SQL Tuning

Tuing的好文章

3) Sometimes you may have more than one subqueries in your main query. Try to minimize the number of subquery block in your query. 
For Example: Write the query as

SELECT name 
FROM employee 
WHERE (salary, age ) = (SELECT MAX (salary), MAX (age) 
FROM employee_details) 
AND dept = 'Electronics'; 

Instead of:

SELECT name 
FROM employee
WHERE salary = (SELECT MAX(salary) FROM employee_details) 
AND age = (SELECT MAX(age) FROM employee_details) 
AND emp_dept = 'Electronics'

http://beginner-sql-tutorial.com/sql-query-tuning.htm