PostgreSQL - ROW_NUMBER Function

In PostgreSQL, the ROW_NUMBER() function is a window function that assigns a unique sequential integer to rows within a result set. It can be used to generate a series of numbers for each row, often in conjunction with the ORDER BY clause within the window definition to determine the order in which to assign numbers.

Syntax:

ROW_NUMBER() OVER (
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...
)

PARTITION BY: This divides the result set into partitions to which the ROW_NUMBER() is applied. If you omit it, the function treats the whole result set as a single partition.
ORDER BY: This determines the order in which the row numbers are assigned.

Example:

Consider the following employees table:

emp_id	emp_name	department
1	Alice	HR
2	Bob	Finance
3	Charlie	HR
4	David	IT
5	Eva	IT

If you want to assign a unique row number to each employee within their respective departments, ordered by their emp_id, you can use the ROW_NUMBER() function:

SELECT 
    emp_id,
    emp_name,
    department,
    ROW_NUMBER() OVER (PARTITION BY department ORDER BY emp_id) as row_num
FROM 
    employees;

The result would be:

emp_id	emp_name	department	row_num
2	Bob	Finance	1
1	Alice	HR	1
3	Charlie	HR	2
4	David	IT	1
5	Eva	IT	2

As you can see, the ROW_NUMBER() function has assigned a unique number to each row, starting from 1 for each partition (in this case, each department).

Usage Notes:

If two or more rows in a partition have the same values in the ORDER BY clause, the ROW_NUMBER() function will assign each of them a unique value arbitrarily. This is one of the main differences between ROW_NUMBER() and the RANK() function, which would assign the same rank to rows with the same values.
ROW_NUMBER() does not require an ORDER BY clause; however, without it, the order of row numbers will be non-deterministic.

In summary, the ROW_NUMBER() function in PostgreSQL is a versatile tool for generating unique sequential numbers for rows within a result set based on specified criteria. It's particularly useful for scenarios such as pagination or finding duplicate rows.

How to use ROW_NUMBER function in PostgreSQL: The ROW_NUMBER function assigns a unique number to each row within a result set.
```
SELECT column1, column2, ROW_NUMBER() OVER (ORDER BY column1) AS row_num
FROM your_table;
```

Generating row numbers with ROW_NUMBER in PostgreSQL:

SELECT employee_name, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank
FROM employees;

PARTITION BY clause with ROW_NUMBER in PostgreSQL:

SELECT department, employee_name, salary, ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rank
FROM employees;

ORDER BY clause with ROW_NUMBER in PostgreSQL:

SELECT product_name, unit_price, ROW_NUMBER() OVER (ORDER BY unit_price) AS row_num
FROM products;

Resetting row numbers with ROW_NUMBER in PostgreSQL: Row numbers automatically reset for each partition. If you want to manually reset, use a window specification.

Handling NULL values with ROW_NUMBER in PostgreSQL:

SELECT column1, column2, ROW_NUMBER() OVER (ORDER BY COALESCE(column1, 0)) AS row_num
FROM your_table;

ROW_NUMBER with window functions in PostgreSQL:

SELECT department, employee_name, salary,
       ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS rank,
       AVG(salary) OVER (PARTITION BY department) AS avg_salary
FROM employees;

ROW_NUMBER with different data types in PostgreSQL: Row numbers are integers. Convert to other data types if needed.

ROW_NUMBER and pagination in PostgreSQL:

SELECT column1, column2
FROM your_table
ORDER BY column1
OFFSET 10 ROWS FETCH NEXT 5 ROWS ONLY;

Using ROW_NUMBER for ranking in PostgreSQL:

SELECT product_name, unit_price,
       RANK() OVER (ORDER BY unit_price) AS rank,
       DENSE_RANK() OVER (ORDER BY unit_price) AS dense_rank
FROM products;

ROW_NUMBER with JOIN operations in PostgreSQL:

SELECT e.employee_name, e.salary, d.department_name,
       ROW_NUMBER() OVER (PARTITION BY d.department_name ORDER BY e.salary DESC) AS rank
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

ROW_NUMBER and filtering results in PostgreSQL: Apply filters after assigning row numbers.
ROW_NUMBER with subqueries in PostgreSQL: Use ROW_NUMBER in subqueries for more complex scenarios.

Combining ROW_NUMBER with other window functions in PostgreSQL:

SELECT department, employee_name, salary,
       ROW_NUMBER() OVER (ORDER BY salary DESC) AS rank,
       RANK() OVER (ORDER BY salary DESC) AS rank2,
       DENSE_RANK() OVER (ORDER BY salary DESC) AS rank3
FROM employees;

ROW_NUMBER and index usage in PostgreSQL: Efficiently use indexes by ensuring the ORDER BY clause aligns with indexed columns.
ROW_NUMBER and unique constraints in PostgreSQL: Consider unique constraints when using ROW_NUMBER for ranking to avoid conflicts.