SQL Interview Questions for Freshers (0-2 Years Experience)

SQL (Structured Query Language) is the standard language for creating, querying, and modifying data in a relational database. A relational database stores data in tables made of rows and columns, with relationships between tables defined by shared key values — rather than one giant flat file.

SQL is the language. MySQL, Oracle Database, PostgreSQL, and SQL Server are database products — software that implements SQL, each with its own extensions and quirks on top of the shared standard. Learning SQL transfers between all of them; learning one product's specific syntax doesn't always transfer perfectly to another.

Four categories: DDL (Data Definition Language) — CREATE, ALTER, DROP, which define schema objects; DML (Data Manipulation Language) — INSERT, UPDATE, DELETE, which change data; DQL (Data Query Language) — SELECT, which reads data; and DCL (Data Control Language) — GRANT, REVOKE, which manage permissions. Transaction control (COMMIT, ROLLBACK) is sometimes grouped as a fifth category, TCL.

A primary key is a column (or set of columns) that uniquely identifies every row in a table and can never be NULL. A table can have only one primary key — though that key can be composite, made up of more than one column together.

CREATE TABLE employees (
  employee_id INT PRIMARY KEY,
  last_name   VARCHAR(50)
);

A foreign key is a column in one table that references the primary key of another table, linking the two. It's how relational databases represent relationships — an employees table might have a department_id foreign key pointing at the departments table.

WHERE filters individual rows before any grouping happens. HAVING filters groups after GROUP BY has run, and is the only place you can filter using an aggregate function like COUNT or SUM.

SELECT department_id, COUNT(*) AS emp_count
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 5;

DISTINCT removes duplicate rows from a query's result, based on every column selected. SELECT DISTINCT department_id FROM employees returns each department only once, no matter how many employees are in it.

CHAR(n) is fixed-length — it always stores exactly n characters, padding shorter values with spaces. VARCHAR/VARCHAR2(n) is variable-length — it stores only the characters actually used, up to a maximum of n. VARCHAR types are almost always the right default; CHAR is reserved for genuinely fixed-width data like a two-letter country code.

DELETE removes selected or all rows, one at a time, is DML, and can be rolled back — the table itself remains. TRUNCATE removes every row instantly, is DDL (and auto-commits in most databases), and generally cannot be rolled back — the table remains, empty. DROP removes the entire table — structure and data — permanently.

A JOIN combines rows from two or more tables based on a related column. The main types: INNER JOIN returns only rows with a match in both tables; LEFT JOIN returns every row from the left table plus matches from the right (NULL where there's no match); RIGHT JOIN is the mirror image; FULL OUTER JOIN returns every row from both tables.

SELECT e.first_name, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

Normalization is organizing a database's tables to reduce duplicate data and avoid update anomalies — typically by splitting information into related tables connected by foreign keys, following a series of rules called normal forms. For example, storing a customer's address once in a customers table instead of repeating it on every one of their orders.

NULL means "unknown" or "no value at all" — it is not the same as 0 (a known numeric value) or '' (a known, empty piece of text). Because NULL is unknown, column = NULL never matches anything, even another NULL; you must use IS NULL or IS NOT NULL to test for it.

Both prevent duplicate values in a column. A table can have only one PRIMARY KEY, and it can never be NULL. A table can have several UNIQUE constraints, and unique columns are still allowed to contain NULLs (since NULL is never considered equal to another NULL).

Even though you write SELECT first, the database evaluates a query in this order: FROM → WHERE → GROUP BY → HAVING → SELECT → ORDER BY. Knowing this order explains why you can't filter on an aggregate in WHERE, and why a column alias defined in SELECT usually can't be used in WHERE.

An alias is a temporary name given to a column or table, using AS (often optional) — used to make output more readable or to shorten long table names in a query with multiple joins.

SELECT e.first_name AS name, e.salary AS pay
FROM employees AS e;

A subquery is a query nested inside another query, usually to compute a value the outer query then filters or compares against. It can appear in SELECT, FROM, or WHERE.

SELECT first_name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);