SQL Window Functions: How to Get the Top Row Per Group

By the end of this page you can answer the question that stops most people the first week they write SQL: which row is the best one in each category. You will know OVER and PARTITION BY , the three ranking functions and how each treats a tie, a running total, and LAG for comparing a row to the one before it. It is about twenty-five minutes. Here is what to actually do with it. The next time you write GROUP BY genre and get back a best rating without the name attached to it, stop rewriting the GROUP BY . Add ROW_NUMBER() OVER (PARTITION BY genre ORDER BY rating DESC) to the plain query instead, then keep the rows numbered 1. That is the whole move, and it replaces a query most people never get working. The short version: a window function adds a calculated column to each row while leaving every row in place. Grouping collapses rows. A window looks at them. One idea decides everything else on this page, so it gets the picture. Both halves do the same arithmetic over the same four rows, and only one of them still has four rows at the end. The original carries a diagram here. In words: Two panels side by side, each starting from the same stack of four identical row shapes. The left panel is labelled GROUP BY. Its four rows funnel down through a single arrow into one row at the bottom, and the four original rows are shown faded to indicate they are gone from the result. Only one row remains. The right panel is labelled OVER. Its four rows stay exactly where they are, at full strength, and each one gains a small badge on its right hand side holding a number: one, two, three, four. Nothing funnels and nothing is faded. The contrast is the whole idea: the left panel ends with a single summary row and no way to say which original row it came from, while the right panel ends with all four rows still present, each carrying its own calculated value. The worked example is real. Every number on this page comes from a published portfolio project: finding the genuinely overlooked g