In an API query, when would you use an inner join versus a left join?

Explain inner join versus left join for API results

Explains join behavior using API-shaped data, missing related rows, and result-cardinality trade-offs.

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I would use an inner join when the related row is required for the result to make sense. For example, listing paid invoices with their customer can use an inner join if every invoice must have a customer. I would use a left join when the main row still matters even if the related row is missing, such as listing users with an optional last login record. The choice changes which rows appear, not only which columns appear.

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Join semantics, missing related data, cardinality, API projection, and practical query reasoning.

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Join questions show whether a candidate can reason about result sets instead of memorizing syntax. They also reveal whether API behavior stays intentional when related data is optional.
I would use an inner join when the related row is required for the result to make sense. For example, listing paid invoices with their customer can use an inner join if every invoice must have a customer. I would use a left join when the main row still matters even if the related row is missing, such as listing users with an optional last login record. The choice changes which rows appear, not only which columns appear.
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I would start from the business meaning of missing data. An inner join filters out parent rows without a match, which can hide records unexpectedly if the relationship is optional or data is incomplete. A left join keeps the parent row and returns nulls for the related columns, so the API can show a default value or an empty nested object. I would also watch one-to-many joins because they can multiply rows and force aggregation, grouping, or separate queries for a clean response shape.

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Choosing join type by habit rather than by required or optional data.
Forgetting that inner joins remove parent rows with no match.
Returning duplicated parent rows from a one-to-many join.
Letting null related data break API serialization or display.

In an API query, when would you use an inner join versus a left join?

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