EXPERT RESPONSE
The difference is that WHERE operates on individual rows,
while HAVING operates on groups.
You can have WHERE without HAVING, you can have HAVING without WHERE,
you can have both WHERE and HAVING, and you can have neither WHERE nor
HAVING. But you can't have HAVING without grouping, even if the group
consists of the entire result set.
A good example of the difference between WHERE and HAVING is
given in my earlier answer
Why WHERE cannot be used after GROUP BY.
Typically you will have at least one column to GROUP BY, such as:
select cust_id
, count(distinct order_id) as orders
from sales
where order_date >= '2005-01-01'
group
by cust_id
having sum(order_amt) > 10000
This query returns the cust_id and number of orders
for every customer who has order totals of $10,000 or more
(the HAVING condition) for orders made this year (the WHERE condition).
An example of the entire result set as a single group is:
select count(*) as orders
from sales
where order_date >= '2005-01-01'
This query returns the number of orders this year. The aggregate
function COUNT(*) operates on the single group consisting of all the
rows that satisfy the WHERE clause. Note that it would be unusual to have
a HAVING clause in this situation.
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