|Lesson 6||Advanced group operators: DISTINCT, UNIQUE, HAVING|
|Objective||Correctly place group functions DISTINCT, UNIQUE, or HAVING clauses |
Group Operators DISTINCT, UNIQUE, HAVING
DISTINCT or UNIQUE
clause is a group function that does not require you to use the
clause. Add this to a query and you eliminate any duplicate rows returned from the query.
You can use either
, both have exactly the same meaning.
For example, in the query below, you are looking for a list of
values and you are not interested in listing the same
more than once, even if it appears in more than one sale:
SELECT DISTINCT PRODUCT_ID FROM SALE_ITEM
You can also use
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
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)
GROUP BY LASTNAME
HAVING COUNT(LASTNAME) > 1
Another use might be to find customers that have spent more than $100 at the pet store:
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
In the next lesson, you will learn how to use pseudocolumns to your advantage.
Advanced Group Operators - Exercise