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SQL Join Types in Claims Reporting

MediumSQL & Data Manipulation00:00
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Problem

Explain SQL join types

You are asked to explain the major SQL join types and when you would use each in reporting work. Cover INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, CROSS JOIN, and SELF JOIN, including what rows each returns, how unmatched rows appear, and how NULL values affect interpretation.

Use practical finance or healthcare reporting examples such as claims, payments, providers, or budget reconciliation. A strong answer should also mention common mistakes, especially how filtering a right-table column in the WHERE clause after a LEFT JOIN can unintentionally remove unmatched rows.

What This Tests

  • Understanding of row-matching behavior across join types
  • Ability to choose the correct join for a reporting question
  • Awareness of NULL handling in outer joins
  • Recognition of common join mistakes in analyst workflows

Reporting Context

In Providence-style reporting, join choice directly affects metrics such as unpaid claims counts, payment reconciliation, provider attribution, and department-level financial rollups. The interviewer is looking for both SQL correctness and business interpretation.