You write a query that's logically correct, meaning it has no syntactical errors, all referenced objects exist, and there's no apparent reason for a logical error. However, when you run the query against SQL Server, the query fails with runtime errors. You might be looking at a situation in which the physical execution plan that SQL Server chooses differs from the logical interpretation of the query per ANSI-standard SQL. Certain differences between logical and physical query processing might lead to query failures. Let's walk through a couple of examples that demonstrate those differences and explore the reasoning behind them.