Count distinct pairs by keeping their columns together. A small T-SQL example shows why concatenated labels can collide. Choose whether incomplete pairs belong in the count.
Derived Table, CTE or View: Choose the Scope
A derived table, CTE and view can express the same grouped result while offering different scopes. Compare complete outputs with duplicates and NULLs, then choose a statement-local expression or a saved definition for the task.
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.