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
