**Performance Insights:**

• COUNT(*) is highly optimized and doesn't need to check for NULL

• COUNT(column) must examine each value to check for NULL (slightly slower)

• Filtering NULL with WHERE IS NOT NULL before aggregation can improve performance

• COALESCE adds minimal overhead

COUNT(*) is highly optimized - uses index metadata

COUNT(column) must check each value for NULL (slightly slower)

Filter NULL with WHERE IS NOT NULL before aggregation when appropriate

Avoid unnecessary COALESCE in massive table scans

Thinking AVG treats NULL as 0

Expecting SUM of all NULL to be 0

Using COUNT(column) when COUNT(*) is needed

Forgetting COALESCE when business requires treating NULL as 0

Not handling NULL results in calculations