REMEMBER the difference:
We can limit the groups in a result set by adding HAVING condition afterwords.
We can limit it before grouping them using WHERE.
Learn more about HAVING in this article:
Why do we need HAVING Clause in SQL?
Learn more about WHERE in this article:
Some Useful Operators in The SQL WHERE Clause
Now, let's just go through some examples to understand the difference better. We will go through search query that uses HAVING and WHERE one by one.
Let's look at the search query that uses HAVING clause:
//Search using HAVING Clause SELECT Seller_name, COUNT(*) AS Invoice_quantity, ROUND(AVG(invoice_total),2) AS Invoice_average FROM sellers JOIN invoices ON sellers.seller_id=invoices.seller_id GROUP BY seller_name HAVING AVG(invoice_total) > 500 ORDER BY invoice_quantity DESC;
In this query, we are just pulling up seller names, number of invoices using count() function and getting the average of invoices. After that we are joining the sellers table with invoices table and using group by to group the sellers together.
Now, after that I have used HAVING clause so that means the query groups the sellers together and then checks for the condition. If the AVG for invoice total is greater than 500 then and then the sellers will show up in the search results set.
Let's look at the search query that uses WHERE clause:
//Search using WHERE Clause SELECT Seller_name, COUNT(*) AS Invoice_quantity, ROUND(AVG(invoice_total),2) AS Invoice_average FROM sellers JOIN invoices ON sellers.seller_id=invoices.seller_id WHERE invoice_total > 500 GROUP BY seller_name ORDER BY invoice_quantity DESC;
In this query I am fetching the same data and then joining the same tables. But immediately after joining the tables, I have added WHERE clause, before the Group By clause. So, that means it limits the invoices included in the groups if the invoices are not greater than $500.
In other words we can say that the search condition in WHERE was being applied to every row but in HAVING, it was applied to the group of rows.
Also there are few more differences between them.
Notice the first query. We are using HAVING with AVG.
We can't use aggregate functions with WHERE clause but we can use it with Having clause. The reason is because we put WHERE before Group By so it won't limit the grouped rows.
Another difference is that HAVING clause can ONLY refer to columns included in the SELECT statement. While, WHERE can refer to any column.