How can we prevent the creation of unions for tables that have largely similar content?

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One way to prevent the creation of unions for tables that have largely similar content is to normalize the database by breaking down larger tables into smaller ones based on their specific attributes. By doing this, we can avoid duplication of data and ensure that each table represents only one type of entity. Another way is to use inheritance in database design to create a hierarchy of tables sharing common attributes, reducing the need for unions. Additionally, we can enforce strict data consistency rules that require data in one table to match that in other tables, minimizing the need for unions. Finally, careful planning and analysis of the database schema before implementation can help identify potential areas where unions may be required and facilitate design decisions that prevent their need.