SQL_Exercise3

desc user_type;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| usertype | varchar(20) | YES  |     | NULL    |       |
| user     | varchar(20) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

desc user_category;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| user     | varchar(20) | YES  |     | NULL    |       |
| article  | varchar(20) | YES  | UNI | NULL    |       |
| category | varchar(30) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

desc article_comments;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| article  | varchar(20) | YES  |     | NULL    |       |
| comments | varchar(30) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

1. select article from user_category where user='user1';

2. select article_comments.article, comments from article_comments inner join user_category on article_comments.article=user_category.article where user='user3';

3. select article from article_comments where ISNULL(comments);