MySQL GROUP BY Clause

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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.



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