The main difference between WHERE and HAVING is the filtering timing: 1. WHERE filters rows before grouping, acting on the original data, and cannot use the aggregate function; 2. HAVING filters the results after grouping, and acting on the aggregated data, and can use the aggregate function. For example, when using WHERE to screen high-paying employees in the query, then group statistics, and then use HAVING to screen departments with an average salary of more than 60,000, the order of the two cannot be changed. WHERE always executes first to ensure that only rows that meet the conditions participate in the grouping, and HAVING further filters the final output based on the grouping results.
The difference between WHERE and HAVING in SQL comes down to when each filter is applied — especially in relation to grouping. In short:
-
WHEREfilters rows before they are grouped. -
HAVINGfilters groups or aggregated results after the grouping is done.
If you're using GROUP BY , that's where this distinction really matters.
Filtering Before Grouping with WHERE
Use WHERE when you want to narrow down which rows go into the grouping process. It works on individual rows, not aggregated values.
For example:
SELECT department, COUNT(*) AS employee_count FROM employees WHERE salary > 50000 GROUP BY department;
Here, only employees earning more than $50k are included before the database groups them by department. The filtering happens early, so it affects the final counts.
You can't use aggregate functions like COUNT() , SUM() , etc., in a WHERE clause — because those values ??don't exist yet at that stage of the query.
Key points:
- Filters raw data.
- Cannot reference aggregate functions.
- Runs before grouping.
Filtering After Grouping with HAVING
Once you've grouped your data using GROUP BY , you might want to filter based on the result of an aggregation — that's where HAVING steps in.
Example:
SELECT department, COUNT(*) AS employee_count FROM employees GROUP BY department HAVING COUNT(*) > 10;
This query returns only departments with more than 10 employees. The count is calculated first, then filtered.
Unlike WHERE , HAVING can include aggregate functions. You can also reference column aliases defined in the SELECT .
Common uses:
- Filter based on group size (
COUNT,SUM, etc.) - Compare aggregated values ??(eg,
HAVING AVG(salary) > 60000) - Use with or without
GROUP BY
When to Use Which?
To decide between WHERE and HAVING , ask yourself:
- Do I need to filter individual rows before grouping? → Use
WHERE. - Do I need to filter groups or aggregated results after grouping? → Use
HAVING.
Sometimes both can be used together:
SELECT department, AVG(salary) AS avg_salary FROM employees WHERE status = 'active' GROUP BY department HAVING AVG(salary) > 60000;
In this case:
-
WHEREnarrows the dataset to active employees only. -
HAVINGfurther filters the resulting groups to show only those with an average salary over $60k.
So basically, it's about timing: WHERE filters early, HAVING filters late — especially useful after aggregations.
The above is the detailed content of What is the difference between WHERE and HAVING clauses in SQL?. For more information, please follow other related articles on the PHP Chinese website!
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