What is DQL

What is DQL?

Definition (English)

DQL (Data Query Language) is a category of SQL commands used to retrieve data from one or more database tables.

Definition (Hindi)

DQL (Data Query Language) SQL का वह भाग है जिसका उपयोग Database से Data को पढ़ने (Retrieve), देखने (View) और खोजने (Search) के लिए किया जाता है।
 

Simple Definition

DQL = Read Data from Database

DDL → Creates Structure

DML → Changes Data

DQL → Reads Data

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Real-Life Example

Suppose a college database contains 50,000 students.

You want to find:

• All CSE students
• Students whose marks are above 80
• Students sorted by marks
• Top 10 students

Instead of searching manually, SQL retrieves the data instantly.

Sample Table

We'll use this table throughout the chapter.

StudentID	Name	Age	Department	Marks
101	Rahul	20	CSE	85
102	Aman	21	IT	78
103	Neha	19	ECE	92
104	Priya	20	CSE	88
105	Rohit	22	ME	70

 

SELECT Statement

What is SELECT?

The SELECT statement retrieves data from one or more tables.

Syntax  

SELECT column_name
FROM table_name;

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Select All Columns  

SELECT *
FROM Student;

StudentID	Name	Age	Department	Marks
101	Rahul	20	CSE	85
102	Aman	21	IT	78
103	Neha	19	ECE	92
104	Priya	20	CSE	88
105	Rohit	22	ME	70

* means all columns.

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Select Specific Columns

SELECT Name, Marks
FROM Student;

Name	Marks
Rahul	85
Aman	78
Neha	92
Priya	88
Rohit	70

FROM Clause

What is FROM?

The FROM clause specifies the table from which data will be retrieved.

SELECT Name
FROM Student;

Syntax

SELECT Name
FROM Student;

Here,

• SELECT → What data?
• FROM → From which table?

WHERE Clause

What is WHERE?

The WHERE clause filters records based on a condition.

Syntax

SELECT *
FROM Student
WHERE condition;

 

Example 1

SELECT *
FROM Student
WHERE Department='CSE';

Result

Name	Department
Rahul	CSE
Priya	CSE

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Example 2

SELECT *
FROM Student
WHERE Marks>80;

Result

Name	Marks
Rahul	85
Neha	92
Priya	88

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Comparison Operators

Operator	Meaning
=	Equal
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal
!= or <>	Not equal

Example

SELECT *
FROM Student
WHERE Age>=20;

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DISTINCT Keyword

What is DISTINCT?

DISTINCT removes duplicate values.

Suppose Department column contains:

Department
CSE
IT
ECE
CSE
IT

Query

SELECT DISTINCT Department
FROM Student;

Result

Department
CSE
IT
ECE

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ORDER BY Clause

What is ORDER BY?

ORDER BY sorts the result.

Ascending Order (Default)

SELECT *
FROM Student
ORDER BY Marks ASC;

Result

70 → 78 → 85 → 88 → 92

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Descending Order

SELECT *
FROM Student
ORDER BY Marks DESC;

Result

92 → 88 → 85 → 78 → 70

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Order by Multiple Columns

SELECT *
FROM Student
ORDER BY Department ASC,
Marks DESC;

First sorts by Department, then by Marks within each department.

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LIMIT Clause (MySQL/PostgreSQL)

What is LIMIT?

LIMIT returns only a specified number of rows.

SELECT *
FROM Student
LIMIT 3;

Returns the first 3 records.

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SQL Server Equivalent

SELECT TOP 3 *
FROM Student;

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Column Alias (AS)

What is Alias?

Alias gives a temporary name to a column.

SELECT Name AS StudentName,
Marks AS Score
FROM Student;

Result

StudentName	Score
Rahul	85
Aman	78

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Arithmetic Expressions

SQL can perform calculations.

SELECT Name,
Marks+5 AS NewMarks
FROM Student;

Result

Name	NewMarks
Rahul	90
Aman	83

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Logical Operators

AND

Both conditions must be true.

SELECT *
FROM Student
WHERE Department='CSE'
AND Marks>80;

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OR

At least one condition must be true.

SELECT *
FROM Student
WHERE Department='IT'
OR Department='ECE';

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NOT

Reverses the condition.

SELECT *
FROM Student
WHERE NOT Department='CSE';

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IN Operator

Used to match multiple values.

SELECT *
FROM Student
WHERE Department IN ('CSE','IT');

Equivalent to

Department='CSE'
OR Department='IT'

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BETWEEN Operator

Used to select values within a range.

SELECT *
FROM Student
WHERE Marks BETWEEN 80 AND 90;

Result

Rahul (85)

Priya (88)

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LIKE Operator

Used for pattern matching.

Starts With

SELECT *
FROM Student
WHERE Name LIKE 'R%';

Matches

Rahul

Rohit

---

Ends With

SELECT *
FROM Student
WHERE Name LIKE '%a';

Matches

Neha

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Contains

SELECT *
FROM Student
WHERE Name LIKE '%oh%';

Matches

Rohit

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Wildcards

Wildcard	Meaning
%	Any number of characters
_	Exactly one character

Examples

LIKE 'A%'

Starts with A

LIKE '%n'

Ends with n

LIKE '_a%'

Second letter is "a"

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IS NULL

Find NULL values.

SELECT *
FROM Student
WHERE Marks IS NULL;

---

IS NOT NULL

SELECT *
FROM Student
WHERE Marks IS NOT NULL;

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Complete Example

SELECT Name,
Marks
FROM Student
WHERE Department='CSE'
AND Marks>80
ORDER BY Marks DESC;

Execution Flow

Student Table
      │
      ▼
WHERE Department='CSE'
      │
      ▼
Marks > 80
      │
      ▼
ORDER BY Marks DESC
      │
      ▼
Display Name and Marks

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Common Mistakes

Missing Quotes

Incorrect

WHERE Name=Rahul

Correct

WHERE Name='Rahul'

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Using = with NULL

Incorrect

WHERE Marks=NULL

Correct

WHERE Marks IS NULL

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Forgetting ORDER BY Direction

ORDER BY Marks

Default is ASC.