Sql GROUP BY CASE statement with aggregate function

Embracing Subqueries and CTEs for Complex Aggregates

Sometimes, you need to perform aggregate operations before you can group the results. This is especially true when dealing with complex grouping conditions. But, not to worry! SQL offers the subquery and Common Table Expression (CTE) features to help you out.

Subquery example:

SELECT 
  AgeGroup, 
  SUM(PeopleCount) as TotalPeople -- because more the merrier
FROM (
  SELECT
    CASE 
      WHEN age < 20 THEN 'Youth'
      WHEN age < 60 THEN 'Adult'
      ELSE 'Senior' -- ripe with age
    END as AgeGroup,
    COUNT(*) as PeopleCount -- total humans here!
  FROM 
    citizens
  GROUP BY 
    AgeGroup 
)AS SubGrouped
GROUP BY AgeGroup; -- We love consistency, don't we?


CTE example:

WITH GroupedCitizens AS (
  SELECT
    CASE 
      WHEN age < 20 THEN 'Youth'
      WHEN age < 60 THEN 'Adult'
      ELSE 'Senior'
    END as AgeGroup,
    COUNT(*) as PeopleCount
  FROM citizens
  GROUP BY AgeGroup   -- making groups, one citizen at a time
)
SELECT AgeGroup, SUM(PeopleCount) as TotalPeople   -- Because, we keep a count
FROM GroupedCitizens
GROUP BY AgeGroup; 


Both the subquery and CTE approaches enable you to pre-calculate the aggregates and group by the required expression.

Tips, Tricks, and the Untold Story of GROUP BY with CASE

You might feel the temptation to group by the alias directly following an aggregate function. Don't give in — SQL won't support this. You need to mirror the CASE expression in your GROUP BY clause.

Consider potential performance implications when dealing with complicated queries. Overuse of subqueries can cause your query run speed to drag — strive for a balance between sophistication and execution speed.

Diving Deeper: Advanced Tips and Techniques

One Group, Multiple Conditions

Design complex grouping conditions with more than one CASE condition.

SELECT
  CASE
    WHEN age < 18 THEN 'Minor'  -- too young for responsibilities
    WHEN age < 65 THEN 'Working Age' -- work, work, work!
    ELSE 'Retiree'   -- time to enjoy life
  END as AgeGroup,
  CASE 
    WHEN income < 50000 THEN 'Low Income'
    WHEN income < 100000 THEN 'Middle Income'
    ELSE 'High Income'
  END as IncomeGroup, -- show off your wealth
  COUNT(*) as Population -- headcount, anytime, anywhere
FROM
  citizens
GROUP BY
  AgeGroup, IncomeGroup; -- Because we like order, don't we?


Maintaining Accuracy in Your Aggregate Functions

Always verify the results of SUM products within CASE statements match your expectations for grouping. Incorrect results can spoil the party.

Taming Complex Aggregate Functions

In the world of complex aggregates, subqueries or CTEs can help tackle complicated calculations separately before grouping—leading to clean SQL code and accurate results.