It allows you to "group by/aggregate/etc" on a row by row basis rather than once over the entire table. So whle GROUP BY defines a set to aggregate over the ent

It allows you to "group by/aggregate/etc" on a row by row basis rather than once over the entire table. So whle GROUP BY defines a set to aggregate over the entire table ( and hence you get 1 row back ), window functions creates a criteria by which each row gets a set ( aka window ) to aggregate over for that particular row. In addition, you can define a "frame" over that "window" for even more refined aggregation. I suppose window functions are used more in the OLAP space than OLTP, but it is a very useful part of SQL. In my opinion, windows function is the most impressive and important part of the language. People can define, describe and provide examples but with SQL, it won't sink in until you try it out yourself and have the "Ah ha" moment. If you have a database server and some test data, try writing a few window function queries or try some online examples.