Why isn't the GROUP BY clause in this MySQL statement throwing errors?

Question

I am using w3s as my example in here

SELECT Shippers.ShipperName,COUNT(Orders.OrderID) AS NumberOfOrders,
    Orders.OrderDate 
FROM Orders
LEFT JOIN Shippers
ON Orders.ShipperID=Shippers.ShipperID
GROUP BY ShipperName;

I am confused why the above statement isn't throwing an error. How does MySQL know which OrderDate to use when we are aggregating by all OrderID ?

Answer 1

How does mysql know which OrderDate to use when we are aggregating by all OrderID?

It doesn't. It just picks one, because it assumes you would have grouped by all the necessary columns, and any columns that weren't in the GROUP BY and weren't subject to any aggregate functions would have the same values for each group.

It's non-standard behavior that works as an optimization, allowing the server to "leak" one of the values through from each group in the source rows into the result-set, reducing the size of the data the GROUP BY has to manage. Which source row's value is used for each group is undefined, so this is intended to be used only in queries where the non-grouped columns are functionally dependent on the grouped columns... because, in that case, "which" row doesn't matter, because they're all the same within each group.

MySQL extends the standard SQL use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the [queries excluding non-aggregated columns are] legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. [emphasis added]

https://dev.mysql.com/doc/refman/5.6/en/group-by-handling.html

You can disable this behavior by including ONLY_FULL_GROUP_BY in @@SQL_MODE .