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12.11.1. GROUP BY (Aggregate) Functions

This section describes group (aggregate) functions that operate on sets of values. Unless otherwise stated, group functions ignore NULL values.

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

The SUM() and AVG() aggregate functions do not work with temporal values. (They convert the values to numbers, which loses the part after the first non-numeric character.) To work around this problem, you can convert to numeric units, perform the aggregate operation, and convert back to a temporal value. Examples:

SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(time_col))) FROM tbl_name;
SELECT FROM_DAYS(SUM(TO_DAYS(date_col))) FROM tbl_name;
  • AVG([DISTINCT] expr)

    Returns the average value of expr. The DISTINCT option can be used to return the average of the distinct values of expr.

    AVG() returns NULL if there were no matching rows.

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

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

    This function returns 18446744073709551615 if there were no matching rows. (This is the value of an unsigned BIGINT value with all bits set to 1.)

  • BIT_OR(expr)

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

    This function returns 0 if there were no matching rows.

  • BIT_XOR(expr)

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

    This function returns 0 if there were no matching rows.

  • COUNT(expr)

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

    COUNT() returns 0 if there were no matching rows.

    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 or not they contain NULL values.

    COUNT(*) is optimized 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;
    

    This optimization applies only to MyISAM tables only, because an exact row count is stored for this storage engine and can be accessed very quickly. For transactional storage engines such as InnoDB and BDB, storing an exact row count is more problematic because multiple transactions may be occurring, each of which may affect the count.

  • COUNT(DISTINCT expr,[expr...])

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

    COUNT(DISTINCT) returns 0 if there were no matching rows.

    mysql> SELECT COUNT(DISTINCT results) FROM student;
    

    In MySQL, you can obtain the number of distinct expression combinations that do not contain NULL by giving a list of expressions. In standard SQL, you would have to do a concatenation of all expressions inside COUNT(DISTINCT ...).

  • GROUP_CONCAT(expr)

    This function returns a string result with the concatenated non-NULL values from a group. It returns NULL if there are no non-NULL values. The full syntax is as follows:

    GROUP_CONCAT([DISTINCT] expr [,expr ...]
                 [ORDER BY {unsigned_integer | col_name | expr}
                     [ASC | DESC] [,col_name ...]]
                 [SEPARATOR str_val])
    
    mysql> SELECT student_name,
        ->     GROUP_CONCAT(test_score)
        ->     FROM student
        ->     GROUP BY student_name;
    

    Or:

    mysql> SELECT student_name,
        ->     GROUP_CONCAT(DISTINCT test_score
        ->               ORDER BY test_score DESC SEPARATOR ' ')
        ->     FROM student
        ->     GROUP BY student_name;
    

    In MySQL, you can get the concatenated values of expression combinations. You can eliminate duplicate values by using DISTINCT. If you want to sort values in the result, you should use ORDER BY clause. To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by in the ORDER BY clause. The default is ascending order; this may be specified explicitly using the ASC keyword. SEPARATOR is followed by the string value that should be inserted between values of result. The default is a comma (‘,’). You can eliminate the separator altogether by specifying SEPARATOR ''.

    You can set a maximum allowed length with the group_concat_max_len system variable. (The default value is 1024.) The syntax to do this at runtime is as follows, where val is an unsigned integer:

    SET [SESSION | GLOBAL] group_concat_max_len = val;
    

    If a maximum length has been set, the result is truncated to this maximum length.

    See also CONCAT() and CONCAT_WS(): Section 12.3, “String Functions”.

  • MIN([DISTINCT] expr), MAX([DISTINCT] 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 Section 7.4.5, “How MySQL Uses Indexes”. The DISTINCT keyword can be used to find the minimum or maximum of the distinct values of expr, however, this produces the same result as omitting DISTINCT.

    MIN() and MAX() return NULL if there were no matching rows.

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

    For MIN(), MAX(), and other aggregate functions, MySQL currently compares ENUM and SET columns by their string value rather than by the string's relative position in the set. This differs from how ORDER BY compares them. This is expected to be rectified in a future MySQL release.

  • STD(expr) STDDEV(expr)

    Returns the population standard deviation of expr. This is an extension to standard SQL. The STDDEV() form of this function is provided for compatibility with Oracle. The standard SQL function STDDEV_POP() can be used instead.

    These functions return NULL if there were no matching rows.

  • STDDEV_POP(expr)

    Returns the population standard deviation of expr (the square root of VAR_POP()). You can also use STD() or STDDEV(), which are equivalent but not standard SQL.

    STDDEV_POP() returns NULL if there were no matching rows.

  • STDDEV_SAMP(expr)

    Returns the sample standard deviation of expr (the square root of VAR_SAMP().

    STDDEV_SAMP() returns NULL if there were no matching rows.

  • SUM([DISTINCT] expr)

    Returns the sum of expr. If the return set has no rows, SUM() returns NULL. The DISTINCT keyword can be used in MySQL 5.1 to sum only the distinct values of expr.

    SUM() returns NULL if there were no matching rows.

  • VAR_POP(expr)

    Returns the population standard variance of expr. It considers rows as the whole population, not as a sample, so it has the number of rows as the denominator. You can also use VARIANCE(), which is equivalent but is not standard SQL.

    VAR_POP() returns NULL if there were no matching rows.

  • VAR_SAMP(expr)

    Returns the sample variance of expr. That is, the denominator is the number of rows minus one.

    VAR_SAMP() returns NULL if there were no matching rows.

  • VARIANCE(expr)

    Returns the population standard variance of expr. This is an extension to standard SQL. The standard SQL function VAR_POP() can be used instead.

    VARIANCE() returns NULL if there were no matching rows.


 
 
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