Glossary · Phase 7 · Software engineering

Window function

Also written: window functions

A SQL calculation across a set of rows, like a rank or a running total, that keeps every row instead of grouping them.

GROUP BY collapses rows. A window function calculates over a set of rows but keeps every row, adding the result beside it. You write it with OVER (...): PARTITION BY says which rows belong together (like a group), and ORDER BY sets the order within them.
From the lesson SQL for analysis

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SQL is written SELECT first but worked out in this order, which is why a WHERE can't use a column alias but ORDER BY can.

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SQL for analysis · Subqueries, CASE, common table expressions, outer joins and window functions on real order data. About 45 minutes.

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Other terms from the same lesson

anti-join: Finding rows with no match in another table, usually a LEFT JOIN followed by WHERE ... IS NULL.
common table expression: A named step in a SQL query, written with WITH, that later steps can use like a table.
subquery: A query inside another query, which runs first and hands its result to the outer one.

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