MySQL relations with restrictions - mysql

Question

I have this table structure and simple relationships:
and sample data in the table:
Company
Company names are unique and should not be repeated:
+------------+---------------+
| Company_ID | Company_name |
+------------+---------------+
| 1 | Company_name1 |
+------------+---------------+
| 2 | Company_name2 |
+------------+---------------+
Location
(Locations should be assigned to a specific company):
+-------------+------------+-------------------------+
| Location_ID | Company_ID | Location_name |
+-------------+------------+-------------------------+
| 1 | 1 | Company1_Location_name1 |
+-------------+------------+-------------------------+
| 2 | 1 | Company1_Location_name2 |
+-------------+------------+-------------------------+
| 3 | 2 | Company2_Location_name1 |
+-------------+------------+-------------------------+
| 4 | 2 | Company2_Location_name2 |
+-------------+------------+-------------------------+
Data
The data in the table should depend on the selected company, and the locations should be only those that occur in the company:
+---------+-------------+------------+------------+------+
| Data_ID | Location_ID | Company_ID | data_value | date |
+---------+-------------+------------+------------+------+
| 1 | 1 | 1 | 5 | date |
+---------+-------------+------------+------------+------+
| 2 | 2 | 1 | 2 | date |
+---------+-------------+------------+------------+------+
| 3 | 3 | 2 | 3 | date |
+---------+-------------+------------+------------+------+
| 4 | 2 | 1 | 1 | date |
+---------+-------------+------------+------------+------+
| 5 | 4 | 2 | 6 | date |
+---------+-------------+------------+------------+------+
| 6 | 4 | 2 | 7 | date |
+---------+-------------+------------+------------+------+
The main dependencies that should be met:
Company names should be unique and attempts to add the same company should be blocked
Location names should be assigned to a specific company, but they may repeat and a location may appear in several companies but have a different Location_ID
Adding values to the date table should depend on:
company (we choose a specific company for which we add values)
locations (locations must depend on company)
For example:
When adding values for a company with Company_ID = 1, I should only be able to add Location_ID that occur under that company.
If I want to add a value in the data table for Company_name1 then the only available values for the Location_ID column in the data table, should be: Company1_Location_name1 and Company1_Location_name2 and I can't have values there from another company (i.e. Company2_Location_name1 and Company2_Location_name2)
At the moment it works badly:
when adding values to the data table I can select a company, but then I have locations available and I can add values that do not make sense - for Company_name1 I can add a location from Company_name2 but it should be blocked.
How can I solve such a problem? Add some additional table which will be responsible for particular pairing?

Answer 1

Depends what database you use.
A simpler way would be to just create a unique constraint on the table field, this will also enforce it for updates too and remove the need for a trigger. Just do:
Example for MSSQL:
ALTER TABLE [dbo].[Company]
ADD CONSTRAINT [Company_name] UNIQUE NONCLUSTERED
(
[CompanyID], [Company_name]
)
and then you'll be in business. You will be not able to add 2 company with the same name.
You can find another examples here : Trigger to prevent Insertion for duplicate data of two columns
This is exacly what you are looking for :)
#EDIT 1
OK so if you want example for MARIADB here we go :
Create unique Contraint - Using a CREATE TABLE statement
The syntax for creating a unique constraint using a CREATE TABLE statement in MariaDB is:
CREATE TABLE table_name
(
column1 datatype [ NULL | NOT NULL ],
column2 datatype [ NULL | NOT NULL ],
...
CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);
table_name
The name of the table that you wish to create.
column1, column2
The columns that you wish to create in the table.
constraint_name
The name of the unique constraint.
uc_col1, uc_col2, ... uc_col_n
The columns that make up the unique constraint.
In your example :
CREATE TABLE Company
( Company_ID INT(11) PRIMARY KEY AUTO_INCREMENT,
Company_name VARCHAR(250) NOT NULL,
CONSTRAINT company_name_unique UNIQUE (Company_name)
);
In this example, we've created a unique constraint on the Company table called company_name_unique. It consists of only one field - the Company_name field.