What is the difference between HAVING and WHERE in SQL?

sql

having

where

query-optimization

byNikita Barsukov·Aug 3, 2024

In a nutshell, WHERE filters rows before they're grouped, removing records that don't meet the condition. In contrast, HAVING operates after the GROUP BY operation, allowing you to filter based on aggregated values like SUM, AVG, or COUNT.

Behold, an example:

SELECT department, SUM(sales) AS total_sales
FROM orders
WHERE department <> 'Returns'  -- "Returns"? We don't need that negativity here.
GROUP BY department
HAVING SUM(sales) > 5000       -- Because size does matter.

Diving into WHERE and HAVING

Let's touch upon WHERE and HAVING in greater detail. Think of WHERE as pre-aggregation filtering applied to individual rows. It's like the gatekeeper that paces the inflow of data before grouping, reducing the load for the grouping stage.

Meanwhile, HAVING is the bouncer keeping an eye on aggregated groups post-GROUP BY. Whereas WHERE can't touch aggregate functions like SUM, AVG, or COUNT, HAVING works these like a charm after the GROUP BY has rolled out.

Now, if you pull GROUP BY out from the equation, HAVING dances pretty much like WHERE, though it isn't their best performance as HAVING ain't optimized for this routine.

In the sequence of query executions, WHERE stage-dives before the GROUP BY act, while HAVING plays the closing note. Be clear on this to keep your SQL queries on the right rhythm.

Common traps and how to avoid them

SQL might get a bit rocky with these clauses. Watch your steps around these potential pitfalls:

Using HAVING without GROUP BY might force your database to perform an imaginary dance routine.
Filtering on aggregate functions using the WHERE clause will result in your database tossing the "not cool, dude" card.
Misjudging the execution sequence can lead to incorrect results – remember, WHERE kicks off before GROUP BY and HAVING concludes the party.

Here are some survival tips:

Go lean and mean on data with WHERE before you move to aggregation.
Use HAVING to fine-tune the grouped results.
Distrust is good – verify your query logic with a subset of your data first.

SQL query optimization: Do it better

Riding the WHERE and HAVING wave optimally can significantly enhance your SQL queries' performance. Here's some sage advice:

Indexes: Creating them on columns used in a WHERE clause can boost your query's speed like a rocket.
Complex conditions in HAVING? Think again: HAVING acting on aggregated data might find complex conditions, well, complex!
Using temporary tables: For challenging datasets, consider applying the WHERE filter, create a temporary table, then aggregate and filter data using HAVING. Your database might thank you.

Correlating Subqueries: Where and Having

The dance of WHERE and HAVING doesn't end at aggregate functions. WHERE can't use these directly since it acts on each row independently. However, you can cleverly use a correlated subquery with WHERE to check conditions based on aggregate values. Considering HAVING is naturally aligned with aggregate functions, it doesn't need such fancy footwork. But knowing how these clauses interact with subqueries? That's some SQL stardom right there!

explain-codes / Sql / What is the difference between HAVING and WHERE in SQL?

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