𝗦𝗤𝗟 𝗤𝘂𝗲𝗿𝗶𝗲𝘀 𝗗𝗼𝗻'𝘁 𝗦𝘁𝗮𝗿𝘁 𝗪𝗶𝘁𝗵 𝗦𝗘𝗟𝗘𝗖𝗧

We usually start with SQL by entering SELECT, but it is not executed first. SELECT actually gets executed as the 5th step.

This explains why some SQL statements work while others don’t.

Here is the actual execution order:

𝟭. 𝗙𝗥𝗢𝗠/𝗝𝗢𝗜𝗡 – Tables are merged to form the basis of the data set. All ON conditions are assessed at this point

𝟮. 𝗪𝗛𝗘𝗥𝗘 – Rows are filtered before any grouping takes place. You cannot refer to any aggregates at this point because none have been created yet

𝟯. 𝗚𝗥𝗢𝗨𝗣 𝗕𝗬 – The remaining rows are grouped. After this step, you're working with groups rather than individual rows

𝟰. 𝗛𝗔𝗩𝗜𝗡𝗚 – Groups get filtered. This is where you can filter on aggregate results like 𝗖𝗢𝗨𝗡𝗧(*) > 𝟱

𝟱. 𝗦𝗘𝗟𝗘𝗖𝗧 – Columns are computed and returned. Window functions run here too

𝟲. 𝗢𝗥𝗗𝗘𝗥 𝗕𝗬 – Results are sorted. This runs after SELECT, so you can use aliases in your sorting

𝟳. 𝗟𝗜𝗠𝗜𝗧 – Output is capped to the specified row count

This order answers common debugging questions. Can you filter on a window function result in a WHERE clause? No, window functions run in SELECT, which happens after WHERE.

Can you use a GROUP BY alias in ORDER BY? Yes, ORDER BY runs last. This is also why 𝗣𝗼𝘀𝘁𝗴𝗿𝗲𝘀, 𝗠𝘆𝗦𝗤𝗟, and 𝗦𝗤𝗟 𝗦𝗲𝗿𝘃𝗲𝗿 all reject 𝗪𝗛𝗘𝗥𝗘 𝗖𝗢𝗨𝗡𝗧(*) > 𝟱, as the aggregate doesn't exist at that stage.

Database engines optimize heavily and may reorder operations for performance. But this logical order determines what's valid SQL.