SELECT * FROM products WHERE category = 'electronics' OR category = 'computers' AND price < 1000;

SELECT * FROM products WHERE (category = 'electronics' OR category = 'computers') AND price < 1000;

Without parentheses, AND binds tighter than OR, giving: (electronics) OR (computers AND price<1000). Wrong logic!

Always use parentheses with mixed AND/OR

High

Missing parentheses causes wrong evaluation order

SELECT * FROM customers WHERE age > 18 AND age < 18;

SELECT * FROM customers WHERE age = 18; -- or remove impossible condition

age cannot be both >18 AND <18 simultaneously. This is a logic error.

Check for contradictory conditions

High

Impossible condition returns no results