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How to use Correlated Subquery In SQL

1. Using Subquery with Row Number

3. Using Common Table Expression (CTE) with Window Function

The process that we apply is that we use the correlated subquery to count the rows with same region and which revenue is general or equal to the current row’s revenue.

The subquery sums the number of rows, meeting the conditions specified inside the WHERE clause’s parenthesis for every row in the outer query.

SELECT *
FROM sales_data t1
WHERE (
    SELECT AVG(t2.revenue)
    FROM sales_data t2
    WHERE t2.region = t1.region
) > 2;

Output:

Explanation: This statement joins the outer query with a sub-query that counts the number of equally ordered rows with a group and an order column less than or equal to the current row. It subsequently selects such rows as these correspond with the condition that is less than or equal to N.

How to Restrict Results to top N Rows per Group in SQLite?

Assume a situation where the data to be retrieved is grouped by specific criteria and the rows are desired to be filtered so that data from the top N rows in each group can be obtained, SQLite databases will be the tools used.

This can be particularly important where, for example, items need to be ranked categorically or to identify the top performers in different groups. While database systems that offer specialized functions for this task exist, SQLite doesn’t provide such core capabilities.

In this article, We will learn about How to Restrict results to the top N rows per group in SQLite by understanding various methods along with the examples and so on.

Similar Reads

How to Restrict Results to Top N Rows per Group?

When working with large datasets, it’s often necessary to extract the top N rows per group based on certain criteria. SQLite provides several methods to achieve this, including the use of subqueries and window functions. Below are the methods that help us to extract the Top N Rows per Group in SQLite....

1. Using Subquery with Row Number

This method makes use of a subquery that produces a row number (row_num) for each row of its region in accordance with its revenue, i.e. higher revenue rows are assigned lower numbers....

2. Using Correlated Subquery

The process that we apply is that we use the correlated subquery to count the rows with same region and which revenue is general or equal to the current row’s revenue....

3. Using Common Table Expression (CTE) with Window Function

This technique utilizes CTE that combines two expressions in a Common Table Expression – first, to generate row numbers within the regions based on revenue, just like Method 1 uses....

Conclusion

However, SQLite is not directly supported to limit the result of the top N rows per groups using only SQL commands. You should use the more advanced SQL commands such as subqueries, window functions, and Common Table Expressions, to fulfill this requirement....

Tags:
#SQLite Query #Databases #SQLite

1. Using Subquery with Row Number

3. Using Common Table Expression (CTE) with Window Function