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MySQL 5.7 Reference Manual  / ...  / Functions and Operators  / Aggregate Functions  /  Aggregate Function Descriptions

12.19.1 Aggregate Function Descriptions

This section describes aggregate functions that operate on sets of values. They are often used with aGROUP BY clause to group values into subsets.

Table 12.25 Aggregate Functions

NameDescriptionIntroduced
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
JSON_ARRAYAGG() Return result set as a single JSON array5.7.22
JSON_OBJECTAGG() Return result set as a single JSON object5.7.22
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, aggregate functions ignoreNULL values.

If you use an aggregate function in a statement containing noGROUP BY clause, it is equivalent to grouping on all rows. For more information, seeSection 12.19.3, “MySQL Handling of GROUP BY”.

For numeric arguments, the variance and standard deviation functions return aDOUBLE value. TheSUM() andAVG() functions return aDECIMAL value for exact-value arguments (integer orDECIMAL), and aDOUBLE value for approximate-value arguments (FLOAT orDOUBLE).

TheSUM() andAVG() 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))) FROMtbl_name;SELECT FROM_DAYS(SUM(TO_DAYS(date_col))) FROMtbl_name;

Functions such asSUM() orAVG() that expect a numeric argument cast the argument to a number if necessary. ForSET orENUM values, the cast operation causes the underlying numeric value to be used.

TheBIT_AND(),BIT_OR(), andBIT_XOR() aggregate functions perform bit operations. They requireBIGINT (64-bit integer) arguments and returnBIGINT values. Arguments of other types are converted toBIGINT and truncation might occur. For information about a change in MySQL 8.0 that permits bit operations to take binary string type arguments (BINARY,VARBINARY, and theBLOB types), seeSection 12.12, “Bit Functions and Operators”.

  • AVG([DISTINCT]expr)

    Returns the average value ofexpr. TheDISTINCT option can be used to return the average of the distinct values ofexpr.

    If there are no matching rows,AVG() returnsNULL.

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

  • BIT_AND(expr)

    Returns the bitwiseAND of all bits inexpr. 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).

  • BIT_OR(expr)

    Returns the bitwiseOR of all bits inexpr. 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).

  • BIT_XOR(expr)

    Returns the bitwiseXOR of all bits inexpr. 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).

  • COUNT(expr)

    Returns a count of the number of non-NULL values ofexpr in the rows retrieved by aSELECT statement. The result is aBIGINT value.

    If there are no matching rows,COUNT() returns0.

    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 containNULL values.

    For transactional storage engines such asInnoDB, 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.

    Prior to MySQL 5.7.18,InnoDB processesSELECT COUNT(*) statements by scanning the clustered index. As of MySQL 5.7.18,InnoDB processesSELECT COUNT(*) statements by traversing the smallest available secondary index unless an index or optimizer hint directs the optimizer to use a different index. If a secondary index is not present, the clustered index is scanned.

    ProcessingSELECT COUNT(*) statements takes some time if index records are 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, useSHOW TABLE STATUS.

    InnoDB handlesSELECT COUNT(*) andSELECT COUNT(1) operations in the same way. There is no performance difference.

    ForMyISAM tables,COUNT(*) is optimized to return very quickly if theSELECT retrieves from one table, no other columns are retrieved, and there is noWHERE clause. For example:

    mysql> SELECT COUNT(*) FROM student;

    This optimization only applies toMyISAM 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 asNOT NULL.

  • COUNT(DISTINCTexpr,[expr...])

    Returns a count of the number of rows with different non-NULLexpr values.

