SQL Logical Operators

There are three Logical Operators namely, AND, OR, and NOT. These operators compare two conditions at a time to determine whether a row can be selected for the output. When retrieving data using a SELECT statement, you can use logical operators in the WHERE clause, which allows you to combine more than one condition.

Logical Operators	Description
OR	For the row to be selected at least one of the conditions must be true.
AND	For a row to be selected all the specified conditions must be true.
NOT	For a row to be selected the specified condition must be false.

"OR" Logical Operator:

If you want to select rows that satisfy at least one of the given conditions, you can use the logical operator, OR.
For example: if you want to find the names of students who are studying either Maths or Science, the query would be like,
SELECT first_name, last_name, subject
FROM student_details
WHERE subject = 'Maths' OR subject = 'Science'

The output would be something like,

first_name	last_name	subject
-------------	-------------	----------
Anajali	Bhagwat	Maths
Shekar	Gowda	Maths
Rahul	Sharma	Science
Stephen	Fleming	Science

The following table describes how logical "OR" operator selects a row.

Column1 Satisfied?	Column2 Satisfied?	Row Selected
YES	YES	YES
YES	NO	YES
NO	YES	YES
NO	NO	NO

"AND" Logical Operator:

If you want to select rows that must satisfy all the given conditions, you can use the logical operator, AND.
For Example: To find the names of the students between the age 10 to 15 years, the query would be like:
SELECT first_name, last_name, age
FROM student_details
WHERE age >= 10 AND age <= 15;

The output would be something like,

first_name	last_name	age
-------------	-------------	------
Rahul	Sharma	10
Anajali	Bhagwat	12
Shekar	Gowda	15

The following table describes how logical "AND" operator selects a row.

Column1 Satisfied?	Column2 Satisfied?	Row Selected
YES	YES	YES
YES	NO	NO
NO	YES	NO
NO	NO	NO

"NOT" Logical Operator:

If you want to find rows that do not satisfy a condition, you can use the logical operator, NOT. NOT results in the reverse of a condition. That is, if a condition is satisfied, then the row is not returned.
For example: If you want to find out the names of the students who do not play football, the query would be like:
SELECT first_name, last_name, games
FROM student_details
WHERE NOT games = 'Football'

The output would be something like,

first_name	last_name	games
----------------	----------------	-----------
Rahul	Sharma	Cricket
Stephen	Fleming	Cricket
Shekar	Gowda	Badminton
Priya	Chandra	Chess

The following table describes how logical "NOT" operator selects a row.

Column1 Satisfied?	NOT Column1 Satisfied?	Row Selected
YES	NO	NO
NO	YES	YES

Nested Logical Operators:
You can use multiple logical operators in an SQL statement. When you combine the logical operators in a SELECT statement, the order in which the statement is processed is
1) NOT
2) AND
3) OR

For example: If you want to select the names of the students who age is between 10 and 15 years, or those who do not play football, the
SELECT statement would be
SELECT first_name, last_name, age, games
FROM student_details
WHERE age >= 10 AND age <= 15
OR NOT games = 'Football'

The output would be something like,

first_name	last_name	age	games
-------------	-------------	--------	------------
Rahul	Sharma	10	Cricket
Priya	Chandra	15	Chess

In this case, the filter works as follows:
Condition 1: All the students you do not play football are selected.
Condition 2: All the students whose are aged between 10 and 15 are selected.
Condition 3: Finally the result is, the rows which satisfy atleast one of the above conditions is returned.

NOTE:The order in which you phrase the condition is important, if the order changes you are likely to get a different result.