colithium colithium - 1 year ago 96
MySQL Question

MySQL - Selecting a Column not in Group By

I'm trying to add features to a preexisting application and I came across a MySQL view something like this:

FROM table_name
GROUP BY table_name.personID;

OK so there's a few aggregate functions. You can select personID because you're grouping by it. But it also is selecting a column that is not in an aggregate function and is not a part of the GROUP BY clause. How is this possible??? Does it just pick a random value because the values definitely aren't unique per group?

Where I come from (MSSQL Server), that's an error. Can someone explain this behavior to me and why it's allowed in MySQL?

Answer Source

It's true that this feature permits some ambiguous queries, and silently returns a result set with an arbitrary value picked from that column. In practice, it tends to be the value from the row within the group that is physically stored first.

These queries aren't ambiguous if you only choose columns that are functionally dependent on the column(s) in the GROUP BY criteria. In other words, if there can be only one distinct value of the "ambiguous" column per value that defines the group, there's no problem. This query would be illegal in Microsoft SQL Server (and ANSI SQL), even though it cannot logically result in ambiguity:

SELECT AVG(table1.col1), table1.personID, persons.col4
FROM table1 JOIN persons ON (table1.personID =
GROUP BY table1.personID;

Also, MySQL has an SQL mode to make it behave per the standard: ONLY_FULL_GROUP_BY

FWIW, SQLite also permits these ambiguous GROUP BY clauses, but it chooses the value from the last row in the group.

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