ON vs WHERE in a LEFT JOIN

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Accepted Answer

Putting a filter in ON vs WHERE has completely different semantics for outer joins. In Query A, the condition d.name = 'Sales' is part of the join - employees not in Sales still appear but get NULL for the dept columns. In Query B, WHERE filters after the join, so only rows where d.name = 'Sales' survive - effectively turning the LEFT JOIN into an INNER JOIN. This is one of the most common outer join mistakes.