_id IN (SELECT tag_id FROM tag WHERE tag = 'category1')
AND tag.tag = 'category2';

To check for other categories you just repeat the WHERE clause:

SELECT title 
FROM post 
JOIN tag ON post.post_id = tag.post_id 
WHERE tag.tag_id IN (SELECT tag_id FROM tag WHERE tag = 'category1')
AND tag.tag_id IN (SELECT tag_id FROM tag WHERE tag = 'category2')
AND tag.tag = 'category3';

This would return results for titles that have all 3 categories.