Learning MySQL
MySQL Functions
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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 function on the grouped column.
To understand GROUP BY clause consider an employee_tbl table which is having 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)
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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 following result:
mysql> SELECT COUNT(*) FROM employee_tbl;
+---------------------------+
| COUNT(*) |
+---------------------------+
| 7 |
+---------------------------+
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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)
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We will see more functionality related to GROUP BY in other functions like SUM, AVG etc.
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