WHERE vs HAVING Clause in SQL

In SQL, the HAVING and WHERE clauses have a similar function with a key difference.

Both functions allow a user to filter data with respect to a certain condition. The difference between the two clauses has to do with when each is used.

Basically, the WHERE can only be used in non-aggregated data (ie. data that has not been aggregated by GROUP BY). On the other hand, HAVING is used after a GROUP BY.

I mean, the main difference between them is that you can use WHERE if you want to filter data before grouping, I mean before you group records using GROUP BY clause and use HAVING when you want to filter groups.

This distinction is made by Query Engine on most of popular database like MySQL, Microsoft SQL Server, Oracle, and PostgreSQL

For example

select
  * 
from
  `demo`.`employee_lq1` 
  
 where hire_date > '2010-01-01'

will only show records where hire_date > 2010-01-01, here we have used WHERE clause because there is no grouping.

HAVING (and GROUP BY)

HAVING is often used following a GROUP BY, which aggregates data by a certain feature. In case, we need grouping like’ position_id ‘, we can use group by and having clause together .

select
  position_id,
  count(*) as znumber
from
  `demo`.`employee_lq1` 
  
 where hire_date > '2010-01-01'
 group by position_id
 having
 znumber > 1

The main differences between the WHERE clause and the HAVING clause in SQL are as follows:

Timing of Filtering:

The WHERE clause is used to filter data before grouping. In other words, WHERE filters out rows that meet the conditions before executing the GROUP BY statement.

The HAVING clause is used to filter data after grouping. It operates on the grouped result set, not individual records.

Use Cases:

Use the WHERE clause when you need to filter the original data without considering grouping.

Use the HAVING clause when you need to filter based on aggregated results of grouping.

With Aggregation Functions:

The WHERE clause cannot be directly used with aggregate functions (such as COUNT(), SUM(), AVG(), etc.) because it operates on individual records.

The HAVING clause can be used with aggregate functions as it operates on summarized data after grouping.

Performance Considerations:

Since WHERE filters data before grouping, it can reduce the amount of data entering the grouping operation, potentially improving query performance.

HAVING filters data after grouping, so it processes already grouped data, which may lead to more data processing and impact performance. However, in some cases, using HAVING is necessary, especially when filtering based on grouped results.

In conclusion, WHERE and HAVING play different roles in SQL queries, and the choice of which one to use depends on specific needs and scenarios.

Feedback

For inquiries or feedback, please contact the SQLynx team at service@sqlynx.com.