How to create primary key using multiple columns & Auto increment id? - mysql

Question

This is my tables structures
mysql> DESCRIBE sections;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| sec_id | int(5) | NO | PRI | NULL | auto_increment |
| sec_name | varchar(45) | YES | | NULL | |
| sec_type | tinyint(4) | YES | | NULL | |
+----------+-------------+------+-----+---------+----------------+
mysql> DESCRIBE subjects;
+-----------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+----------------+
| sub_id | int(5) | NO | PRI | NULL | auto_increment |
| sub_name | varchar(45) | YES | | NULL | |
| sections_sec_id | int(5) | NO | MUL | NULL | |
+-----------------+-------------+------+-----+---------+----------------+
mysql> DESCRIBE chapters;
+-----------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+----------------+
| chp_id | int(5) | NO | PRI | NULL | auto_increment |
| chp_name | varchar(45) | YES | | NULL | |
| subjects_sub_id | int(5) | NO | MUL | NULL | |
| sections_sec_id | int(5) | NO | MUL | NULL | |
+-----------------+-------------+------+-----+---------+----------------+
mysql> DESCRIBE questions;
+-----------------+-----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-----------+------+-----+---------+----------------+
| que_id | int(11) | NO | PRI | NULL | auto_increment |
| que_text | text | YES | | NULL | |
| que_created | timestamp | YES | | NULL | |
| chapters_chp_id | int(5) | NO | MUL | NULL | |
| subjects_sub_id | int(5) | NO | MUL | NULL | |
| sections_sec_id | int(5) | NO | MUL | NULL | |
+-----------------+-----------+------+-----+---------+----------------+
mysql> DESCRIBE answers;
+------------------+-----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------+-----------+------+-----+---------+----------------+
| ans_id | int(11) | NO | PRI | NULL | auto_increment |
| ans_text | text | YES | | NULL | |
| ans_created | timestamp | YES | | NULL | |
| questions_que_id | int(11) | NO | MUL | NULL | |
| chapters_chp_id | int(5) | NO | MUL | NULL | |
| subjects_sub_id | int(5) | NO | MUL | NULL | |
| sections_sec_id | int(5) | NO | MUL | NULL | |
+------------------+-----------+------+-----+---------+----------------+
Let's assume 'Answers' table
I created the 'ans_id' as the primary key of 'Answers' table, created a another UNIQUE key using (ans_id,questions_que_id,chapters_chp_id,subjects_sub_id,sections_sec_id)
What i want do.
create primary key for 'Answers' table using (ans_id,questions_que_id,chapters_chp_id,subjects_sub_id,sections_sec_id) columns and create Trigger before INSERT to create 'auto_increment' id.
Finally i want to accomplish this

Answer 1

Definition: Primary key is minimal set of attributes needed to determine every row in a table.
UNIQUE index should be {questions_que_id} + {chapters_chp_id} + {subjects_sub_id} +{sections_sec_id} and make sure it is combined as one index with these 4 columns, if you set it as different 4 indexes than you will probably achieve undesired outcome, as Zohar already pointed. From my experience, it could be unpractical to use composite keys like this. For basic table operations (CRUD) you will have to provide a lot of parameters.
Maybe to try this way: autoincremental numeric id as a primary key, the rest of important attributes as UNIQUE index. Then you could count already inserted answers to make sure right number will be assigned to ans_id. Hint: it's maybe easier to achive this from your application layer, not as a trigger or procedure for MySQL. If your users will provide answers as step-by-step, then just save a timestamp and you will know which answer is older so you don't need ans_id at all.