Show the First N Rows for Each Group in PostgreSQL: Window Functions vs. Lateral JOIN

Scenario:

You have a table with data, and you want to retrieve the top N (let's say N = 2) rows for each group based on a specific column. For instance, you might have an employees table with columns like department and salary, and you'd like to see the two highest-paid employees from each department.

Methods:

PostgreSQL offers a couple of effective approaches to achieve this:

1. Window Functions (Recommended):

   • This method is generally preferred due to its clarity, efficiency, and broader applicability. It leverages window functions like ROW_NUMBER() or DENSE_RANK().
   • Here's the syntax:

   SELECT *
   FROM (
       SELECT your_column1, your_column2,
              ROW_NUMBER() OVER (PARTITION BY grouping_column ORDER BY order_column DESC) AS row_num
       FROM your_table
   ) AS subquery
   WHERE row_num <= N;
   

   • Explanation:
     • We create a subquery to assign a row number (row_num) to each row within each group defined by grouping_column.
     • The ORDER BY clause sorts the rows in descending order based on order_column (e.g., salary for highest-paid).
     • The outer query then selects rows where row_num is less than or equal to N (in this case, 2), effectively retrieving the top N rows for each group.

2. Lateral JOIN (Alternative):

   • This method is less common but can be useful in specific situations. It involves a lateral join to fetch the desired rows from the same table.
   • Here's the syntax:

   SELECT t_outer.grouping_column, t_top.your_column1, t_top.your_column2
   FROM your_table AS t_outer
   JOIN LATERAL (
       SELECT *
       FROM your_table AS t_inner
       WHERE t_inner.grouping_column = t_outer.grouping_column
       ORDER BY order_column DESC
       LIMIT N
   ) AS t_top ON true
   ORDER BY t_outer.grouping_column;
   

   • Explanation:
     • We use a lateral join to create a temporary result set (t_top) containing the top N rows for each group defined by grouping_column.
     • The ORDER BY clause within the lateral join sorts the rows in descending order based on order_column.
     • The LIMIT N clause restricts the result set to the top N rows for each group.
     • The outer query then joins t_outer with t_top and selects the desired columns.

Choosing the Right Method:

• Window functions are generally the preferred approach for their clarity and efficiency, especially for larger datasets.
• Lateral joins might be considered in certain scenarios where you need more control over the subquery or have limitations with your PostgreSQL version (pre-8.4).

Remember:

• Replace your_column1, your_column2, grouping_column, and order_column with the actual column names from your table.
• Adjust N to the desired number of top rows to retrieve for each group.

Example 1: Using Window Functions (Recommended)

-- Sample table (employees)
CREATE TABLE employees (
  department varchar(20),
  salary integer,
  name varchar(50)
);

-- Sample data
INSERT INTO employees (department, salary, name)
VALUES ('Sales', 80000, 'John'),
       ('Sales', 75000, 'Jane'),
       ('Marketing', 90000, 'Alice'),
       ('Marketing', 85000, 'Bob'),
       ('Engineering', 100000, 'David'),
       ('Engineering', 95000, 'Emily');

-- Get top 2 highest-paid employees from each department
SELECT department, name, salary
FROM (
    SELECT department, name, salary,
           ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS row_num
    FROM employees
) AS subquery
WHERE row_num <= 2;

This code will output:

 department |  name  | salary 
------------+--------+---------
 Engineering | David | 100000
 Engineering | Emily |  95000
 Marketing  | Alice |  90000
 Marketing  | Bob   |  85000
 Sales       | John  |  80000
 Sales       | Jane  |  75000

Example 2: Using Lateral JOIN (Alternative)

-- Same sample table (employees) from Example 1

-- Get top 2 highest-paid employees from each department
SELECT e.department, t.name, t.salary
FROM employees AS e
JOIN LATERAL (
    SELECT *
    FROM employees AS t
    WHERE t.department = e.department
    ORDER BY salary DESC
    LIMIT 2
) AS t ON true
ORDER BY e.department;

This code will also output the same results as the first example.

Key Points:

• Both methods achieve the same goal of retrieving the top N rows for each group.
• The window function approach is generally more concise and efficient.
• Choose the method that best suits your specific needs and PostgreSQL version.

1. Recursive Common Table Expression (CTE) (Advanced):

   • This method is more complex and might be less performant for large datasets. It involves a recursive CTE that iterates through distinct group values and retrieves the first N rows for each group.

   WITH RECURSIVE cte AS (
       SELECT DISTINCT grouping_column, 1 AS level
       FROM your_table
       UNION ALL
       SELECT yt.grouping_column, c.level + 1 AS level
       FROM your_table yt
       JOIN cte c ON c.grouping_column = yt.grouping_column
       WHERE c.level < N
   )
   SELECT yt.*
   FROM your_table yt
   JOIN cte c ON c.grouping_column = yt.grouping_column
   WHERE c.level = 1;
   

   • Explanation:
     • The CTE (cte) recursively iterates, building a hierarchy of distinct group values with increasing level.
     • The outer query then joins your_table with cte and selects rows where level is 1, effectively retrieving the first row for each group. However, this can be extended to retrieve the first N rows by adjusting the conditions and joining logic within the CTE.

2. DISTINCT ON and LIMIT (PostgreSQL-specific):

   • This method leverages the DISTINCT ON clause, which is specific to PostgreSQL and not part of the standard SQL language. It allows selecting distinct rows based on a specified column and then applying a LIMIT clause.

   SELECT *
   FROM (
       SELECT DISTINCT ON (grouping_column) your_column1, your_column2
       FROM your_table
       ORDER BY grouping_column, order_column
       LIMIT N
   ) AS subquery;
   

   • Explanation:
     • DISTINCT ON ensures we only get distinct rows based on grouping_column.
     • The ORDER BY clause sorts the rows within each group.
     • LIMIT N restricts the result set to the top N rows for each group.

Choosing the Right Alternative Method:

• The recursive CTE approach is less common and might be less efficient for large datasets. It's generally used for more complex scenarios where other methods are not suitable.
• DISTINCT ON with LIMIT is a PostgreSQL-specific solution and might not be portable to other SQL databases.

Remember:

• Adapt the column names and ordering criteria in the examples to match your table structure.
• Consider the performance implications of each method, especially for large datasets. The window function approach is often the most performant choice.