**Performance Insights:**

• Aggregates typically require scanning all matching rows

• COUNT(*) is highly optimized - often uses index metadata

• Indexes on WHERE clause columns dramatically improve performance

• Multiple aggregates in one query are computed in a single pass (efficient)

COUNT(*) is highly optimized - often uses index metadata

Create indexes on columns used in WHERE clauses with aggregates

Filter with WHERE to reduce rows before aggregation

Multiple aggregates in one query are computed in a single pass

Mixing aggregate and non-aggregate columns without GROUP BY causes errors

Assuming AVG treats NULL as zero (it ignores NULL completely)

Using COUNT(column) when COUNT(*) is needed (or vice versa)

Expecting SUM of all NULL values to return 0 (it returns NULL)

Not using DISTINCT with COUNT when counting unique values