SQL Window Functions – Apply Calculations Over Sets and Frames

SQL window functions are powerful tools that allow you to perform calculations across sets of rows in a query result set. Instead of doing aggregate computations over entire tables, window functions apply an aggregate or analytic logic to each row along with the related set of rows.

This enables flexible analysis without requiring complex self-joins or subqueries. Window functions partition result sets into movable windows or frames over which calculations can be executed. They are invaluable for many complex reporting and analytics tasks.

What are Window Functions?

Window functions perform a calculation on a set of rows related to the current row. They allow you to apply aggregate-like logic to multiple rows without collapsing into a single output row like GROUP BY does.

Some key characteristics of window functions:

• Apply a calculation to each row in the result set along with some number of surrounding rows
• Calculate aggregates and rankings without grouping the data
• Avoid complex joins, subqueries, and nested SQL
• Offer flexibility for sophisticated reporting and analytics

Instead of mashing rows together, window functions slide a movable frame across sorted rows while applying logic over each section.

How Window Functions Work

The key to understanding window functions is the concept of window frames. As the window slides down the rows, you can access a subset of rows within the frame for each row.

This could be the current row, previous and next rows, a fixed number of rows before and after, or other ranges depending on the frame definition. Calculations execute over this moveable row subset.

Several components work together to control window functionality:

PARTITION BY – Splits rows into groups or partitions to scope the window ORDER BY – Sorts rows to define the window order Frame – Specifies subset of rows in the window from current row Function – Applies logic across the window frame

These pieces allow flexible slicing and analyzing of results without complex SQL.

Common Uses for Window Functions

Window functions shine for calculations that require:

• Ranking – Rank items by a metric in ordered results
• Running totals – Progressive totals as window slides down
• Percentages – Percent of partition or whole table
• Time intelligence – Compare time periods and slices
• Lag/lead analysis – Relate rows to prior or next rows
• Partitioned analytics – Analyze segments independently

They serve many common (and uncommon!) business analysis needs without difficult coding.

Types of Window Functions

Several categories of window functions exist, each serving distinct purposes:

Ranking Functions

Ranking window functions order results and produce a ranking value relative to the window frame for each row:

• ROW_NUMBER() – Number rows sequentially
• RANK() – Rank items with gaps in ranking values
• DENSE_RANK() – Rank items with consecutive integers
• NTILE(n) – Divide rows into n number of buckets

Analytic Functions

Analytic functions perform advanced calculations comparing row values to other rows in the window:

• LAG()/LEAD() – Access prior/next row values
• FIRST_VALUE()/LAST_VALUE() – First and last values in window frame
• PERCENT_RANK() – Percentage rank of current row

Aggregate Functions

Standard aggregates like SUM() and AVG() can also be used as window functions. They aggregate over the rows within the frame rather than the entire set.

Window Function Syntax and Components

Window functions use the basic syntax:

FUNCTION(expression) OVER(
  [PARTITION BY partition_groups]
  [ORDER BY sorting_columns] 
  [frame_definition]
)

The key components that control window processing:

PARTITION BY – Groups rows into partitions and performs windowing on each one independently.

PARTITION BY country, region

ORDER BY – Specifies ordering of rows to align windows and define processing order.

ORDER BY date ROWS BETWEEN CURRENT ROW AND 1 FOLLOWING

Frame – Subset of rows in a window the function applies to. Defines size of sliding partition.

ROWS – Specifies logical offset rows before and after current row.

RANGE – Defines window based on value range before and after.

Examples and Use Cases

Window functions open many possibilities. A few examples:

Rank contacts by revenue:

SELECT 
  name, revenue, 
  RANK() OVER(ORDER BY revenue DESC) revenue_rank  
FROM contacts;

Running total revenue over time:

SELECT 
  date, revenue,
  SUM(revenue) OVER(ORDER BY DATE) running_revenue
FROM financials;

Lag – compare to prior row value:

SELECT
  date, value, 
  LAG(value, 1) OVER(ORDER BY date) prev_value
FROM performance;

Employee salary as percentage of department:

SELECT
  name, salary, dept,
  ROUND(RATIO_TO_REPORT(salary) OVER(PARTITION BY dept) * 100) pct
FROM employees;

Benefits of Using Window Functions

Window functions provide vital solutions for reporting, analytics, and business intelligence. Benefits include:

• Simpler code – Avoid complex JOINS, UNIONs, and subqueries
• Granular calculations – Analyze slices without collapsing rows
• Ranking – Rank ordered results with gaps/densification
• Time intelligence – Relate series of time periods
• Partitioned analysis – Analyze segments independently
• Row comparisons – Look at previous/next row values

By applying aggregate-like logic per row over movable frames, window functions solve many complex requirements without extra work.

FAQ

What is the difference between window functions and aggregates?

Aggregates perform a calculation over an entire set and return a single result row. Window functions apply a calculation to each row along with related rows, without collapsing the result set.

Can window functions be used with GROUP BY?

Yes, window functions can be used with GROUP BY. The GROUP BY groups query results into aggregate rows. Window functions then perform their own calculation on the groups.

What SQL engines support window functions?

Most major databases support window functions including Oracle, SQL Server, PostgreSQL, MySQL 8+, BigQuery, Redshift, and Snowflake. Syntax may vary slightly.

What are some limitations of window functions?

Window function processing requires sorting all queried rows, so performance degrades on huge datasets. Databases are optimized for aggregates over full-table window calculations. Advanced partitioning and querying approaches can improve performance.

Conclusion

SQL window functions enable sophisticated analysis by applying aggregate-like logic across sets of rows. They slide a frame across groups of rows while performing calculations related to each row.

Mastering functions like ranking, intervals, lag/lead analysis provides flexibility for data analysis that is difficult or inefficient with plain aggregates or self-joins.

Window functions open the door to explore data in new ways. By dividing and conquering subsets of rows, they make once complex operations elegantly simple and efficient.