Difference Between Having and Where in Sql

What is the difference between HAVING and WHERE in SQL?

HAVING specifies a search condition for a
group or an aggregate function used in SELECT statement.

Source

Difference between HAVING and WHERE Clause

Functionally, the two are equivalent.

The WHERE clause is saying:

Filter the data and then aggregate the results.

The HAVING clause is saying:

Aggregate the data and then filter the results.

Both return the same result, because the filtering is on the columns used for aggregation. Usually, HAVING uses aggregation functions; these are not allowed in the WHERE.

In general, the WHERE clause is going to be faster, because less data is being aggregated. You should use WHERE in this case.

Difference between HAVING and WHERE in SQL

The simple way to think about it is to consider the order in which the steps are applied.

Step 1: Where clause filters data

Step 2: Group by is implemented (SUM / MAX / MIN / ETC)

Step 3: Having clause filters the results

So in your 2 examples:

SELECT agentId, SUM(quantity) total_sales 
FROM sales s, houses h
WHERE s.houseId = h.houseId AND h.type = "condo"
GROUP BY agentId
ORDER BY total_sales;

Step 1: Filter by HouseId and Condo

Step 2: Add up the results
(number of houses that match the houseid and condo)

SELECT agentId, SUM(quantity) total_sales 
FROM sales s, houses h
GROUP BY agentId
HAVING s.houseId = h.houseId AND h.type = "condo"
ORDER BY total_sales;

Step 1: No Filter

Step 2: Add up quantity of all houses

Step 3: Filter the results by houseid and condo.

Hopefully this clears up what is happening.

The easiest way to decide which you should use is:
- Use WHERE to filter the data
- Use HAVING to filter the results of an aggregation (SUM / MAX / MIN / ETC)

Difference between HAVING and WHERE clause in SQL

where filters on the select ... from

having filters on the aggregate results from the group by ...

So, looking at your example again:

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value

Here, WHERE column_name operator value says "Return results for table_name where
'column_name operator value' is true".

Only after all the results from these conditions are found, it groups by column_name.

Then HAVING aggregate_function(column_name) operator value says "For the resulting aggregate groups, run 'aggregate_function(column_name)' and return only results where 'aggregate_function(column_name) operator value' is true."

SQL - HAVING vs. WHERE

WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly.

As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.

While you're at it, you may want to re-write your query using ANSI version of the join:

SELECT  L.LectID, Fname, Lname
FROM Lecturers L
JOIN Lecturers_Specialization S ON L.LectID=S.LectID
GROUP BY L.LectID, Fname, Lname
HAVING COUNT(S.Expertise)>=ALL
(SELECT COUNT(Expertise) FROM Lecturers_Specialization GROUP BY LectID)

This would eliminate WHERE that was used as a theta join condition.

MySql - HAVING vs WHERE

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.

Which is better : click

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