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If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.

COUNT(expr)

Returns a count of the number of non-NULL values in the rows retrieved by a SELECT statement:

mysql> SELECT student.student_name,COUNT(*)
    ->        FROM student,course
    ->        WHERE student.student_id=course.student_id
    ->        GROUP BY student_name;

COUNT(*) is somewhat different in that it returns a count of the number of rows retrieved, whether they contain NULL values.

COUNT(*) is optimised to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause. For example:

mysql> SELECT COUNT(*) FROM student;

COUNT(DISTINCT expr,[expr...])

Returns a count of the number of different non-NULL values:

mysql> SELECT COUNT(DISTINCT results) FROM student;

In MySQL you can get the number of distinct expression combinations that don't contain NULL by giving a list of expressions. In ANSI SQL you would have to do a concatenation of all expressions inside COUNT(DISTINCT ...).

AVG(expr)

Returns the average value of expr:

mysql> SELECT student_name, AVG(test_score)
-> FROM student
-> GROUP BY student_name;

MIN(expr)

MAX(expr)

Returns the minimum or maximum value of expr. MIN() and MAX() may take a string argument; in such cases they return the minimum or maximum string value. See MySQL indexes.

mysql> SELECT student_name, MIN(test_score), MAX(test_score)
-> FROM student
-> GROUP BY student_name;

SUM(expr)

Returns the sum of expr. Note that if the return set has no rows, it returns NULL!

STD(expr)

STDDEV(expr)

Returns the standard deviation of expr. This is an extension to ANSI SQL. The STDDEV() form of this function is provided for Oracle compatibility.

BIT_OR(expr)

Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

BIT_AND(expr)

Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit (BIGINT) precision.

MySQL has extended the use of GROUP BY. You can use columns or calculations in the SELECT expressions that don't appear in the GROUP BY part. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on customer.name in the following query:

mysql> SELECT order.custid,customer.name,MAX(payments)
    ->        FROM order,customer
    ->        WHERE order.custid = customer.custid
    ->        GROUP BY order.custid;

In ANSI SQL, you would have to add customer.name to the GROUP BY clause. In MySQL, the name is redundant if you don't run in ANSI mode.

Don't use this feature if the columns you omit from the GROUP BY part aren't unique in the group! You will get unpredictable results.

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it isn't unique. The following gives the value of column from the row containing the smallest value in the sort column:

SUBSTR(MIN(CONCAT(RPAD(sort,6,' '),column)),7)

See example-Maximum-column-group-row.

Note that if you are using MySQL Version 3.22 (or earlier) or if you are trying to follow ANSI SQL, you can't use expressions in GROUP BY or ORDER BY clauses. You can work around this limitation by using an alias for the expression:

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

In MySQL Version 3.23 you can do:

mysql> SELECT id,FLOOR(value/100) FROM tbl_name ORDER BY RAND();