    If there are no matching rows,COUNT(DISTINCT) returns0.

    mysql> SELECT COUNT(DISTINCT results) FROM student;

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

  • GROUP_CONCAT(expr)

    This function returns a string result with the concatenated non-NULL values from a group. It returnsNULL 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 ...]]             [SEPARATORstr_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 theDISTINCT clause. To sort values in the result, use theORDER BY clause. To sort in reverse order, add theDESC (descending) keyword to the name of the column you are sorting by in theORDER BY clause. The default is ascending order; this may be specified explicitly using theASC keyword. The default separator between values in a group is comma (,). To specify a separator explicitly, useSEPARATOR followed by the string literal value that should be inserted between group values. To eliminate the separator altogether, specifySEPARATOR ''.

    The result is truncated to the maximum length that is given by thegroup_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 ofmax_allowed_packet. The syntax to change the value ofgroup_concat_max_len at runtime is as follows, whereval 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 isTEXT orBLOB unlessgroup_concat_max_len is less than or equal to 512, in which case the result type isVARCHAR orVARBINARY.

    IfGROUP_CONCAT() is invoked from within themysql client, binary string results display using hexadecimal notation, depending on the value of the--binary-as-hex. For more information about that option, seeSection 4.5.1, “mysql — The MySQL Command-Line Client”.

    See alsoCONCAT() andCONCAT_WS():Section 12.8, “String Functions and Operators”.

  • JSON_ARRAYAGG(col_or_expr)

    Aggregates a result set as a singleJSON array whose elements consist of the rows. The order of elements in this array is undefined. The function acts on a column or an expression that evaluates to a single value. ReturnsNULL if the result contains no rows, or in the event of an error.

    mysql> SELECT o_id, attribute, value FROM t3;+------+-----------+-------+| o_id | attribute | value |+------+-----------+-------+|    2 | color     | red   ||    2 | fabric    | silk  ||    3 | color     | green ||    3 | shape     | square|+------+-----------+-------+4 rows in set (0.00 sec)mysql> SELECT o_id, JSON_ARRAYAGG(attribute) AS attributes    -> FROM t3 GROUP BY o_id;+------+---------------------+| o_id | attributes          |+------+---------------------+|    2 | ["color", "fabric"] ||    3 | ["color", "shape"]  |+------+---------------------+2 rows in set (0.00 sec)

    Added in MySQL 5.7.22.

  • JSON_OBJECTAGG(key,value)

    Takes two column names or expressions as arguments, the first of these being used as a key and the second as a value, and returns a JSON object containing key-value pairs. ReturnsNULL if the result contains no rows, or in the event of an error. An error occurs if any key name isNULL or the number of arguments is not equal to 2.

    mysql> SELECT o_id, attribute, value FROM t3;+------+-----------+-------+| o_id | attribute | value |+------+-----------+-------+|    2 | color     | red   ||    2 | fabric    | silk  ||    3 | color     | green ||    3 | shape     | square|+------+-----------+-------+4 rows in set (0.00 sec)mysql> SELECT o_id, JSON_OBJECTAGG(attribute, value)    -> FROM t3 GROUP BY o_id;+------+---------------------------------------+| o_id | JSON_OBJECTAGG(attribute, value)      |+------+---------------------------------------+|    2 | {"color": "red", "fabric": "silk"}    ||    3 | {"color": "green", "shape": "square"} |+------+---------------------------------------+2 rows in set (0.00 sec)

    Duplicate key handling.  When the result of this function is normalized, values having duplicate keys are discarded. In keeping with the MySQLJSON data type specification that does not permit duplicate keys, only the last value encountered is used with that key in the returned object (“last duplicate key wins”). This means that the result of using this function on columns from aSELECT can depend on the order in which the rows are returned, which is not guaranteed.

