Exists vs JOIN and use of EXISTS clause

Important concepts and usage patterns

To determine which to use, consider defining characteristics of EXISTS and JOIN. EXISTS could be your best friend in 1:n relationships when you just want to know if the party's on (read: related rows exist). No more scanning after the first Cinderella shoe fits.

JOIN, on the other hand, can get you every piece of gossip (read: data) from all related tables. But beware, it might invite unwanted triples (or duplicates). And to get rid of them, DISTINCT is your only saviour at the cost of performance.

If I had to cast my vote on readability, JOIN would lead the race - resembling a neat spreadsheet, it's every data lover's comfort food!

Understanding when to use what

When no-shows matter

In scenarios where you are keen on knowing if someone didn't turn up for the party, EXISTS can be an effective tool. Think of it as the DJ who stops playing once the last guest leaves.

-- Sometimes you gotta discontinue products enjoying their retirement!
UPDATE products
SET discontinued = 1
WHERE NOT EXISTS (
  SELECT 1 FROM orders WHERE product_id = products.id
);


Dealing with complex conditions

EXISTS is also great at handling crowded subqueries. You'll appreciate its talent when filtering based on conditions that resemble a maze.

-- Find products that have mastered the art of big orders
SELECT * FROM products p
WHERE EXISTS (
  SELECT 1 FROM orders o WHERE o.product_id = p.id AND o.quantity > 100
);


Recognizing typical situations

Choose EXISTS when you need a fast confirmation on permissions, settings, or features associated with a user. JOIN is more suited for compiling detailed reports or bringing cross-table statistics to light.

Considering performance

Remember, the efficiency of EXISTS and JOIN may vary depending on the size of your table. EXISTS can outperform in some scenarios, but JOIN can throw a surprise with the right battlefield setup (indexes).

Performance improvement strategies

Indexing matters

Both EXISTS and JOIN send thank-you notes for optimal indexing. Make the columns used in join conditions and subqueries your prime candidates for indexing.

Know your optimizer

Understand that smart kids like SQL Server or PostgreSQL have their unique ways to brew coffee. They might just optimize your IN clauses as EXISTS - so refer to those DBMS manuals to know them better!

Analyzing the plan

They say a good execution plan can reveal a lot about your query - like whether a JOIN or an EXISTS would give you the best performance. So don’t forget to review them!

Getting hands dirty with code

EXISTS: Just tell me if there's a bear, alright?
  (=> Quick confirmation 🌲🐻)

JOIN: Show me all the bear tracks you found
  (=> Detailed report 🐾=🐻)

Think of EXISTS as the efficient first responder. They see the bear tracks, confirm the bear EXISTS, and quit!

🐾 EXISTS? 🌲 -> 🐻 (Confirmed! Let's bail...)

JOIN is the forensic investigator. Every single track out there? They've got it!

🐾👣🐾 JOIN 🐾🐻 -> 🌲🌲🌲 + 🐻 (Started from the forest, now we're here!)

EXISTS often sprints faster as it stops when the job is done, while JOIN doesn't rest until it has examined all data.