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Group by and having Clauses

You will want to find trends in your data that will require the database server to cook the data a bit before you retrieve your result set. One such mechanism is the group by clause, which is used to group data by column values. For example, rather than looking at a list of employees and the departments to which they are assigned, you might want to look at a list of departments along with the number of employees assigned to each department. When using the group by clause, you may also use the having clause, which allows you to filter group data in the same way the where clause lets you filter raw data.Here's a quick look at a query that counts all the employees in each department and returns thenames of those departments having more than two employees:
mysql> SELECT, count(e.emp_id) num_employees
-> FROM department d INNER JOIN employee e
-> ON d.dept_id = e.dept_id
-> HAVING count(e.emp_id) > 2;
| name | num_employees |
| Administration | 3 |
| Operations | 14 |
2 rows in set (0.00 sec)

GROUP BY syntax

1) ALL indicates that every row will be considered in the grouping, even if the row doesn’t meet the criteria set in the WHERE clause

2) expression is the name of the column(s) that make up the grouping. If there is more than one column that makes up the grouping, each column must be separated by comma.

3) CUBE|ROLLUP specifies that additional summary rows are returned with the result. This is covered in more detail in the third course in this series because it relates to on-line Analytical Processing (OLAP).