SQL Query Optimization Techniques

1) SQL query became fast if you  select actual columns instead of selecting all values (*)
2) HAVING clause is used to filter the rows after all the rows are selected. It is just like a filter. Do not use HAVING clause for any other purposes. 
For Example: Write the query as
SELECT subject, count(subject) FROM student_details WHERE subject != 'Science' AND subject != 'Maths' GROUP BY subject;
Instead of:
SELECT subject, count(subject) FROM student_details GROUP BY subject HAVING subject!= Vancouver' AND subject!= 'Toronto';


3) 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';


4) Use operator EXISTS, IN and table joins appropriately in your query. 
    a) Usually IN has the slowest performance. 
    b) IN is efficient when most of the filter criteria is in the sub-query. 
    c) EXISTS is efficient when most of the filter criteria is in the main query.
For Example: Write the query as
Select * from product p where EXISTS 
(select * from order_items o where o.product_id = p.product_id)
Instead of:
Select * from product p where product_id IN (select product_id from order_items


5) Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationship. For Example: Write the query as
SELECT d.dept_id, d.dept FROM dept d 
WHERE EXISTS ( SELECT 'X' FROM employee e WHERE e.dept = d.dept);
Instead of:
SELECT DISTINCT d.dept_id, d.dept FROM dept d,employee e WHERE e.dept = e.dept;


6) Use of WHERE clause.
Symbol operator: For Example: Write the query as
SELECT id, first_name, age FROM student_details WHERE age > 10;
Instead of:
SELECT id, first_name, age FROM student_details WHERE age != 10; 


Wildcard vs sub-string: Write the query as:
SELECT id, first_name, age FROM student_details WHERE first_name LIKE 'Chan%';
Instead of:
SELECT id, first_name, age FROM student_details WHERE SUBSTR(first_name,1,3) = 'Cha';


Write the query as:
SELECT id, first_name, age FROM student_details WHERE first_name LIKE NVL ( :name, '%');
Instead of:

SELECT id, first_name, age FROM student_details WHERE first_name = NVL ( :name, first_name);

Write the query as:

SELECT id, first_name, age FROM student_details WHERE first_name LIKE NVL ( :name, '%');

Instead of:

SELECT id, first_name, age FROM student_details WHERE first_name = NVL ( :name, first_name); 

Write the query as:

SELECT product_id, product_name FROM product 

WHERE unit_price BETWEEN MAX(unit_price) and MIN(unit_price)

Instead of:

SELECT product_id, product_name FROM product WHERE unit_price >= MAX(unit_price) 

and unit_price <= MIN(unit_price) 

Write the query as:

SELECT id, name, salary FROM employee WHERE dept = 'Electronics' AND location = 'Bangalore';

Instead of:

SELECT id, name, salary FROM employee WHERE dept || location= 'Electronics Bangalore';

Use non-column expression on one side of the query because it will be processed earlier.

Write the query as:

SELECT id, name, salary FROM employee WHERE salary < 25000;

Instead of:

SELECT id, name, salary FROM employee WHERE salary + 10000 < 35000;

Write the query as:

SELECT id, first_name, age FROM student_details WHERE age > 10;

Instead of:

SELECT id, first_name, age FROM student_details WHERE age NOT = 10;



7) Utilize Union instead of OR
Indexes lose their speed advantage when using them in OR-situations
Write the query as:

SELECT * FROM TABLE WHERE COLUMN_A = 'value' OR COLUMN_B = 'value'

Instead of:

SELECT * FROM TABLE WHERE COLUMN_A = 'value'
UNION
SELECT * FROM TABLE WHERE COLUMN_B = 'value'

8) Use DECODE to avoid the scanning of same rows or joining the same table repetitively. DECODE can also be made used in place of GROUP BY or ORDER BY clause.

For Example: Write the query as

SELECT id FROM employee WHERE name LIKE 'Ramesh%' and location = 'Bangalore';

Instead of:

SELECT DECODE(location,'Bangalore', id, NULL) id FROM employee WHERE name LIKE 'Ramesh%';

10) To store large binary objects, first place them in the file system and add the file path in the database.

11) To write queries which provide efficient performance follow the general SQL standard rules.

     a) Use single case for all SQL verbs

     b) Begin all SQL verbs on a new line

     c) Separate all words with a single space 

     d) Right or left aligning verbs within the initial SQL verb

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

http://hungred.com/useful-information/ways-optimize-sql-queries/