MySQL Handling of GROUP BY--官方文档

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. 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. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values within each group the server chooses.

A similar MySQL extension applies to the HAVING clause. In standard SQL, a query that includes a GROUP BYclause cannot refer to nonaggregated columns in the HAVING clause that are not named in the GROUP BY clause. A MySQL extension permits references to such columns to simplify calculations. This extension assumes that the nongrouped columns will have the same group-wise values. Otherwise, the result is indeterminate.

To disable the MySQL GROUP BY extension, enable the ONLY_FULL_GROUP_BY SQL mode. This enables standard SQL behavior: Columns not named in the GROUP BY clause cannot be used in the select list or HAVING clause unless enclosed in an aggregate function.

ONLY_FULL_GROUP_BY also affects use of aliases in the HAVING clauses. For example, the following query returnsname values that occur only once in table orders:

SELECT name, COUNT(name) FROM orders
  GROUP BY name
  HAVING COUNT(name) = 1;

MySQL extends this behavior to permit the use of an alias in the HAVING clause for the aggregated column:

SELECT name, COUNT(name) AS c FROM orders
  GROUP BY name
  HAVING c = 1;

Enabling ONLY_FULL_GROUP_BY disables this MySQL extension and a non-grouping field 'c' is used in HAVING clause error occurs because the column c in the HAVING clause is not enclosed in an aggregate function (instead, it is an aggregate function).

The select list extension also applies to ORDER BY. That is, you can refer to nonaggregated columns in the ORDER BY clause that do not appear in the GROUP BY clause. (However, as mentioned previously, ORDER BY does not affect which values are chosen from nonaggregated columns; it only sorts them after they have been chosen.) This extension does not apply if the ONLY_FULL_GROUP_BY SQL mode is enabled.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it is not unique. If the sortcolumn contains integers no larger than 6 digits, the following query gives the value of column from the row containing the smallest sort value:

SUBSTR(MIN(CONCAT(LPAD(sort,6,'0'),column)),7)

See Section 3.6.4, “The Rows Holding the Group-wise Maximum of a Certain Column”.

If you are trying to follow standard SQL, you cannot use expressions in GROUP BY clauses. As a workaround, use an alias for the expression:

SELECT id, FLOOR(value/100) AS val
  FROM tbl_name
  GROUP BY id, val;

MySQL permits expressions in GROUP BY clauses, so the alias is unnecessary:

SELECT id, FLOOR(value/100)
  FROM tbl_name
  GROUP BY id, FLOOR(value/100);

http://dev.mysql.com/doc/refman/5.0/en/group-by-handling.html