A relational database consists of at least two tables along with a definition of the relationships between the two tables.
At this level of complexity, it is straightforward to keep the data consistent. The term used is 'data integrity'. For example if a record in one table is deleted, then a related record in the other table may have to be deleted as well because they share a relationship. If the deletion is not done properly then you end up with 'orphan' records that have no relevance and are just taking up storage.
Another problem might be trying to insert data into a field of the wrong data type, for example inserting a string into a boolean field will cause an error.
What is needed is a piece of software that can keep track of all the rules implicit in the database and to maintain data integrity.
This software is called a 'Relational Database Management System' or RDBMS.
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