Basic SQL Interview Questions for Freshers / Entry Level

Basic-level SQL interview questions for Freshers / Entry Level positions.

1. What is SQL and what are its main components?

Answer: SQL (Structured Query Language) is used to communicate with databases. Main components: DDL (CREATE, ALTER, DROP - structure), DML (SELECT, INSERT, UPDATE, DELETE - data), DCL (GRANT, REVOKE - permissions), TCL (COMMIT, ROLLBACK - transactions).

-- DDL: Create structure
CREATE TABLE students (id INT, name VARCHAR(100));

-- DML: Manipulate data
INSERT INTO students VALUES (1, 'John');
SELECT * FROM students;

-- DCL: Control access
GRANT SELECT ON students TO app_user;

-- TCL: Transaction control
BEGIN; UPDATE students SET name = 'Jane' WHERE id = 1; COMMIT;

2. What is the difference between CHAR and VARCHAR?

Answer: CHAR is fixed-length (pads with spaces), VARCHAR is variable-length (stores actual length). CHAR(10) always uses 10 bytes, VARCHAR(10) uses actual string length + overhead. Use CHAR for fixed-length data (country codes), VARCHAR for variable (names).

CREATE TABLE example (
  country_code CHAR(2),     -- Always 2 chars: 'US', 'UK'
  name VARCHAR(100)         -- Variable: 'John' uses 4 chars
);

-- CHAR pads with spaces
SELECT LENGTH(country_code) FROM example; -- Returns 2
SELECT country_code = 'US' FROM example;  -- Works (space-padded comparison)

3. What is NULL in SQL and how do you check for it?

Answer: NULL represents unknown or missing value. It's not equal to zero or empty string. Use IS NULL or IS NOT NULL to check (= NULL doesn't work). NULL in any calculation returns NULL. Use COALESCE() or IFNULL() to provide defaults.

-- Checking for NULL
SELECT * FROM users WHERE phone IS NULL;
SELECT * FROM users WHERE phone IS NOT NULL;

-- WRONG: This never matches
SELECT * FROM users WHERE phone = NULL;

-- Handling NULL in calculations
SELECT COALESCE(phone, 'N/A') FROM users;
SELECT salary + COALESCE(bonus, 0) AS total FROM employees;