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### 12.17.1 Aggregate (GROUP BY) Function Descriptions

This section describes group (aggregate) functions that operate on sets of values.

Table 12.21 Aggregate (GROUP BY) Functions

Name Description
`AVG()` Return the average value of the argument
`BIT_AND()` Return bitwise AND
`BIT_OR()` Return bitwise OR
`BIT_XOR()` Return bitwise XOR
`COUNT()` Return a count of the number of rows returned
`COUNT(DISTINCT)` Return the count of a number of different values
`GROUP_CONCAT()` Return a concatenated string
`MAX()` Return the maximum value
`MIN()` Return the minimum value
`STD()` Return the population standard deviation
`STDDEV()` Return the population standard deviation
`STDDEV_POP()` Return the population standard deviation
`STDDEV_SAMP()` Return the sample standard deviation
`SUM()` Return the sum
`VAR_POP()` Return the population standard variance
`VAR_SAMP()` Return the sample variance
`VARIANCE()` Return the population standard variance

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. For more information, see Section 12.17.3, “MySQL Handling of GROUP BY”.

For numeric arguments, the variance and standard deviation functions return a `DOUBLE` value. The `SUM()` and `AVG()` functions return a `DECIMAL` value for exact-value arguments (integer or `DECIMAL`), and a `DOUBLE` value for approximate-value arguments (`FLOAT` or `DOUBLE`).

The `SUM()` and `AVG()` aggregate functions do not work with temporal values. (They convert the values to numbers, losing everything after the first nonnumeric character.) To work around this problem, 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;``````

Functions such as `SUM()` or `AVG()` that expect a numeric argument cast the argument to a number if necessary. For `SET` or `ENUM` values, the cast operation causes the underlying numeric value to be used.

The `BIT_AND()`, `BIT_OR()`, and `BIT_XOR()` aggregate functions perform bit operations. They require `BIGINT` (64-bit integer) arguments and return `BIGINT` values. Arguments of other types are converted to `BIGINT` and truncation might occur.

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

If there are no matching rows, `AVG()` returns `NULL`.

``````mysql> SELECT student_name, AVG(test_score)
FROM student
GROUP BY student_name;``````
• Returns the bitwise `AND` of all bits in `expr`. The calculation is performed with 64-bit (`BIGINT`) precision.

If there are no matching rows, `BIT_AND()` returns a neutral value (all bits set to 1).

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

If there are no matching rows, `BIT_OR()` returns a neutral value (all bits set to 0).

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

If there are no matching rows, `BIT_XOR()` returns a neutral value (all bits set to 0).

• Returns a count of the number of non-`NULL` values of `expr` in the rows retrieved by a `SELECT` statement. The result is a `BIGINT` value.

If there are no matching rows, `COUNT()` returns `0`.

``````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.

For transactional storage engines such as `InnoDB`, storing an exact row count is problematic. Multiple transactions may be occurring at the same time, each of which may affect the count.

`InnoDB` does not keep an internal count of rows in a table because concurrent transactions might see different numbers of rows at the same time. Consequently, `SELECT COUNT(*)` statements only count rows visible to the current transaction.

To process a `SELECT COUNT(*)` statement, `InnoDB` scans an index of the table, which takes some time if the index is not entirely in the buffer pool. For a faster count, create a counter table and let your application update it according to the inserts and deletes it does. However, this method may not scale well in situations where thousands of concurrent transactions are initiating updates to the same counter table. If an approximate row count is sufficient, use `SHOW TABLE STATUS`.

`InnoDB` handles ```SELECT COUNT(*)``` and `SELECT COUNT(1)` operations in the same way. There is no performance difference.

For `MyISAM` tables, `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 only applies to `MyISAM` tables, because an exact row count is stored for this storage engine and can be accessed very quickly. `COUNT(1)` is only subject to the same optimization if the first column is defined as ```NOT NULL```.

• Returns a count of the number of rows with different non-`NULL` `expr` values.

If there are no matching rows, `COUNT(DISTINCT)` returns `0`.

``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 ...)`.

• 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. To eliminate duplicate values, use the `DISTINCT` clause. To sort values in the result, use the `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. The default separator between values in a group is comma (`,`). To specify a separator explicitly, use `SEPARATOR` followed by the string literal value that should be inserted between group values. To eliminate the separator altogether, specify `SEPARATOR ''`.

The result is truncated to the maximum length that is given by the `group_concat_max_len` system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of `max_allowed_packet`. The syntax to change the value of `group_concat_max_len` at runtime is as follows, where `val` is an unsigned integer:

``SET [GLOBAL | SESSION] group_concat_max_len = val;``

The return value is a nonbinary or binary string, depending on whether the arguments are nonbinary or binary strings. The result type is `TEXT` or `BLOB` unless `group_concat_max_len` is less than or equal to 512, in which case the result type is `VARCHAR` or `VARBINARY`.

• Returns the maximum value of `expr`. `MAX()` may take a string argument; in such cases, it returns the maximum string value. See Section 8.3.1, “How MySQL Uses Indexes”. The `DISTINCT` keyword can be used to find the maximum of the distinct values of `expr`, however, this produces the same result as omitting `DISTINCT`.

If there are no matching rows, `MAX()` returns `NULL`.

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

For `MAX()`, 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.

• Returns the minimum value of `expr`. `MIN()` may take a string argument; in such cases, it returns the minimum string value. See Section 8.3.1, “How MySQL Uses Indexes”. The `DISTINCT` keyword can be used to find the minimum of the distinct values of `expr`, however, this produces the same result as omitting `DISTINCT`.

If there are no matching rows, `MIN()` returns `NULL`.

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

For `MIN()`, 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.

• Returns the population standard deviation of `expr`. `STD()` is a synonym for the standard SQL function `STDDEV_POP()`, provided as a MySQL extension.

If there are no matching rows, `STD()` returns `NULL`.

• Returns the population standard deviation of `expr`. `STDDEV()` is a synonym for the standard SQL function `STDDEV_POP()`, provided for compatibility with Oracle.

If there are no matching rows, `STDDEV()` returns `NULL`.

• 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.

If there are no matching rows, `STDDEV_POP()` returns `NULL`.

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

If there are no matching rows, `STDDEV_SAMP()` returns `NULL`.

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

If there are no matching rows, `SUM()` returns `NULL`.

• 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.

If there are no matching rows, `VAR_POP()` returns `NULL`.

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

If there are no matching rows, `VAR_SAMP()` returns `NULL`.

• Returns the population standard variance of `expr`. `VARIANCE()` is a synonym for the standard SQL function `VAR_POP()`, provided as a MySQL extension.

If there are no matching rows, `VARIANCE()` returns `NULL`.