SQL Optimization

1) The sql query becomes faster if you use the actual columns names in SELECT statement instead of than '*'. 

If we use '*' , then sql takes time to expand it. Though time taken is very less but in large and complex queries, it can be a considerable factor.

For Example: Write the query as

SELECT id, first_name, last_name, age, subject FROM student_details;

Instead of:

SELECT * FROM student_details;

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) 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 employeeWHERE 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) Try to use UNION ALL in place of UNION.
For Example: Write the query as

SELECT id, first_name FROM student_details_class10 SELECT id, first_name FROM sports_team;

Instead of:

SELECT id, first_name, subject FROM student_details_class10 UNION SELECT id, first_name FROM sports_team;

7) Be careful while using conditions in WHERE clause.
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;

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 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, first_name, age FROM student_details WHERE age > 10;

Instead of:

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

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

9) 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