Use of HAVING clause in SQL

Overview : 

Having clause in SQL is used to place aggregate functions  to filter data because where condition does not allow the use of aggregate function.

Syntax :

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;

Demo :

Suppose we have a database of items as shown below. 

Now if we need to find out if there is any duplicate bar-code in these items then we can use having clause.

Query :

select count(item_code), item_name, item_purchase_rate, item_sale_rate from item group by item_code, item_name, item_purchase_rate, item_sale_rate having count(item_code)  > 0