5 SQL Interview Questions That Trip Up Beginners

Short summary

Five SQL gotchas that surprise beginners: WHERE runs before GROUP BY so aggregates don't exist yet—use HAVING instead; a table has one primary key, which can be composite; INNER JOIN with zero matches returns nothing (use OUTER JOIN to preserve unmatched rows); NULL isn't comparable with = or <>, use IS NULL instead; DELETE removes specific rows and can roll back, but TRUNCATE removes all rows, resets identity, and is faster but can't typically roll back.

• •WHERE filters before GROUP BY executes; use HAVING for aggregate conditions like COUNT() > 5
• •Primary keys are singular per table but can be composite (multiple columns)
• •INNER JOIN returns zero rows if unmatched; use LEFT/RIGHT/FULL OUTER to preserve unmatched data

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