Before you start building your database keep these answers ready i) where do you need to link the table, and ii) what if the table is removed?. Instead of using foreign keys are you still relying on the informal links between databases? To ensure you have set the foreign keys correctly let’s check if you have avoided these foreign key mistakes;
The data that you have linked to a reference table will only be entered into the database if you use a foreign key, which avoids sneaky errors.
Excluding foreign key indexes
Some of you all might have not indexed your foreign key, well it might not seem like a big issue as there won’t be any error message, but what happens when things start to scale up?
Creating indexes for the foreign keys will improve your performance for queries between the parent and child table joins. This also allows these queries to avoid triggering a full table scan.
Note: Primary indexes are created automatically when you create a table’s primary key, so its indexes must be created manually irrespective of your database size. This will help you when you operate at scale. You will notice a significant hike in the performance of the queries when you index the foreign keys between the parent and child table joins.
Data Type Mismatch
Make sure you are using the same data types while using a foreign key to refer to a column in another table.
If not you will get the error message while you create a table; ‘Foreign key mismatch’ or ‘Could not create constraint’
If you are using SQLite database you will still be able to create a table with a foreign key data type mismatch, which will later trigger errors. You just need to ensure that when creating a table, all columns linked to each other with foreign keys share the same data type.
Note: Altering an existing table with a new foreign key constraint, your DB will return an error when you try to create a foreign-key constraint that links columns with different data types.
Avoid linking to a nonexistent table
Based on the type of database management system you use, it triggers an error such as;
ERROR: relation “referenced_table” does not exist.
This is because when you try to link it to a nonexistent table/column and when you try to drop a table that is referenced by another table’s foreign key. You will get an error such as;
ERROR: “table_1” is referenced by a foreign key from table “table_2”.
A modern database system validates constraints when they’re added to a table that will restrict a user to break the database’s referential integrity by removing data that is required by another table.