**Performance Tips:**

- DISTINCT requires sorting or hashing (expensive)

- Index columns used in DISTINCT

- Avoid DISTINCT on large result sets

- Use GROUP BY instead when aggregating

- Consider if duplicates are data quality issue

DISTINCT requires sorting or hashing (expensive)

Index columns in DISTINCT clause

Avoid DISTINCT on large result sets

Use GROUP BY for complex aggregations

Consider EXISTS instead of DISTINCT in subqueries

Using DISTINCT to hide data quality issues

Applying DISTINCT unnecessarily (performance cost)

Expecting DISTINCT on one column when selecting multiple

Not understanding DISTINCT with NULL values

Using DISTINCT when GROUP BY more appropriate