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SQL COUNT Function Guide

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Basic Concepts: COUNT() function returns total number of rows in a table
Default behavior includes NULL values and duplicates
DISTINCT keyword excludes NULL values from count
Usage Options: COUNT(*) and COUNT(1) have identical performance
COUNT(ALL expression) counts non-NULL values
COUNT(DISTINCT expression) counts unique non-NULL values
COUNT_BIG returns values larger than INT data type limit
Advanced Features: GROUP BY clause allows breaking down counts by groups
HAVING clause enables COUNT in WHERE clause
COUNT OVER(PARTITION BY) returns counts per partition
@@ROWCOUNT shows number of rows affected by queries
Performance Considerations: Nonclustered index preferred for large tables
Columnstore index available in SQL Server 2016+
SET STATISTICS TIME and MAXDOP options improve performance
NULL values require special handling in COUNT function