Difference between WHERE vs HAVING clause in SQL - GROUP BY Comparison with Example (original) (raw)
What is the difference between WHERE and HAVING clause in SQL is one of the most popular questions asked on SQL and database interviews, especially to beginners? Since programming jobs, required more than one skill, it’s quite common to see a couple of SQL Interview questions in Java and .NET interviews. By the way unlike any other question, not many Java programmers or dot net developers, who are supposed to have knowledge of basic SQL, fail to answer this question. Though almost half of the programmer says that WHERE is used in any SELECT query, while HAVING clause is only used in SELECT queries, which contains aggregate function or group by clause, which is correct.
Though both WHERE and HAVING clause is used to specify filtering condition in SQL, there is subtle difference between them. Real twist comes into interview, when they are asked to explain result of a SELECT query, which contains both WHERE and HAVING clause, I have seen many people getting confused there.
Key point, which is also the main difference between WHERE and HAVING clause in SQL is that, condition specified in WHERE clause is used while fetching data (rows) from table, and data which doesn't pass the condition will not be fetched into result set, on the other hand HAVING clause is later used to filter summarized data or grouped data.
In short if both WHERE and HAVING clause is used in a SELECT query with aggregate function or GROUP BY clause, it will execute before HAVING clause. This will be more clear, when we will see an example of WHERE, HAVING, JOIN and GROUP BY clause together.
WHERE vs HAVING Clause Example in SQL
In this example of the WHERE and HAVING clause, we have two tables Employee and Department. Employee contains details of employees e.g. id, name, age, salary and department id, while Department contains id and department name. In order to show, which employee works for which department we need to join two tables on DEPT_ID to get the department name.
Our requirement is to find how many employees are working in each department and the average salary of the department. In order to use the WHERE clause, we will only include employees who are earning more than 5000. Before executing our query which contains WHERE HAVING, and GROUP BY clause, let see data from Employee and Department table:
SELECT * FROM Employee;
| EMP_ID | EMP_NAME | EMP_AGE | EMP_SALARY | DEPT_ID |
|---|---|---|---|---|
| 1 | Virat | 23 | 10000 | 1 |
| 2 | Rohit | 24 | 7000 | 2 |
| 3 | Suresh | 25 | 8000 | 3 |
| 4 | Shikhar | 27 | 6000 | 1 |
| 5 | Vijay | 28 | 5000 | 2 |
SELECT * FROM Department;
| DEPT_ID | DEPT_NAME |
|---|---|
| 1 | Accounting |
| 2 | Marketing |
| 3 | Sales |
SELECT d.DEPT_NAME, count(e.EMP_NAME) as NUM_EMPLOYEE, avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAME;
| DEPT_NAME | NUM_EMPLOYEE | AVG_SALARY |
|---|---|---|
| Accounting | 1 | 8000 |
| Marketing | 1 | 7000 |
| Sales | 2 | 8000 |
From the number of employees (NUM_EMPLOYEE) column you can see that only Vijay who work for Marketing department is not included in result set because his earning 5000. This example shows that, condition in WHERE clause is used to filter rows before you aggregate them and then HAVING clause comes in picture for final filtering, which is clear from following query, now Marketing department is excluded because it doesn't pass condition in HAVING clause I mean AVG_SALARY > 7000
SELECT d.DEPT_NAME, count(e.EMP_NAME) as NUM_EMPLOYEE, avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAME HAVING AVG_SALARY > 7000;
| DEPT_NAME | NUM_EMPLOYEE | AVG_SALARY |
|---|---|---|
| Accounting | 1 | 8000 |
| Sales | 2 | 8000 |
Difference between WHERE and HAVING in SQL
Apart from this key difference we have seen in this article, here are few more differences between WHERE and HAVING clause, which is worth remembering and can be used to compare both of them :
- Apart from SELECT queries, you can use the WHERE clause with UPDATE and DELETE clause but the HAVING clause can only be used with SELECT query. For example following query, which involves WHERE clause will work but others which uses HAVING clause will not work :
update DEPARTMENT set DEPT_NAME="NewSales" WHERE DEPT_ID=1 ; // works fine
update DEPARTMENT set DEPT_NAME="NewSales" HAVING DEPT_ID=1 ; // error
Incorrect syntax near the keyword 'HAVING'.: update DEPARTMENT set DEPT_NAME='NewSales' HAVING DEPT_ID=1
WHERE clause is used for filtering rows and it applies on each and every row, while HAVING clause is used to filter groups in SQL.
One syntax level difference between WHERE and HAVING clause is that, former is used before GROUP BY clause, while later is used after GROUP BY clause.
When WHERE and HAVING clause are used together in a SELECT query with aggregate function, WHERE clause is applied first on individual rows and only rows which pass the condition is included for creating groups. Once group is created, HAVING clause is used to filter groups based upon condition specified.
Here is a list of difference between WHERE and HAVING clause in point format:
That's all on the difference between WHERE and HAVING clause in SQL. As I said this is very popular question and you can't afford not to prepare it. Always remember key difference between WHERE and HAVING clause in SQL, if WHERE and HAVING clause is used together, first WHERE clause is applied to filter rows and only after grouping HAVING clause is applied.
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P.S. - If you are new to the SQL and Database and looking for free SQL and database courses to learn SQL fundamentals then you can also check out my list of free Udemy courses to learn SQL. These are really great SQL courses that are available for free on Udemy and Coursera and you can use them to build your SQL skills.