What is the difference between DISTINCT and GROUP BY?

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Accepted Answer

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   What is the difference between DISTINCT and GROUP BY?
  Level: Intermediate — A subtle question, since both can remove duplicates but serve different goals.

  Both DISTINCT and GROUP BY can eliminate duplicate values, but their intent is different:

  
     DISTINCT: Simply removes duplicate rows from the result set. It is used purely for uniqueness, not for calculations.
     GROUP BY: Groups rows so you can run aggregate functions (COUNT, SUM, AVG) on each group. Removing duplicates is just a side effect.
  

  
     Rule of thumb: If you just want unique values, use DISTINCT. If you want unique values plus aggregated data (like a count per group), use GROUP BY.
  

   Side-by-Side Comparison
  
    
      
        
          Feature
           DISTINCT
           GROUP BY
        
      
      
        
          Main purpose
          Remove duplicate rows
          Group rows for aggregation
        
        
          Aggregate functions
          Not used
          Commonly used
        
        
          Allows HAVING?
          No
          Yes
        
        
          Output
          Unique rows
          One row per group
        
        
          Sorting side effect
          None guaranteed
          May group/order internally
        
      
    
  

   Both Give the Same Result Here
  For a simple unique-value list, these two queries return identical output — but DISTINCT is clearer for that intent:
-- Using DISTINCT
SELECT DISTINCT dept_id FROM employees;

-- Using GROUP BY (same result)
SELECT dept_id FROM employees GROUP BY dept_id;

   Where GROUP BY Wins — Aggregation
-- DISTINCT can only list unique departments
SELECT DISTINCT dept_id FROM employees;

-- GROUP BY can also COUNT employees per department
SELECT dept_id, COUNT(*) AS total_employees
FROM employees
GROUP BY dept_id
HAVING COUNT(*) > 3;    -- and filter groups (DISTINCT can't do this)

  
     Interviewer tip: The one-liner they want — "DISTINCT only removes duplicate rows, while GROUP BY groups rows to perform aggregate calculations. Use DISTINCT for uniqueness and GROUP BY when you need per-group summaries."

Feature	DISTINCT	GROUP BY
Main purpose	Remove duplicate rows	Group rows for aggregation
Aggregate functions	Not used	Commonly used
Allows HAVING?	No	Yes
Output	Unique rows	One row per group
Sorting side effect	None guaranteed	May group/order internally

What is the difference between DISTINCT and GROUP BY?

Answer with Explanation

What is the difference between DISTINCT and GROUP BY?

Side-by-Side Comparison

Both Give the Same Result Here

Where GROUP BY Wins — Aggregation