Limit by percentage of rows

In Oracle SQL, you can limit the result set by a percentage of rows using a combination of features like FETCH FIRST, ROWNUM, and the PERCENT keyword. However, depending on the Oracle version you are using, the methods to achieve this can vary. Let’s break down how you can limit the number of rows by a percentage in Oracle SQL in detail.

1. Oracle 12c and Later: FETCH FIRST with PERCENT

Oracle 12c introduced the FETCH FIRST N PERCENT clause, which allows you to directly limit the number of rows returned by a percentage of the total result set. This method is simple, clear, and part of the SQL standard.

Syntax:

SELECT *

FROM employees

ORDER BY salary DESC

FETCH FIRST 20 PERCENT ROWS ONLY;

·        Explanation: This query will return the top 20% of rows ordered by salary in descending order. The FETCH FIRST 20 PERCENT ROWS ONLY clause will automatically calculate 20% of the total number of rows and return that portion of the result set.

·        Advantages: This method is efficient and easy to use. You don’t need to manually calculate the number of rows or write complex subqueries.

Example with ORDER BY:

SELECT * 

FROM employees

ORDER BY salary DESC

FETCH FIRST 10 PERCENT ROWS ONLY;

·        This will return the top 10% of employees sorted by salary.

Example with OFFSET:

You can also combine this with the OFFSET clause for pagination-like behavior:

SELECT *

FROM employees

ORDER BY salary DESC

OFFSET 10 PERCENT ROWS FETCH NEXT 10 PERCENT ROWS ONLY;

·        This query skips the first 10% of rows and then fetches the next 10% of rows based on salary.

2. Oracle 11g and Earlier: Limiting Rows by Percentage Using ROWNUM or COUNT

In versions of Oracle prior to 12c, the syntax for limiting by percentage is not as straightforward as FETCH FIRST N PERCENT. You would need to use a more manual approach, usually by calculating the total number of rows and then selecting that percentage. You can achieve this using ROWNUM or by calculating the total row count in a subquery.

Method 1: Using ROWNUM with COUNT (Subquery)

1.     Step 1: First, calculate the total number of rows.

2.     Step 2: Calculate the desired number of rows based on a percentage.

3.     Step 3: Use ROWNUM to limit the rows based on the calculated number.

Example:

SELECT *

FROM (

  SELECT * FROM employees ORDER BY salary DESC

)

WHERE ROWNUM <= (SELECT ROUND(COUNT(*) * 0.20) FROM employees);

·        Explanation:

o   The subquery calculates the total count of rows in the employees table and multiplies it by 0.20 to get 20% of the total.

o   The outer query retrieves the rows where ROWNUM is less than or equal to 20% of the total row count.

o   The ROUND() function is used to round the result to the nearest whole number.

·        Note: This approach works, but it is not as efficient as the Oracle 12c FETCH FIRST approach, especially when working with large datasets.

Method 2: Using ROWNUM and COUNT in a Join (Advanced)

Alternatively, you can join the table with a derived table that contains the total row count to get a similar result.

Example:

SELECT e.*

FROM employees e, 

  (SELECT ROUND(COUNT(*) * 0.20) AS row_count FROM employees) t

WHERE ROWNUM <= t.row_count

ORDER BY e.salary DESC;

·        Explanation:

o   The subquery t calculates 20% of the total rows in the employees table.

o   The ROWNUM condition ensures that only the first 20% of the rows (ordered by salary) are returned.

3. Using Analytic Functions and ROW_NUMBER() for More Control

For more advanced scenarios where you need to limit the rows by percentage but also require ranking or partitioning (e.g., fetching a percentage of rows within each department), you can use analytic functions like ROW_NUMBER() or RANK().

Example with ROW_NUMBER() and COUNT:

SELECT *

FROM (

  SELECT e.*, 

         ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_num,

         COUNT(*) OVER () AS total_rows

  FROM employees e

)

WHERE row_num <= ROUND(total_rows * 0.20);

·        Explanation:

o   The ROW_NUMBER() function is used to number the rows in descending order of salary.

o   The COUNT() function is used to calculate the total number of rows in the employees table.

o   The outer query filters the rows where the row number is less than or equal to 20% of the total row count.

4. Performance Considerations

·        Oracle 12c FETCH FIRST N PERCENT is the most efficient and straightforward way to limit rows by percentage, as it is optimized for performance.

·        Manual Percentage Calculation with ROWNUM can be less efficient, especially when dealing with large result sets, because it requires a subquery to calculate the row count and can lead to increased overhead.

·        Analytic Functions like ROW_NUMBER() and RANK() provide flexibility when dealing with more complex use cases (e.g., partitioning data), but they may have a slight performance cost compared to the direct FETCH method.

5. Edge Cases and Considerations

·        Rounding: When calculating the percentage of rows, the result may not always be a whole number. The ROUND() function can be used to ensure you get a whole number when calculating a percentage of rows. If the percentage doesn’t result in an integer, it will round to the nearest whole number.

·        Performance on Large Tables: If you're working with very large datasets, using COUNT(*) for calculating percentages may lead to performance issues because it requires scanning the entire table. In such cases, using Oracle 12c's FETCH FIRST N PERCENT is highly recommended for performance.

·        No Sorting by Percentage: When using percentages, the rows are typically returned based on some sort of sorting criteria (like ORDER BY). Ensure that the data is appropriately sorted before applying the percentage limit to get meaningful results (e.g., top N salaries, highest-rated products).

Summary

·        Oracle 12c and later: Use FETCH FIRST N PERCENT ROWS ONLY for a simple, efficient way to fetch a percentage of rows.

·        Oracle 11g and earlier: You’ll need to manually calculate the percentage using ROWNUM and COUNT(*). You can either use subqueries or analytic functions like ROW_NUMBER() to achieve the same effect.

·        Analytic Functions: If you need more control or need to limit the percentage of rows within specific groups (e.g., per department), you can use ROW_NUMBER() or RANK() with COUNT() in a subquery or derived table.

Using these methods, you can easily retrieve a percentage of rows based on your sorting criteria or specific business needs.