Oracle does not extend the older SQL Standard that states that all items in the select list not contained in an aggregate function must be included in the group by clause.
The MySQL Docs state:
In standard SQL, a query that includes a GROUP BY clause cannot refer to nonaggregated columns in the select list that are not named in the GROUP BY clause. For example, this query is illegal in standard SQL because the name column in the select list does not appear in the GROUP BY:
SELECT o.custid, c.name, MAX(o.payment)
FROM orders AS o, customers AS c
WHERE o.custid = c.custid
GROUP BY o.custid;
For the query to be legal, the name column must be omitted from the select list or named in the GROUP BY clause.
MySQL extends the 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 preceding query is 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.
So to answer your question as to why MySQL does this the most pertinent extract is:
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.
I always advocate to steer clear of this particular MySQL extension unless you fully understand it.
Imagine the following simple table (T):
ID | Column1 | Column2 |
----|---------+----------|
1 | A | X |
2 | A | Y |
In MySQL you can write
SELECT ID, Column1, Column2
FROM T
GROUP BY Column1;
This actually breaks the SQL Standard, but it works in MySQL, however the trouble is it is non-deterministic, the result:
ID | Column1 | Column2 |
----|---------+----------|
1 | A | X |
Is no more or less correct than
ID | Column1 | Column2 |
----|---------+----------|
2 | A | Y |
So what you are saying is give me one row for each distinct value of Column1
, which both results sets satisfy, so how do you know which one you will get? Well you don't, it seems to be a fairly popular misconception that you can add and ORDER BY
clause to influence the results, so for example the following query:
SELECT ID, Column1, Column2
FROM T
GROUP BY Column1
ORDER BY ID DESC;
Would ensure that you get the following result:
ID | Column1 | Column2 |
----|---------+----------|
2 | A | Y |
because of the ORDER BY ID DESC
, however this is not true (as demonstrated here).
The MySQL documents state:
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.
So even though you have an order by this does not apply until after one row per group has been selected, and this one row is non-determistic.
The SQL-Standard does allow columns in the select list not contained in the GROUP BY or an aggregate function, however these columns must be functionally dependant on a column in the GROUP BY. From the SQL-2003-Standard:
15) If T is a grouped table, then let G be the set of grouping columns of T. In each contained
in , each column reference that references a column of T shall reference some column C that
is functionally dependent on G or shall be contained in an aggregated argument of a
whose aggregation query is QS.
For example, ID in the sample table is the PRIMARY KEY, so we know it is unique in the table, so the following query conforms to the SQL standard and would run in MySQL and fail in many DBMS currently (At the time of writing Postgresql is the closest DBMS I know of to correctly implementing the standard - Example here):
SELECT ID, Column1, Column2
FROM T
GROUP BY ID;
Since ID is unique for each row, there can only be one value of Column1
for each ID, one value of Column2
there is no ambiguity about what to return for each row.