The DISTINCT or UNIQUE clause is a group function that does not require you to use the GROUP BY clause. Add this to a query and you eliminate any duplicate rows returned from the query. You can use either DISTINCT or UNIQUE, both have exactly the same meaning.
For example, in the query below, you are looking for a list of PRODUCT_ID values and you are not interested in listing the same PRODUCT_ID more than once, even if it appears in more than one sale:

SELECT DISTINCT PRODUCT_ID FROM SALE_ITEM

You can also use DISTINCT in combination with other grouping functions. For example, if you want to count the number of rows found in the query above, you could use this query:

SELECT COUNT(DISTINCT PRODUCT_ID) FROM SALE_ITEM

The HAVING clause is an optional addition to a query that uses grouping functions. It is added to the WHERE clause immediately after the GROUP BY clause, so that you can filter your query results based on the group function's data.
For example, let us say you want to add a unique index on the LASTNAME column of the CUSTOMER table, but the index cannot be created because duplicates exist somewhere in the table's data. How can you find the duplicates? The following query uses the HAVING clause to quickly identify the duplicate rows.

SELECT LASTNAME, COUNT(LASTNAME)
FROM CUSTOMER
GROUP BY LASTNAME
HAVING COUNT(LASTNAME) > 1

SELECT FIRSTNAME, LASTNAME, SUM(TOTAL_SALE_AMOUNT)
FROM CUSTOMER C, CUSTOMER_SALE CS
WHERE C.CUST_ID = CS.CUST_ID
GROUP BY FIRSTNAME, LASTNAME
HAVING SUM(TOTAL_SALE_AMOUNT) > 100

Click the Exercise link below to place group functions into queries.
Advanced Group Operators - Exercise