MySQL efficient data to junction table query - mysql

Question

The problem:
I want to move the links of the categories from the table companies_1 into the company_categories table. The company_id in the company_categories table need to be equal to the id of the companies_2 table. The records of the companies_1 and the companies_2 table are linked by the "name"-column.
The current code below took me over a night, still unfinished! I want to learn to be more efficient and speed this progress up. I feel like there is very much to optimize because there are A LOT of company records.
Another issue was that i found no way how to check where my query was while looping (resulting in no way to check the progress). Because the progress took so long i killed the query and I'm searching for a better way to solve this issue.
The information:
There is a table with companies like:
----------------------------------------
| companies_1 |
----------------------------------------
| id | category_id | name |
----------------------------------------
| 1 | 1 | example-1 |
| 2 | 2 | example-1 |
| 3 | 1 | example-2 |
| 4 | 2 | example-2 |
| 5 | 3 | example-2 |
| 6 | 1 | example-3 |
----------------------------------------
A table with the DISTINCT company names:
-------------------------
| companies_2 |
-------------------------
| id | name |
-------------------------
| 1 | example-1 |
| 2 | example-2 |
| 3 | example-3 |
-------------------------
A categories table, like:
-------------------------
| categories |
-------------------------
| id | name |
-------------------------
And a junction table, like:
---------------------------------
| company_categories |
---------------------------------
| company_id | category_id |
---------------------------------
The current code:
This code works, but is far from efficient.
DELIMITER $$
DROP PROCEDURE IF EXISTS fill_junc_table$$
CREATE PROCEDURE fill_junc_table()
BEGIN
DECLARE r INT;
DECLARE i INT;
DECLARE i2 INT;
DECLARE loop_length INT;
DECLARE company_old_len INT;
DECLARE _href VARCHAR(255);
DECLARE cat_id INT;
DECLARE comp_id INT;
SET r = 0;
SET i = 0;
SET company_old_len = 0;
SELECT COUNT(*) INTO loop_length FROM companies;
WHILE i < loop_length DO
SELECT href INTO _href FROM company_old LIMIT i,1;
SELECT id INTO comp_id FROM companies WHERE site_href=_href;
SELECT COUNT(*) INTO company_old_len FROM company_old WHERE href=_href;
SET i2 = 0;
WHILE i2 < company_old_len DO
SELECT category_id INTO cat_id FROM company_old WHERE href=_href LIMIT i2,1;
INSERT INTO company_categories (company_id, category_id) VALUES (comp_id, cat_id);
SET r = r + 1;
SET i2 = i2 + 1;
END WHILE;
SET i = i + 1;
END WHILE;
SELECT r;
END$$
DELIMITER ;
CALL fill_junc_table();
Edit (new idea):
I am going to test another way to solve this problem by fully copying the companies_1 table with the following columns (company_id empty on copy):
---------------------------------------------
| company_id | category_id | name |
---------------------------------------------
Then, I will loop through the companies_2 table to fill the correct company_id related to the name-column.
I hope you can give your thoughts about this. When I finish my test I will leave the result over here for others.

Answer 1

To clarify, I don't see any PIVOT transformation in the company_categories. What I see is you want a JUNCTION TABLE because it seems that companies and categories tables have many-to-many relationship.
In your case, you have company which has multiple categories. And you also have categories assigned to multiple companies.
Now base from your requirement:
I want to move the links of the categories from the table companies_1
into the company_categories table. The company_id in the
company_categories table need to be equal to the id of the companies_2
table. The records of the companies_1 and the companies_2 table are
linked by the "name"-column.
I arrived with this query:
INSERT INTO company_categories (company_id, category_id)
SELECT C2.id
, C1.category_id
FROM companies_1 C1
INNER JOIN companies_2 C2 ON C2.name = C1.name
Let me know if this works. The nested loops that you created will really take a while.
As #DanielE pointed out, this query will work in the assumption that company_categories is empty. We will need to use UPDATE otherwise.

Answer 2

Why not just update companies_1?
ALTER TABLE companies_1 ADD (company_id INT)
UPDATE companies_1 SET company_id = (SELECT id FROM companies_2 WHERE name=companies_1.name)
ALTER TABLE companies_1 DROP name, RENAME TO company_categories
SELECT * FROM `company_categories`
Output
id category_id company_id
1 1 1
2 2 1
3 1 2
4 2 2
5 3 2
6 1 3