what is the difference between the having and the where clause?

Answer

Major differences between the having and where clause are:

1>where clause is not used with aggregate function like sum, avg etc but having we can

2>having clause is slow filtering the results of aggregate functions in the result set of the query as compare to where clause used to comparing values in the base table as it can be  call on index

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.

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:

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;

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.