Question

I have been scratching my head on this one for a while, so I figured I’d ask stack overflow (Note: I am an SQL novice trying to learn more SQL, so please be respectful and explanatory):

I have one sql table that looks like this called "posts":

id | user
--------------------------------
0  | tim
1  | tim
2  | bob

And another called "tags" that stores the tags on the posts (in the "posts" table) in text:

id | postID | tag
--------------------------------
0  | 0      | php
1  | 2      | php
2  | 0      | mysql
3  | 1      | mysql
4  | 1      | sql
5  | 3      | perl

(To clarify, the concept where: id=0 is tagged php,mysql; id=1 is tagged sql,mysql; id=2 is tagged php; id=3 is tagged perl.)

How could I write a WHERE statement to get posts tagged x, but not y (x and y will be defined by php)?

For example, how could I get all posts tagged mysql but not php?

EDIT

Could you also explain how to add multiple tags to search for (for example get all tagged mysql and recursion but not php)

Answer 1

Naively, simply and portably:

SELECT *
FROM posts
WHERE EXISTS (SELECT * FROM tags WHERE tags.postid = posts.id AND tags.tag = 'x')
AND NOT EXISTS (SELECT * FROM tags WHERE tags.postid = posts.id AND tags.tag = 'y')

Now depending upon the execution plan, you can do other things to optimize this.