# Group By and Having

The **group by** clause is used to group records by an attribute/set of attributes. An aggregate operation (count, max, min, avg etc.) can be performed after grouping.

```
select * from r group by attribute
```

Note that all the attributes in the select clause (that aren't part of an aggregate function) must be present in the group by clause.

```
select dept_name, ID, avg (salary)
from instructor
group by dept_name;
```

is an erroneous query because ID isn't in the group by clause (salary can be exempted because it is part of the avg aggregate function).

The **having** clause is a predicate for grouping.

Note that the **where** clause is applied before grouping and the **having** clause is applied after grouping.

```
select * from r where P1 group by attribute having P2
```


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