Why should you care?

Backend and data jobs require SQL. Performance Diagnostics appears in interviews and daily work.

See it live — copy this example

Run in pgAdmin or psql.

-- Performance Diagnostics
SELECT id, name, email
FROM users
WHERE is_active = 1
ORDER BY name;

Run Example »

Run this SQL in your database tool (SSMS, pgAdmin, MySQL Workbench) or use the Try code button on the block above.

What happened?

• The example shows Performance Diagnostics in action.
• Read each line, then edit one part and run again.

Try it yourself

1. Open SSMS, pgAdmin, or MySQL Workbench.
2. Paste the query and run it.
3. Change the WHERE clause and see different rows.
4. Change one value in the example and run it again.
5. Break the code on purpose and read the error message.

Remember

You saw how Performance Diagnostics works. Practice by editing the example. Use Next when you can explain it in your own words.