SQL

Question 1

Insert query

Answer 1

INSERT INTO employees (id, name, age, department, salary) VALUES (1, ‘John Doe’, 30, ‘IT’, 50000);

Question 2

UPDATE

Answer 2

UPDATE employees
SET salary = 55000
WHERE id = 1;

Question 3

DROP

Answer 3

DROP TABLE employees;

Question 4

CREATE query

Answer 4

CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(100),
age INT,
department VARCHAR(50),
salary DECIMAL(10,2)
);

Question 5

Join syntax

Answer 5

SELECT Customers.customer_id, Customers.first_name, Orders.item
FROM Customers
JOIN Orders
ON Customers.customer_id = Orders.customer_id

Question 6

char vs varchar

Answer 6

char 255, varchar 65535

Question 7

binary

Answer 7

stream of data in binary form

Question 8

SQL vs MYSQL

Answer 8

SQL from microsoft, MYSQL from oracle

Question 9

VARCHAR vs TEXT

Answer 9

name VARCHAR(255), – Stores up to 255 characters inline
description TEXT stores here and there,

Varchar best for indexing , and faster in search

for big text like description, comment text is best because of less search

Question 10

decimal datatype with digits

Answer 10

DECIMAL(10,2)

Question 11

date datatypes

Answer 11

Data Type Description Example

DATE Stores only date (YYYY-MM-DD) ‘2025-03-07’

DATETIME Stores date & time (YYYY-MM-DD HH:MM:SS) ‘2025-03-07 14:30:00’

TIMESTAMP Similar to DATETIME, but auto-updates with time zone support Used for recording changes

TIME Stores only time (HH:MM:SS) ‘14:30:00’

YEAR Stores a year (YYYY) ‘2025’

Question 12

Cross join possible combination

Answer 12

m*n

Question 13

Primary key syntax

Answer 13

CREATE TABLE Students (
student_id INT PRIMARY KEY, – Unique identifier for each student
name VARCHAR(50),
age INT,
class VARCHAR(10)
);

Question 14

Foreign key syntax

Answer 14

CREATE TABLE Marks (
mark_id INT PRIMARY KEY, – Unique ID for each mark entry
student_id INT, – Foreign Key referring to Students table
subject VARCHAR(50),
marks INT,
FOREIGN KEY (student_id) REFERENCES Students(student_id) – Linking to Students table
);

Question 15

Composite key syntax

Answer 15

CREATE TABLE Enrollments (
student_id INT,
course_id INT,
enrollment_date DATE,
PRIMARY KEY (student_id, course_id) – Composite Key (student_id + course_id)
);

Question 16

Candidate Key and its syntax

Answer 16

CREATE TABLE Students (
student_id INT UNIQUE, – Candidate Key
email VARCHAR(100) UNIQUE, – Candidate Key
name VARCHAR(50),
PRIMARY KEY (student_id) – Only one is chosen as the Primary Key
);

Question 17

inno db engine

Answer 17

If primary key not declared mysql create its own column and asign as primary key by itself, Reason: Having primary key or unique key is mandatory in DBMS, But not compulsory to declare

Question 18

What field type foreign key can refer

Answer 18

Primary key, Unique ( Candidate key )

Question 19

super key as foreign key

Answer 19

CREATE TABLE Employees (
emp_id INT NOT NULL,
email VARCHAR(100) UNIQUE,
name VARCHAR(50),
UNIQUE (emp_id, email) – Super Key without Primary Key
);
CREATE TABLE Salaries (
salary_id INT PRIMARY KEY,
emp_ref_id INT,
emp_email VARCHAR(100),
FOREIGN KEY (emp_ref_id, emp_email) REFERENCES Employees(emp_id, email)
);

Question 20

CHECK

Answer 20

CREATE TABLE students (
student_id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(50) NOT NULL,
age INT CHECK (age >= 18),
email VARCHAR(100) UNIQUE
);

Question 21

AUTO_INCREMENT

Answer 21

We can insert manually but next row will start from manually entered value

Question 22

DDL Data Definition Language

Answer 22

CREATE Creates a new table/database
ALTER Modifies an existing table
DROP Deletes a table/database
TRUNCATE

