SQL vs MySQL: Rules about aggregate operations and GROUP BY

Question

In this book I\'m currently reading while following a course on databases, the following example of an illegal query using an aggregate operator is given:

Answer 1

Based on a link which a_horse_with_no_name provided in a comment, I have arrived at my own answer:

It seems that the MySQL way of using GROUP BY differs from the SQL way, in order to permit leaving out columns, from the GROUP BY clause, when they are functionally dependant on other included columns anyways.

Lets say we have a table displaying the activity of a bank account. It's not a very thought-out table, but it's the only one we have, and that will have to do. Instead of keeping track of an amount, we imagine an account starts at '0', and all transactions to it is recorded instead, so the amount is the sum of the transactions. The table could look like this:

+------------+----------+-------------+
| costumerID | name     | transaction |
+------------+----------+-------------+
|       1337 | h4x0r    |         101 |
|         42 | John Doe |         500 |
|       1337 | h4x0r    |        -101 |
|         42 | John Doe |        -200 |
|         42 | John Doe |         500 |
|         42 | John Doe |        -200 |
+------------+----------+-------------+

It is clear that the 'name' is functionally dependant on the 'costumerID'. (The other way around would also be possible in this example.)

What if we wanted to know the costumerID, name and current amount of each customer?

In such a situation, two very similar queries would return the following right result:

+------------+----------+--------+
| costumerID | name     | amount |
+------------+----------+--------+
|         42 | John Doe |    600 |
|       1337 | h4x0r    |      0 |
+------------+----------+--------+

This query can be executed in MySQL, and is legal according to SQL.

SELECT costumerID, name, SUM(transaction) AS amount
FROM Activity
GROUP BY costumerID, name

This query can be executed in MySQL, and is NOT legal according to SQL.

SELECT costumerID, name, SUM(transaction) AS amount
FROM Activity
GROUP BY costumerID

The following line would make the query return and error instead, since it would now have to follow the SQL way of using aggregation operations and GROUP BY:

SET sql_mode = 'ONLY_FULL_GROUP_BY';

The argument for allowing the second query in MySQL, seems to be that it is assumed that all columns mentioned in SELECT, but not mentioned in GROUP BY, are either used inside an aggregate operation, (the case with 'transaction'), or are functionally dependent on other included columns, (the case with 'name'). In the case of 'name', we can be sure that the correct 'name' is chosen for all group entries, since it is functionally dependant on 'costumerID', and therefore there is only one possibly name for each group of costumerID's.

This way of using GROUP BY seems flawed tough, since it doesn't do any further checks on what is left out from the GROUP BY clause. People can pick and choose columns from their SELECT statement to put in their GROUP BY clause as they see fit, even if it makes no sense to include or leave out any particular column.

The Sailor example illustrates this flaw very well. When using aggregation operators (possibly in conjunction with GROUP BY), each group entry in the returned set has only one value for each of its columns. In the case of Sailors, since the GROUP BY clause is left out, the whole table is put into one single group entry. This entry needs a name and a maximum age. Choosing a maximum age for this entry is a no-brainer, since MAX(S.age) only returns one value. In the case of S.sname though, wich is only mentioned in SELECT, there are now as many choices as there are unique sname's in the whole Sailor table, (in this case two, John and Jane Doe). MySQL doens't have any clue which to choose, we didn't give it any, and it didn't hit the brakes in time, so it has to just pick whatever comes first, (Jane Doe). If the two rows were switched, it would actually give "the right answer" by accident. It just seems plain dumb that something like this is allowed in MySQL, that the result of a query using GROUP BY could potententially depend on the ordering of the table, if something is left out in the GROUP BY clause. Apparently, that's just how MySQL rolls. But still couldn't it at least have the courtesy of warning us when it has no clue what it's doing because of a "flawed" query? I mean, sure, if you give the wrong instructions to a program, it probably wouldn't (or shouldn't) do as you want, but if you give unclear instructions, I certainly wouldn't want it to just start guessing or pick whatever comes first... -_-'

Answer 2

Is the query above really illegal in SQL, but legal in MySQL

Yes

if so, why is that

I don't know the reasons for the design decisions made in MySQL, but considering that you can get the actual related data from the same row(s) as the aggregate came from (e.g., MAX or MIN) with only slightly more work, I don't see any advantage in returning additional column data from arbitrary rows.

I strongly dislike this "feature" in MySQL and it trips up many people who learn aggregates on MySQL and then move to a different dbms, and suddenly realize they never quite knew what they were doing.

Answer 3

MySQL allows this non-standard SQL syntax because there is at least one specific case in which it makes the SQL nominally easier to write. That case is when you're joining two tables which have a PRIMARY / FOREIGN KEY relationship (whether enforced by the database or not) and you want an aggregate value from the FOREIGN KEY side and multiple columns from the PRIMARY KEY side.

Consider a system with Customer and Orders tables. Imagine you want all the fields from the customer table along with the total of the Amount field from the Orders table. In standard SQL you would write:

 SELECT C.CustomerID, C.FirstName, C.LastName, C.Address, C.City, C.State, C.Zip, SUM(O.Amount)
    FROM Customer C INNER JOIN Orders O ON C.CustomerID = O.CustomerID
    GROUP BY C.CustomerID, C.FirstName, C.LastName, C.Address, C.City, C.State, C.Zip

Notice the unwieldy GROUP BY clause, and imagine what it would look like if there were more columns you wanted from customer.

In MySQL, you could write:

 SELECT C.CustomerID, C.FirstName, C.LastName, C.Address, C.City, C.State, C.Zip, SUM(O.Amount)
    FROM Customer C INNER JOIN Orders O ON C.CustomerID = O.CustomerID
    GROUP BY C.CustomerID

or even (I think, I haven't tried it):

 SELECT C.*, SUM(O.Amount)
    FROM Customer C INNER JOIN Orders O ON C.CustomerID = O.CustomerID
    GROUP BY C.CustomerID

Much easier to write. In this particular case it's safe as well, since you know that only one row from the Customer table will contribute to each group (assuming CustomerID is PRIMARY or UNIQUE KEY).

Personally, I'm not a big fan of this exception to standard SQL syntax (since there are many cases where it's not safe to use this syntax and rely on getting values from any particular row in the group), but I can see where it makes certain kinds of queries easier and (in the case of my second MySQL example) possible.

Answer 4

By the way, it is default MySQL behavior. But it can be changed by setting ONLY_FULL_GROUP_BY server mode in the my.ini file or in the session -

SET sql_mode = 'ONLY_FULL_GROUP_BY';
SELECT * FROM sakila.film_actor GROUP BY actor_id;

Error: 'sakila.film_actor.film_id' isn't in GROUP BY

ONLY_FULL_GROUP_BY - Do not permit queries for which the select list refers to nonaggregated columns that are not named in the GROUP BY clause.