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One-to-One Relationships - The Whiteboard #11

The one-to-one relationship is just a particular type of one-to-many relationship...

Dec 14, 2022

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5 Comments

Muhammad Atif

9 months ago

Should one-to-one tables ALWAYS be combined into one table? Is it ok to have one-to-one if we want to split a big dimension into multiple small dimensions to better organize a data model.

For e.g. instead of having all columns in one big 'Project' table, I split some of the columns into following three tables, so that columns relating to different aspects of project are together in their own tables for better readability of the data model, which are linked with the 'Project' table with one-to-one:

Project Comments
Project Executive Summary
Project Budget

Would you recommend doing this?

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SQLBI

Marco Russo

9 months ago

Reply to Muhammad Atif

In general, one-to-one should be used rarely - for example when two tables with the same cardinality (e.g. different attributes of the same customer) have different data sources and you cannot consolidate them into one table. It's a question of tradeoffs: two or more tables in one-to-one relationship perform worse than a single table.
The readability issue is better solved by using the display folder property, and it is also what users could expect if they are used to Power BI.

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Muhammad Atif

9 months ago

Reply to Marco Russo

Thanks for the reply. As a follow-up question, if we do have a one-to-one in the model, is it advisable to explicitly change the cardinality to one-to-many in Power BI (even though the underlying data has same cardinality)? Will that mitigate the performance issues?

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SQLBI

Marco Russo

9 months ago

Reply to Muhammad Atif

It really does not change: a one-to-one relationship is like a one-to-many relationship in terms of performance. The thing is that in a one-to-one relationship, you could skip the relationship altogether by consolidating data in a single table.

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Muhammad Atif

9 months ago

Reply to Marco Russo

Got it, thank you so much for explaining

1

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