Counting Groups from a GROUP BY Query without Subqueries

As a developer, we often encounter queries that require aggregating data based on certain conditions. One such scenario involves retrieving the count of groups from a GROUP BY query without using subqueries. In this article, we will explore alternative approaches to achieve this.

Understanding GROUP BY and Having Clauses

Before diving into the alternatives, let’s quickly review how GROUP BY and HAVING clauses work.

• The GROUP BY clause groups rows based on one or more columns.
• The HAVING clause filters grouped results based on conditions applied to aggregated values.

In the given example, we want to retrieve the count of students with the same FACULTY, AGE, and MAJOR (non-key and non-unique attributes) where there are more than 10 students.

Original Solution

The original solution uses a subquery:

SELECT COUNT(*)
FROM
   (SELECT FACULTY, AGE, MAJOR, COUNT(*)
    FROM STUDENT
    GROUP BY FACULTY, AGE, MAJOR
    HAVING COUNT(*) > 10) students;

This solution works by first grouping the data by FACULTY, AGE, and MAJOR, then applying the HAVING condition to filter out groups with less than or equal to 10 students. Finally, it counts the remaining groups.

Alternative Solutions

There are a few alternative approaches to achieve this without using subqueries:

Nested Aggregation

One way to rewrite the query is to use nested aggregation:

SELECT COUNT(COUNT(*))
FROM STUDENT
GROUP BY FACULTY, AGE, MAJOR
HAVING COUNT(*) > 10;

This approach involves counting the number of rows in each group (using COUNT(*)) and then aggregating those counts.

Another example can be seen in the HR sample-schema employee table:

SELECT count(*)
FROM (
  select department_id, count(*)
  from  employees
  group by department_id
  having count(*) > 5
);

COUNT(*)
----------
         4

select count(count(*))
from employees
group by department_id
having count(*) > 5;

COUNT(COUNT(*))
---------------
              4

In this example, the outer query counts the number of groups with more than 5 employees, while the inner query counts the total number of rows in each group.

Using Subquery Factoring

Subquery factoring is another approach that involves redefining a subquery as a derived table. The following example demonstrates this:

WITH students (FACULTY, AGE, MAJOR, c) AS (
  SELECT FACULTY, AGE, MAJOR, COUNT(*)
  FROM STUDENT
  GROUP BY FACULTY, AGE, MAJOR
  HAVING COUNT(*) > 10
)
SELECT COUNT(*) 
FROM students;

This approach is similar to the original solution but redefines the subquery as a derived table.

Another example uses a Common Table Expression (CTE):

WITH cte (departmenr_id, c) AS (
  select department_id, count(*)
  from  employees
  group by department_id
  having count(*) > 5
)
select count(*)
from cte;

  COUNT(*)
----------
         4

with cte (departmenr_id, c) as (
  select department_id, count(*)
  from  employees
  group by department_id
)
select count(*)
from cte
where c > 5;

  COUNT(*)
----------
         4

In this example, the CTE is defined with a HAVING condition to filter out groups with less than or equal to 5 employees.

Simplifying the Query

It’s worth noting that you don’t need to select all columns from the original query. In the subquery factoring examples above, we can simplify the query by removing unnecessary columns:

WITH students (c) AS (
  SELECT COUNT(*)
  FROM STUDENT
  GROUP BY FACULTY, AGE, MAJOR
  HAVING COUNT(*) > 10
)
SELECT COUNT(*) 
FROM students;

By removing the FACULTY, AGE, and MAJOR columns, we can simplify the query while still achieving the desired result.

Conclusion

In this article, we explored alternative approaches to retrieve the count of groups from a GROUP BY query without using subqueries. We discussed nested aggregation, subquery factoring, and simplifying the query by removing unnecessary columns. These approaches offer different trade-offs in terms of performance, readability, and complexity, allowing you to choose the best solution for your specific use case.

Last modified on 2024-01-28