    Consider the following:

    mysql> CREATE TABLE t(c VARCHAR(10), i INT);Query OK, 0 rows affected (0.33 sec)mysql> INSERT INTO t VALUES ('key', 3), ('key', 4), ('key', 5);Query OK, 3 rows affected (0.10 sec)Records: 3  Duplicates: 0  Warnings: 0mysql> SELECT c, i FROM t;+------+------+| c    | i    |+------+------+| key  |    3 || key  |    4 || key  |    5 |+------+------+3 rows in set (0.00 sec)mysql> SELECT JSON_OBJECTAGG(c, i) FROM t;+----------------------+| JSON_OBJECTAGG(c, i) |+----------------------+| {"key": 5}           |+----------------------+1 row in set (0.00 sec)mysql> DELETE FROM t;Query OK, 3 rows affected (0.08 sec)mysql> INSERT INTO t VALUES ('key', 3), ('key', 5), ('key', 4);Query OK, 3 rows affected (0.06 sec)Records: 3  Duplicates: 0  Warnings: 0mysql> SELECT c, i FROM t;+------+------+| c    | i    |+------+------+| key  |    3 || key  |    5 || key  |    4 |+------+------+3 rows in set (0.00 sec)mysql> SELECT JSON_OBJECTAGG(c, i) FROM t;+----------------------+| JSON_OBJECTAGG(c, i) |+----------------------+| {"key": 4}           |+----------------------+1 row in set (0.00 sec)

    SeeNormalization, Merging, and Autowrapping of JSON Values, for additional information and examples.

    Added in MySQL 5.7.22.

  • MAX([DISTINCT]expr)

    Returns the maximum value ofexpr.MAX() may take a string argument; in such cases, it returns the maximum string value. SeeSection 8.3.1, “How MySQL Uses Indexes”. TheDISTINCT keyword can be used to find the maximum of the distinct values ofexpr, however, this produces the same result as omittingDISTINCT.

    If there are no matching rows,MAX() returnsNULL.

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

    ForMAX(), MySQL currently comparesENUM andSET columns by their string value rather than by the string's relative position in the set. This differs from howORDER BY compares them.

  • MIN([DISTINCT]expr)

    Returns the minimum value ofexpr.MIN() may take a string argument; in such cases, it returns the minimum string value. SeeSection 8.3.1, “How MySQL Uses Indexes”. TheDISTINCT keyword can be used to find the minimum of the distinct values ofexpr, however, this produces the same result as omittingDISTINCT.

    If there are no matching rows,MIN() returnsNULL.

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

    ForMIN(), MySQL currently comparesENUM andSET columns by their string value rather than by the string's relative position in the set. This differs from howORDER BY compares them.

  • STD(expr)

    Returns the population standard deviation ofexpr.STD() is a synonym for the standard SQL functionSTDDEV_POP(), provided as a MySQL extension.

    If there are no matching rows,STD() returnsNULL.

  • STDDEV(expr)

    Returns the population standard deviation ofexpr.STDDEV() is a synonym for the standard SQL functionSTDDEV_POP(), provided for compatibility with Oracle.

    If there are no matching rows,STDDEV() returnsNULL.

  • STDDEV_POP(expr)

    Returns the population standard deviation ofexpr (the square root ofVAR_POP()). You can also useSTD() orSTDDEV(), which are equivalent but not standard SQL.

    If there are no matching rows,STDDEV_POP() returnsNULL.

  • STDDEV_SAMP(expr)

    Returns the sample standard deviation ofexpr (the square root ofVAR_SAMP().

    If there are no matching rows,STDDEV_SAMP() returnsNULL.

  • SUM([DISTINCT]expr)

    Returns the sum ofexpr. If the return set has no rows,SUM() returnsNULL. TheDISTINCT keyword can be used to sum only the distinct values ofexpr.

    If there are no matching rows,SUM() returnsNULL.

  • VAR_POP(expr)

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

    If there are no matching rows,VAR_POP() returnsNULL.

  • VAR_SAMP(expr)

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

    If there are no matching rows,VAR_SAMP() returnsNULL.

  • VARIANCE(expr)

    Returns the population standard variance ofexpr.VARIANCE() is a synonym for the standard SQL functionVAR_POP(), provided as a MySQL extension.

    If there are no matching rows,VARIANCE() returnsNULL.