How do i query from a Join Table? - mysql

Question

New to SQL, Assuming this is the right way to create a JOIN Table between my two main entities, do I have to hardcode insert data for the join table? How do I query from my join table? I'm using SQL Fiddle so I'm not sure if the links are being produced correctly to my foreign keys.
CREATE TABLE Organization(
`Organization_id` int NOT NULL,
PRIMARY KEY(`Organization_id`)
);
CREATE TABLE QuestionBank(
`Question_id` int NOT NULL,
`Question_text` VARCHAR(255) NOT NULL,
PRIMARY KEY(`Question_id`)
);
CREATE TABLE OrganizationQuestion(
`OrganizationQuestion_id` int NOT NULL,
`Question_id` int NOT NULL,
`Organization_id` int NOT NULL,
PRIMARY KEY(`OrganizationQuestion_id`),
FOREIGN KEY(`Question_id`) REFERENCES QuestionBank(`Question_id`),
FOREIGN KEY(`Organization_id`) REFERENCES Organization(`Organization_id`)
);
INSERT INTO Organization(`Organization_id`) VALUES(1);
INSERT INTO QuestionBank(`Question_id`, `Question_text`) VALUES(1, 'How did he perform?');
INSERT INTO OrganizationQuestion(`OrganizationQuestion_id`, `Question_id`, `Organization_id`)
VALUES(1, 1, 1);

Answer 1

This is your join:
select oq.OrganizationQuestion_id,
oq.Question_id,
oq.Organization_id,
o.Organization_id,
qb.Question_id,
qb.Question_text
from OrganizationQuestion oq
join Organization o
on o.Organization_id = oq.Organization_id
join QuestionBank qb
on qb.Question_id = oq.Question_id
+-------------------------+-------------+-----------------+-----------------+-------------+---------------------+
| OrganizationQuestion_id | Question_id | Organization_id | Organization_id | Question_id | Question_text |
+-------------------------+-------------+-----------------+-----------------+-------------+---------------------+
| 1 | 1 | 1 | 1 | 1 | How did he perform? |
+-------------------------+-------------+-----------------+-----------------+-------------+---------------------+
Which is not terribly interesting because you made almost everything a 1.
Output with EXPLAIN:
+----+-------------+-------+--------+-----------------------------+---------+---------+---------------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-----------------------------+---------+---------+---------------------------------+------+-------------+
| 1 | SIMPLE | oq | ALL | Question_id,Organization_id | NULL | NULL | NULL | 1 | NULL |
| 1 | SIMPLE | o | eq_ref | PRIMARY | PRIMARY | 4 | so_gibberish.oq.Organization_id | 1 | Using index |
| 1 | SIMPLE | qb | eq_ref | PRIMARY | PRIMARY | 4 | so_gibberish.oq.Question_id | 1 | NULL |
+----+-------------+-------+--------+-----------------------------+---------+---------+---------------------------------+------+-------------+
Remember that KEYS (Indexes) are not used in queries with small tables. It takes longer to use the KEYS than just a table scan.
To view indexes on a table:
mysql> show indexes from OrganizationQuestion;
+----------------------+------------+-----------------+--------------+-------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------------+------------+-----------------+--------------+-------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| organizationquestion | 0 | PRIMARY | 1 | OrganizationQuestion_id | A | 1 | NULL | NULL | | BTREE | | |
| organizationquestion | 1 | Question_id | 1 | Question_id | A | 1 | NULL | NULL | | BTREE | | |
| organizationquestion | 1 | Organization_id | 1 | Organization_id | A | 1 | NULL | NULL | | BTREE | | |
+----------------------+------------+-----------------+--------------+-------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
See the MySQL Manual pages for SHOW INDEX and EXPLAIN
Edit 2 completely different question
To disallow content during an INSERT (say, 2 FK id's in one table being the same such a Mail 'sender' and 'recipient')
See the following Answer for generating a
signal sqlstate '45000';