Saturday, 19 November 2011


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 OperatorsDescription
ORFor the row to be selected at least one of the conditions must be true.
ANDFor a row to be selected all the specified conditions must be true.
NOTFor 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_namelast_namesubject
------------------------------------
AnajaliBhagwatMaths
ShekarGowdaMaths
RahulSharmaScience
StephenFlemingScience
The following table describes how logical "OR" operator selects a row.
Column1 Satisfied?Column2 Satisfied?Row Selected
YESYESYES
YESNOYES
NOYESYES
NONONO

"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_namelast_nameage
--------------------------------
RahulSharma10
AnajaliBhagwat12
ShekarGowda15
The following table describes how logical "AND" operator selects a row.
Column1 Satisfied?Column2 Satisfied?Row Selected
YESYESYES
YESNONO
NOYESNO
NONONO

"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_namelast_namegames
-------------------------------------------
RahulSharmaCricket
StephenFlemingCricket
ShekarGowdaBadminton
PriyaChandraChess
The following table describes how logical "NOT" operator selects a row.
Column1 Satisfied?NOT Column1 Satisfied?Row Selected
YESNONO
NOYESYES
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_namelast_nameagegames
----------------------------------------------
RahulSharma10Cricket
PriyaChandra15Chess
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.

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