What's wrong with SQL AND?

Question

MySQL

Good afternoon. I made a selection by category, where everything works, I decided to finalize it, and add AND

$query = "SELECT * FROM products WHERE brand =  '".$_POST["category"]."' AND 'MSI' ";

But for some reason, the selection by category is no longer displayed. I don’t understand where the error could be. If it’s not difficult for someone to tell me what’s wrong. If you remove AND and everything after it, everything works.

Answer 1

1. What is "AND 'MSI'" ? brand can be MSI?
2. If so, why AND? maybe OR is better? Or transferred partition OR MSI?
3. You need to explicitly specify the field every time
(`brand` = '$brand' OR `brand` = 'MSI')