Oracle SQL: How to retrieve the top 5 highest values in a column
In database queries, it is often necessary to limit the results to rows with specific characteristics, such as the highest or lowest value in a specific column. In Oracle SQL, you can use analytic functions such as RANK(), DENSE_RANK(), or ROW_NUMBER() for this purpose.
RANK() and DENSE_RANK() for highest values
Analysis functions like RANK() and DENSE_RANK() assign a rank to each row based on a specified ranking. To retrieve the top 5 highest values in a column, you can use the following query format:
SELECT * FROM ( SELECT <column_name>, RANK() OVER (ORDER BY <column_name> DESC) AS rnk FROM <table_name> ) WHERE rnk <= 5;
RANK() assigns a numerical rank to each row, starting with the highest value of 1. DENSE_RANK() compresses gaps when identical values exist, while assigning ranks.
ROW_NUMBER() for a specific number of rows
ROW_NUMBER() analysis function assigns a serial number to each row. You can use this to retrieve a specific number of rows regardless of their value. For example:
SELECT * FROM ( SELECT <column_name>, ROW_NUMBER() OVER (ORDER BY <column_name> DESC) AS rnk FROM <table_name> ) WHERE rnk <= 5;
Non-analytical solution: use ROWNUM
Although less common, you can also use the ROWNUM pseudo-column to limit the results to a specific number of rows. However, it should be used with caution because ROWNUM is evaluated before the ORDER BY clause, which can lead to unpredictable results.
Example: Get the highest paid employee
To illustrate the use of RANK() and DENSE_RANK() when retrieving the 5 highest paid employees:
SELECT * FROM ( SELECT empno, sal, RANK() OVER (ORDER BY sal DESC) AS rnk FROM emp ) WHERE rnk <= 5;
This query assigns each employee a numerical rank based on their salary, where 1 represents the highest paid employee. It then displays the top 5 employees in order of decreasing salary.
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