Q.Compare the output produced by the query in Example 9.6 and the output of the following query and differentiate between the OR and AND operators.
[Context: Example 9.6 query — SELECT Ename, DeptId FROM EMPLOYEE WHERE Salary>=20000 AND Salary<=50000; The following query — SELECT * FROM EMPLOYEE WHERE Salary > 5000 OR DeptId= 20;]

Boolean Logic Operators: The Grammar of True and False

Think of Boolean logic as the simplest possible decision-making system — one that works with only two values: true and false. Every question it answers is a yes-or-no question. Every statement it processes is either correct or incorrect, with no shades of grey.

You already use Boolean logic every day. When you say "I will go out if it is not raining and I have finished my work", you are combining conditions using the same operators that power search engines, database queries, and the logic circuits inside every computer. The only difference is that Boolean logic makes these rules precise and mechanical.

The Three Fundamental Operators

There are exactly three basic Boolean operators: AND, OR, and NOT. Everything else is built from these.

NOT is the simplest. It flips a value: true becomes false, false becomes true. If a statement is "The sky is blue", then NOT that statement is "The sky is not blue". NOT is a unary operator — it works on a single condition.

AND combines two conditions and returns true only when both are true. Consider the statement "I will take an umbrella if it is raining AND I am walking outside." If it is raining but you are taking a cab, the overall condition is false. If you are walking but it is sunny, it is false. Only when both parts are true do you take the umbrella.

OR combines two conditions and returns true when at least one of them is true. "I will take an umbrella if it is raining OR the forecast says rain." Here, you take the umbrella even if only one condition holds — actual rain or a warning of rain. The only time you leave it behind is when neither condition is true.

Truth Tables: The Complete Picture

A truth table lists every possible combination of inputs and shows the output for each operator. For two inputs A and B, there are exactly four combinations:

This is the complete definition of each operator. There is no ambiguity, no context, no interpretation. Given the inputs, the output is fixed.

Why This Matters

Boolean logic is the foundation of how computers make decisions. Every search you run, every filter you apply in a spreadsheet, every conditional formatting rule — they all use these operators. When you type "cats AND dogs" into a search engine, you get pages that mention both animals. "Cats OR dogs" gives you pages that mention either. "Cats NOT dogs" excludes pages about dogs entirely. …