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Implicit Type Conversion

When a comparison mixes types (a VARCHAR column against a numeric literal), the engine inserts a silent cast; if the cast lands on the column side, the predicate becomes an expression over the column and index seeks are disabled. Matching literal and parameter types to column types avoids the trap.

Questions this Concept answers

Why does an implicit cast applied to the column side of a comparison make the index unusable?

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How Implicit Data Type Conversion in SQL Server Forces a Table Scan Instead of an Index Seek

When the two sides of a comparison in SQL have different data types, the engine performs an implicit conversion to make them comparable, and the side that gets converted determines the cost: converti…