1. What is the DENSE_RANK function in Oracle?
- The DENSE_RANK function in Oracle assigns a unique rank to each row in a result set based on the specified ordering. Unlike RANK, DENSE_RANK does not skip ranks when there are ties.
- The next rank is always the next consecutive number, even if there are multiple rows with the same value.
2. How does DENSE_RANK handle ties?
When multiple rows have the same value for the specified ORDER BY column, they are assigned the same rank. However, unlike RANK, the next rank is not skipped, meaning that the rank sequence is continuous.
Example:
- If two rows are tied at rank 1, the next row will be ranked 2, not 3 (no gap).
3. How does DENSE_RANK differ from RANK?
- RANK: Skips rank numbers when there are ties. For example, if two rows are tied for rank 1, the next row will be ranked 3 (skipping rank 2).
- DENSE_RANK: Does not skip rank numbers for ties. If two rows are tied for rank 1, the next row will be ranked 2 (no gap).
4. What is the syntax for DENSE_RANK?
The basic syntax for DENSE_RANK is:
SELECT column1, column2, DENSE_RANK() OVER (ORDER BY column_name) AS dense_rank
FROM table_name;
- DENSE_RANK(): The function.
- OVER (ORDER BY column_name): Defines the order for ranking.
- AS dense_rank: Alias for the calculated rank.
5. Can I use DENSE_RANK with multiple columns?
Yes, you can rank rows based on multiple columns by using a composite ORDER BY clause. If there are ties in the first column, the second column will be used to break the tie.
SELECT Employee, Department, Salary,
DENSE_RANK() OVER (PARTITION BY Department ORDER BY Salary DESC) AS Rank
FROM Employees;
This ranks employees within each department based on their salary.
6. How do I handle NULL values in DENSE_RANK?
By default, NULL values are treated as the lowest values in the ORDER BY clause. You can change this behavior using NULLS FIRST or NULLS LAST to control where NULL values are placed in the ranking.
SELECT Salesperson, Sales_Amount,
DENSE_RANK() OVER (ORDER BY Sales_Amount DESC NULLS LAST) AS Rank
FROM Sales;
This places NULL values at the end of the ranking sequence.
7. How do I partition data when using DENSE_RANK?
You can use the PARTITION BY clause to divide the data into groups (partitions). The DENSE_RANK function will rank rows within each partition independently.
SELECT Department, Employee, Salary,
DENSE_RANK() OVER (PARTITION BY Department ORDER BY Salary DESC) AS Rank
FROM Employees;
This query ranks employees within each department by their salary.
8. Can I use DENSE_RANK in subqueries or joins?
Yes, DENSE_RANK can be used in subqueries or joins to rank data after combining tables.
SELECT Department, Employee, Salary
FROM (
SELECT Department, Employee, Salary,
DENSE_RANK() OVER (PARTITION BY Department ORDER BY Salary DESC) AS Rank
FROM Employees
) AS Ranked_Employees
WHERE Rank <= 3;
This retrieves the top 3 employees from each department.
9. Can I rank records based on a calculated column or expression?
Yes, you can rank rows based on calculated columns or expressions within the ORDER BY clause.
SELECT Salesperson, Salary, Bonus,
DENSE_RANK() OVER (ORDER BY (Salary + Bonus) DESC) AS Rank
FROM Salespeople;
This ranks salespeople based on the sum of their salary and bonus.
10. What is the performance impact of DENSE_RANK?
The performance of the DENSE_RANK function can be affected by the size of the data and the complexity of the ORDER BY clause. If you're ranking large datasets, indexing the columns used in the ORDER BY and PARTITION BY clauses can improve performance.
11. Can DENSE_RANK be used with date columns?
Yes, you can use DENSE_RANK to rank rows based on date columns. It works by assigning ranks based on the ordering of dates.
SELECT Salesperson, Sales_Date, Sales_Amount,
DENSE_RANK() OVER (ORDER BY Sales_Date) AS Rank
FROM Sales;
This ranks sales records by date.
12. Can I rank the top N rows using DENSE_RANK?
Yes, you can rank and filter the top N rows by using the DENSE_RANK function in conjunction with a WHERE clause.
SELECT Employee, Salary,
DENSE_RANK() OVER (ORDER BY Salary DESC) AS Rank
FROM Employees
WHERE Rank <= 3;
This query retrieves the top 3 employees based on salary.
13. How does DENSE_RANK behave with multiple partitions?
DENSE_RANK will reset the ranking for each partition defined by the PARTITION BY clause. Within each partition, ranking will start from 1.
SELECT Department, Employee, Salary,
DENSE_RANK() OVER (PARTITION BY Department ORDER BY Salary DESC) AS Rank
FROM Employees;
This ranks employees within each department independently.
14. Can I use DENSE_RANK for cumulative rankings?
No, DENSE_RANK does not provide cumulative rankings. It ranks rows based on the order specified without accumulating the previous results. For cumulative ranking, you would need to use a combination of other window functions.
15. What happens if there are no ties in DENSE_RANK?
If there are no ties, DENSE_RANK will behave like ROW_NUMBER, assigning a unique consecutive rank to each row.
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