SQL - Query Optimization

Rewriting Queries

SELECT * 
FROM Customers 
WHERE state = 'New York' AND city = 'New York';

SELECT * 
FROM Customers 
WHERE state = 'New York' 
AND city IN (SELECT city 
              FROM Customers 
              WHERE city = 'New York');

Choosing the right index

CREATE INDEX index_name
ON table_name (column1, column2, ...);

Fine-tuning Database Design