What is a cross join and when should you use it?
This question tests your understanding of Cartesian products. It evaluates when to generate all possible combinations of two datasets.
Why Interviewers Ask This
Cross joins are often misunderstood and misused. Interviewers ask this to ensure you know exactly when to generate a full combination of rows versus a filtered join. It is essential for generating test data or creating grids of possibilities.
How to Answer This Question
Define a cross join as producing the Cartesian product of two tables, resulting in every row of table A paired with every row of table B. Explain that it does not use a join condition. Mention use cases like generating all date-product combinations for forecasting models.
Key Points to Cover
- Produces Cartesian product
- No join condition required
- Used for generating combinations
Sample Answer
A cross join returns the Cartesian product of two tables, meaning every row from the first table is paired with every row from the second. The result size is the product of the row counts of both tables. I use this when…
Common Mistakes to Avoid
- Accidentally creating a massive dataset
- Confusing it with a regular join
- Using it when a specific match is needed
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