Friday, January 11, 2013

Retrieve Top N records from a query


Question: How can I retrieve the Top N records from a query?
For example, what if I wanted to retrieve the first 3 records from my query results. How can I do this?
Answer: To retrieve the Top N records from a query, you can use the following syntax:
SELECT *
FROM (your ordered query) alias_name
WHERE rownum <= Rows_to_return
ORDER BY rownum;
For example, if you wanted to retrieve the first 3 records from the suppliers table, sorted by supplier_name in ascending order, you would run the following query:
SELECT *
FROM (select * from suppliers ORDER BY supplier_name) suppliers2
WHERE rownum <= 3
ORDER BY rownum;
If you wanted to retrieve the first 3 records from the suppliers table, sorted by supplier_name in descending order, you would run the following query:
SELECT *
FROM (select * from suppliers ORDER BY supplier_name DESC) suppliers2
WHERE rownum <= 3
ORDER BY rownum;
If you wanted to retrieve the first 5 records from the suppliers table, sorted by supplier_id in ascending order, you would run the following query:
SELECT *
FROM (select * from suppliers ORDER BY supplier_id) suppliers2
WHERE rownum <= 5
ORDER BY rownum;
If you wanted to retrieve the first 5 records from the suppliers table, sorted by supplier_id in descending order, you would run the following query:
SELECT *
FROM (select * from suppliers ORDER BY supplier_id DESC) suppliers2
WHERE rownum <= 5
ORDER BY rownum;