Question 23

DML Data Manipulation Language

Answer 23

INSERT Adds new records
UPDATE Modifies existing records
DELETE

Question 24

DQL Data Query Language

Answer 24

SELCT

Question 25

DCL Data Control Language

Answer 25

GRANT Gives user access REVOKE Removes user access

Question 26

DCL Data Control Language

Answer 26

GRANT SELECT, INSERT ON students TO 'user1'@'localhost';

Question 27

TCL Transaction Control Language

Answer 27

COMMIT Saves changes permanently ROLLBACK Reverts changes SAVEPOINT Creates a checkpoint

Question 28

AGGREGATE

Answer 28

COUNT() Counts rows COUNT(*) SUM() Adds values SUM(salary) AVG() Finds average AVG(salary) MAX() Finds max value MAX(age) MIN() Finds min value MIN(age)

Question 29

groupby having orderby

Answer 29

GROUP BY Groups data by column HAVING Filters grouped data ORDER BY

Question 30

Closure

Answer 30

A closure is created when a function is defined inside another function and it retains access to the outer function’s variables even after the outer function has finished executing.

Question 31

Closure example

Answer 31

function outerFunction() { let count = 0; // `count` is inside outerFunction's scope function innerFunction() { count++; // innerFunction remembers `count` console.log(count); } return innerFunction; // Returning inner function } const myClosure = outerFunction(); // Outer function executes and returns `innerFunction` myClosure(); // 1 myClosure(); // 2 myClosure(); // 3

Question 32

Hoisting

Answer 32

Variables declared with var are hoisted but not initialized. Their value is undefined until the assignment occurs.

Question 33

Event loop

Answer 33

Promise, async await are used for asynchronous calls

Question 34

java vs javascript

Answer 34

server vs client data type multi thread vs single thread OOPS vs scripting language JVM vs browser (node js) Object model vs Prototype model

Question 35

Prototype

Answer 35

In JavaScript, prototypes are the mechanism by which objects inherit properties and methods from one another. Every JavaScript object has an internal link to another object called its prototype. Object.prototype.type = "abc"; Array.prototype.type = "abc"; var obj 1 = {} obj1._prototype_.add=function(){}; var obj2 = {}; obj2 has add

Question 36

const

Answer 36

Value cannot be declared alone. It must be defined VM127:1 Uncaught SyntaxError: Missing initializer in const declaration const person = {name:"dsde"} can be done : person.name = "dqdqe" cannot be done : person = {name:"dsdewfewde"}

Question 37

Memory of data

Answer 37

primitive - stack non primitive / functions - heap event loops - queue string - constant pool memory why because strings can be frequently recurring and they will get pointed to a common memory

Question 38

mutable

Answer 38

obj1 = obj2 obj1.abc = "hello"; obj2.abc - has hello

Question 39

mutable vs immutable

Answer 39

In JavaScript, mutability refers to whether a value can be changed after it is created. Mutable: Objects, arrays, and functions can be modified after creation. Immutable: Primitive values (like numbers, strings, and booleans) cannot be changed once assigned. Type Mutable? Example Objects ✅ Yes {} Arrays ✅ Yes [] Functions ✅ Yes function() {} Strings ❌ No "hello" Numbers ❌ No 42 Booleans ❌ No true / false

Question 40

ES6

Answer 40

1. let / const 2. arrow 3. template literal 4. default params 5. destructuring(obj & arr) 6. spread and rest operator (...) 7.promises () 8. OOP 9. modules (import vs export) 10. Map and Set

Question 41

arrow vs function

Answer 41

1. no arguments 2. syntax 3. this - difference 4. ES6 5. arrow cannot be used for constructor 6. hoisting not happen in arrow

Question 42

Alter query

Answer 42

ALTER TABLE table_name ADD column_name datatype; ALTER TABLE table_name DROP COLUMN column_name; ALTER TABLE table_name RENAME COLUMN old_name to new_name; ALTER TABLE table_name ALTER COLUMN column_name datatype;