Question

Considering two tables with a many-to-many relation :

Company           Speciality
---------         ---------
id                id
---------         ---------
1                 21
2                 22
3                 23
4


CompanySpeciality
--------------------------
company_id | speciality_id
--------------------------
1          | 21
1          | 22
4          | 21
4          | 23

I want to delete company 4, and associate its specialities to the company 1.
If I use a simple UPDATE statement on CompanySpeciality to set “company_id = 1 WHERE company_id=4”, I’m facing the primary contraint violation because the pair 1|21 already exists.

Is the a way to update the relation table with a single query ? This query should only affect rows that will not be duplicated.

The result would be :

CompanySpeciality
--------------------------
company_id | speciality_id
--------------------------
1          | 21
1          | 22
1          | 23

Answer 1

You don’t want to UPDATE, you want to INSERT and ignore dupes:

INSERT IGNORE INTO CompanySpeciality (company_id, speciality_id)
SELECT 1, speciality_id
FROM CompanySpeciality
WHERE company_id=4