In SQL, what’s the difference between the having clause and the where clause?

Difference between having and where clause

So we can see that the difference between the having and where clause in sql is
that the where clause can not be used with aggregates, but the having clause can.
One way to think of it is that the having clause is an additional filter to the
where clause.

The difference between the having and where clause is best illustrated by an example.
Suppose we have a table called emp_bonus as shown below. Note that the table has multiple
entries for employees A and B.
emp_bonus

Employee     Bonus
A                 1000
B                 2000
A                500
C                700
B                1250

If we want to calculate the total bonus that each employee received, then we would
write a SQL statement like this:

select employee, sum(bonus) from emp_bonus group by employee;

The Group By Clause

In the SQL statement above, you can see that we use the "group by" clause with the
employee column. What the group by clause does is allow us to find the sum of the bonuses
for each employee. Using the ‘group by’ in combination with the ‘sum(bonus)’ statement will
give us the sum of all the bonuses for employees A, B, and C.


Running the SQL above would return this:
Employee     Sum(Bonus)
A                  1500
B                  3250
C                  700

Now, suppose we wanted to find the employees who received more than 1,000 in bonuses
for the year of 2007. You might think that we could write a query like this:

BAD SQL:
select employee, sum(bonus) from emp_bonus
group by employee where sum(bonus) > 1000;

The WHERE clause does not work with aggregates like SUM

The SQL above will not work, because the where clause doesn’t work with aggregates – like sum,
avg, max, etc.. Instead, what we will need to use is the having clause. The having clause was
added to sql just so we could compare aggregates to other values – just how the ‘where’ clause
can be used with non-aggregates. Now, the correct sql will look like this:

GOOD SQL:
select employee, sum(bonus) from emp_bonus
group by employee having sum(bonus) > 1000;