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When we need to use 1-to-1 relationship in database design?

When do we need to use a 1-to-1 relationship in database design? In my opinion, if two tables are in a 1-to-1 relationship, they can be combined into one table. Is this true?

Solution

    1. Vertical partitioning for large tables to reduce I/O and cache requirements -- separate columns that are queried often vs rarely.

    2. Adding a column to a production system when the alter table is "too expensive".

    3. Super-type/subtype pattern.

    4. Vertical partitioning to benefit from table (join) elimination -- providing optimizer supports it (again to reduce I/O and cache) .

    5. Anchor modeling -- similar to 4, but down to 6NF.