MySQL GROUP BY statement

The GROUP BY statement groups the result set based on one or more columns.

We can use COUNT, SUM, AVG, and other functions on grouped columns.

GROUP BY syntax

SELECT column_name, function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

Example demonstration

The following table structure and data are used in the example of this chapter. Before using, we can import the following data into the database.

SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
--  Table structure for `employee_tbl`
-- ----------------------------
DROP TABLE IF EXISTS `employee_tbl`;
CREATE TABLE `employee_tbl` (
  `id` int(11) NOT NULL,
  `name` char(10) NOT NULL DEFAULT '',
  `date` datetime NOT NULL,
  `singin` tinyint(4) NOT NULL DEFAULT '0' COMMENT 'Login times',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `employee_tbl`
-- ----------------------------
BEGIN;
INSERT INTO `employee_tbl` VALUES ('1', 'Xiao Ming', '2016-04-22 15:25:33', '1'), ('2', 'Xiao Wang', '2016-04-20 15:25:47', '3'), ('3', 'Xiaoli', '2016-04-19 15:26:02', '2'), ('4', 'Xiao Wang', '2016-04-07 15:26:14', '4'), ('5', 'Xiao Ming', '2016-04-11 15:26:40', '4'), ('6', 'Xiao Ming', '2016-04-04 15:26:54', '2');
COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

After the import is successful, execute the following SQL statement:

mysql> set names utf8;
mysql> SELECT * FROM employee_tbl;
+----+--------+---------------------+--------+
| id | name   | date                | singin |
+----+--------+---------------------+--------+
|  1 | Xiao Ming | 2016-04-22 15:25:33 |      1 |
|  2 | Xiao Wang | 2016-04-20 15:25:47 |      3 |
|  3 | Xiaoli | 2016-04-19 15:26:02 |      2 |
|  4 | Xiao Wang | 2016-04-07 15:26:14 |      4 |
|  5 | Xiao Ming | 2016-04-11 15:26:40 |      4 |
|  6 | Xiao Ming | 2016-04-04 15:26:54 |      2 |
+----+--------+---------------------+--------+
6 rows in set (0.00 sec)

Next, we use the GROUP BY statement to group the data table by name and count how many records each person has:

mysql> SELECT name, COUNT(*) FROM   employee_tbl GROUP BY name;
+--------+----------+
| name   | COUNT(*) |
+--------+----------+
| Xiaoli |        1 |
| Xiao Ming |        3 |
| Xiao Wang |        2 |
+--------+----------+
3 rows in set (0.01 sec)

WITH ROLLUP

WITH ROLLUP can realize the same statistics (SUM,AVG,COUNT...) based on grouped statistics.

For example, we group the above data tables by name, and then count the number of logins for each person:

mysql> SELECT name, SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------+--------------+
| name   | singin_count |
+--------+--------------+
| Xiaoli |            2 |
| Xiao Ming |            7 |
| Xiao Wang |            7 |
| NULL   |           16 |
+--------+--------------+
4 rows in set (0.00 sec)

Where the record NULL indicates the number of login times of the owner.

We can use coalesce to set a name that can replace NUll. Coalesce syntax:

select coalesce(a,b,c);

Parameter Description: if a==null, select b; if b==null, select c; if a!=null, select a; if a b c is null, return null (meaningless).

In the following example, if the name is empty, we use the total number instead:

mysql> SELECT coalesce(name, 'Total'), SUM(singin) as singin_count FROM  employee_tbl GROUP BY name WITH ROLLUP;
+--------------------------+--------------+
| coalesce(name, 'Total') | singin_count |
+--------------------------+--------------+
| Xiaoli                   |            2 |
| Xiao Ming                   |            7 |
| Xiao Wang                   |            7 |
| Total                   |           16 |
+--------------------------+--------------+
4 rows in set (0.01 sec)