Nullable NOT IN needs a clear rule for unknown identifiers. Compare NULL and empty-set results before changing the query or interpreting a Row Count Spool.
SQL SERVER – Correlated and Noncorrelated – SubQuery Introduction, Explanation and Example
What is the difference between correlated and noncorrelated subqueries? I explain both kinds with simple AdventureWorks examples.
SQL SERVER – Fix: Error 130: Cannot perform an aggregate function on an expression containing an aggregate or a subquery
Error 130 says you cannot perform an aggregate function on an expression containing an aggregate, like AVG(COUNT()). A derived table gets around it.
SQL Server Interview Questions and Answers Part 3
Interview Questions and Answers Part 3 opens with NOLOCK: how the hint skips read locks, why that means dirty reads, and what it does for concurrency.