MySQL - GROUP BY Clause

You can use GROUP BY to group values from a column, and, if you wish, perform calculations on that column. You can use COUNT, SUM, AVG, etc., functions on the grouped column.

To understand GROUP BY clause, consider an employee_tbl table, which is having the following records −

mysql> SELECT * FROM employee_tbl;
+------+------+------------+--------------------+
|  id  | name | work_date  | daily_typing_pages |
+------+------+------------+--------------------+
|   1  | John | 2007-01-24 |        250         |
|   2  | Ram  | 2007-05-27 |        220         |
|   3  | Jack | 2007-05-06 |        170         |
|   3  | Jack | 2007-04-06 |        100         |
|   4  | Jill | 2007-04-06 |        220         |
|   5  | Zara | 2007-06-06 |        300         |
|   5  | Zara | 2007-02-06 |        350         |
+------+------+------------+--------------------+
7 rows in set (0.00 sec)

Now, suppose based on the above table we want to count number of days each employee did work.

If we will write a SQL query as follows, then we will get the following result −

mysql> SELECT COUNT(*) FROM employee_tbl;
+---------------------------+
|          COUNT(*)         |
+---------------------------+
|             7             |
+---------------------------+

But this is not serving our purpose, we want to display total number of pages typed by each person separately. This is done by using aggregate functions in conjunction with a GROUP BY clause as follows −

mysql> SELECT name, COUNT(*)
   -> FROM   employee_tbl 
   -> GROUP BY name;
+------+----------+
| name | COUNT(*) |
+------+----------+
| Jack |     2    |
| Jill |     1    |
| John |     1    |
| Ram  |     1    |
| Zara |     2    |
+------+----------+
5 rows in set (0.04 sec)

We will see more functionality related to GROUP BY in other functions like SUM, AVG, etc.