SQL PARTITION BY Function
Quick summary: The SQL PARTITION BY clause divides rows into groups for window functions.
SQL PARTITION BY Syntax
OVER ( PARTITION BY column1, column2 ... )SQL
SQL PARTITION BY Basic examples
SELECT user_id, COUNT(*) OVER (PARTITION BY user_id) FROM orders;SQL
Output:
Order count per user on each rowPartitions rows by user_id.
SQL PARTITION BY Real-world usage
SELECT department, salary, AVG(salary) OVER (PARTITION BY department) FROM employees;SQL
Output:
Department salary averagesCalculates per-group metrics.
SQL PARTITION BY Edge cases
SELECT COUNT(*) OVER (PARTITION BY NULL) FROM users;SQL
Output:
Same count for all rowsNULL creates a single partition.
SQL PARTITION BY Common mistakes
Confusing PARTITION BY with GROUP BY
PARTITION BY does not collapse rows.
Incorrect
GROUP BY departmentCorrect
OVER (PARTITION BY department)Use PARTITION BY for window functions.
SQL PARTITION BY Frequently Asked Questions
What does PARTITION BY do in SQL?
Divides rows into groups for window functions.
Use case of PARTITION BY?
Group-based calculations.
Used with window functions?
Yes.
Syntax?
OVER (PARTITION BY column).
Difference from GROUP BY?
Does not collapse rows.
Common mistake?
Confusing with GROUP BY.
Used with ROW_NUMBER()?
Yes.
Used with RANK()?
Yes.
Performance?
Depends on partition size.
Supports multiple columns?
Yes.
Used in analytics?
Yes.
Best practice?
Partition logically related data.