Question 1

What is Left Anti Join and its use case?

Accepted Answer

Left Anti Join returns rows from the left table that have NO match in the right table. Implementation: SELECT a.* FROM table_a a LEFT JOIN table_b b ON a.id = b.id WHERE b.id IS NULL. Or NOT EXISTS: SELECT * FROM a WHERE NOT EXISTS (SELECT 1 FROM b WHERE b.id = a.id). Use cases: Find customers who never ordered; records in source but not target (data reconciliation); exclusion logic. Best practice: NOT EXISTS often optimizes better than LEFT JOIN + NULL....

Question 2

What is a cross-join?

Accepted Answer

Cross-join (Cartesian product): Combines every row of table A with every row of table B. Result size: |A| × |B|. Syntax: SELECT * FROM a CROSS JOIN b or FROM a, b. Use cases: Generate all combinations (e.g., dates × products for a grid), expanding dimensions. Danger: Large tables produce huge results. Example: CROSS JOIN calendar to get row per day. Best practice: Use only for small tables or with filters; explicit CROSS JOIN is clearer than omitting ON....

Question 3

What is a semi-join?

Accepted Answer

Semi-join returns rows from the left table that have at least one match in the right table, but doesn't duplicate left rows. Implementation: SELECT * FROM a WHERE EXISTS (SELECT 1 FROM b WHERE b.id = a.id) or IN. Unlike inner join, semi-join returns one row per left match even if multiple right matches exist. Use cases: Filter to customers who have orders; exclude/include based on existence....

Question 4

Write a query to find minimum age.

Accepted Answer

SELECT MIN(age) AS min_age FROM people. Or ORDER BY age ASC LIMIT 1. **Why**: MIN simpler and standard. **Production**: Consider NULL handling—MIN ignores nulls; verify semantics.

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