1. What is SQL?

SQL (Structured Query Language) is a standard programming language used for managing and manipulating data in relational databases (RDBMS).

2. What are the main types of SQL commands?

DDL (Data Definition Language): CREATE, ALTER, DROP.
DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE.
DCL (Data Control Language): GRANT, REVOKE.
TCL (Transaction Control Language): COMMIT, ROLLBACK.

3. What is an RDBMS?

Relational Database Management System. A database system based on the relational model (data stored in tables with rows and columns). Examples: MySQL, PostgreSQL, SQL Server.

4. What is a "Database Table"?

A collection of related data entries consisting of columns and rows.

5. What is a "Primary Key"?

A unique identifier for each record in a table. It cannot contain NULL values.

6. What is a "Foreign Key"?

A field (or collection of fields) in one table that refers to the Primary Key in another table, establishing a relationship between them.

7. What is the difference between `DELETE` and `TRUNCATE`?

DELETE: A DML command; can be rolled back; can have a WHERE clause.
TRUNCATE: A DDL command; faster; cannot be rolled back; removes all rows.

8. What is a "Join"?

An operation used to combine rows from two or more tables based on a related column between them.

9. What are the different types of Joins?

Inner Join: Returns records with matching values in both tables.
Left Join: Returns all records from the left table and matched records from the right.
Right Join: Returns all records from the right table and matched records from the left.
Full Outer Join: Returns all records when there is a match in either left or right table.

10. What is a "Self Join"?

A regular join in which a table is joined with itself.

11. What is a "Cross Join"?

Returns the Cartesian product of the two tables (every row of the first table matched with every row of the second).

12. What is an "Index" in SQL?

A performance-enhancing structure used to retrieve data from a table more quickly.

13. What is the "Unique Key" constraint?

Ensures that all values in a column are distinct. Unlike a primary key, it can accept one NULL value.

14. What is a "View"?

A virtual table based on the result-set of an SQL statement. It doesn't store data itself.

15. What are "Aggregate Functions"?

Functions that perform a calculation on a set of values and return a single value (e.g., COUNT, SUM, AVG, MIN, MAX).

16. What is the difference between `WHERE` and `HAVING`?

WHERE: Filters rows before any groupings are made.
HAVING: Filters groups after groupings are made (used with GROUP BY).

17. What is "Normalization"?

The process of organizing data in a database to reduce redundancy and improve data integrity (e.g., 1NF, 2NF, 3NF).

18. What is "Denormalization"?

The process of adding redundant data to a database to improve read performance.

19. What is a "Storable Procedure"?

A prepared SQL code that you can save and reuse over and over again.

20. What is a "Trigger"?

A stored procedure that automatically runs when an event (like INSERT, UPDATE, DELETE) occurs in the database server.

21. What is the difference between `UNION` and `UNION ALL`?

UNION: Combines the result of two SELECT statements and removes duplicates.
UNION ALL: Combines the result of two SELECT statements and keeps duplicates.

22. What is a "Subquery"?

A query nested inside another query (e.g., inside a SELECT, FROM, or WHERE clause).

23. What is "ACID" in SQL?

A set of properties that ensure database transactions are processed reliably:

Atomicity: All or nothing.
Consistency: Data stays valid.
Isolation: Transactions don't interfere.
Durability: Data persists after completion.

24. What is a "Cursor"?

A database object used to retrieve and manipulate data from a result set one row at a time.

25. What is the difference between `CHAR` and `VARCHAR`?

CHAR: Fixed-length character string.
VARCHAR: Variable-length character string.

26. What is "Collating"?

A set of rules that defines how data is sorted and compared in a database.

27. What is a "Constraint"?

Rules enforced on data columns (e.g., NOT NULL, UNIQUE, CHECK, DEFAULT).

28. What is the `LIKE` operator?

Used in a WHERE clause to search for a specified pattern in a column (using % or _ wildcards).

29. What is the `IN` operator?

Allows you to specify multiple values in a WHERE clause.

30. What is the `BETWEEN` operator?

Selects values within a given range (inclusive).

31. What is "Transaction"?

A single unit of work that contains one or more SQL statements to be executed.

32. What is the "Database Schema"?

A formal description of the data structure and relationships in a database.

33. What is "SQL Injection"?

A security vulnerability where an attacker can interfere with the queries that an application makes to its database.

34. What is the purpose of the `COALESCE()` function?

Returns the first non-null value in a list of arguments.

35. What is the `GROUP BY` clause used for?

Arranges rows that have the same values into summary rows (e.g., finding the number of customers in each country).

36. What is the `ORDER BY` clause?

Used to sort the result-set in ascending (ASC) or descending (DESC) order.

37. What is a "Composite Key"?

A primary key that consists of two or more columns in a table.

38. What is a "Unique Constraint"?

Ensures that all values in a column are unique across the table.

39. What is a "Check Constraint"?

Ensures that all values in a column satisfy a specific condition.

40. What is the `NULLIF()` function?

Returns NULL if two arguments are equal; otherwise, it returns the first argument.

41. What is "Data Integrity"?

The overall accuracy, completeness, and consistency of data throughout its lifecycle.

42. What is the difference between `NULL` and `Blank`?

NULL: Represents the absence of data.
Blank: A string containing no characters (length 0).

43. What is the difference between `Inner` and `Outer` join?

Inner: Only includes records that have matching values in both tables.
Outer: Includes matching records and non-matching records from one or both tables.

44. What is a "Non-Clustered Index"?

An index where the logical order of the index does not match the physical stored order of the rows on disk.

45. What is a "Clustered Index"?

An index that determines the physical order of the data in a table. A table can have only one clustered index.

46. What is the `CASE` statement in SQL?

Used to create conditional logic in SQL queries (like if-then-else).

47. What is the `DISTINCT` keyword?

Used to return only unique (different) values from a specific column.

48. What is the `EXISTS` operator?

Used to test for the existence of any record in a subquery.

49. What is "Database Denormalization"?

The process of strategic duplication of data to improve query performance.

50. What is a "Database Transaction Log"?

An internal record of all changes made to a database, used for recovery and recovery purposes.

SQL Interview Questions

Table of Contents

What is SQL?

What are the main types of SQL commands?

What is an RDBMS?

What is a "Database Table"?

What is a "Primary Key"?

What is a "Foreign Key"?

What is the difference between <code>DELETE</code> and <code>TRUNCATE</code>?

What is a "Join"?

What are the different types of Joins?

What is a "Self Join"?

What is a "Cross Join"?

What is an "Index" in SQL?

What is the "Unique Key" constraint?

What is a "View"?

What are "Aggregate Functions"?

What is the difference between <code>WHERE</code> and <code>HAVING</code>?

What is "Normalization"?

What is "Denormalization"?

What is a "Storable Procedure"?

What is a "Trigger"?

What is the difference between <code>UNION</code> and <code>UNION ALL</code>?

What is a "Subquery"?

What is "ACID" in SQL?

What is a "Cursor"?

What is the difference between <code>CHAR</code> and <code>VARCHAR</code>?

What is "Collating"?

What is a "Constraint"?

What is the <code>LIKE</code> operator?

What is the <code>IN</code> operator?

What is the <code>BETWEEN</code> operator?

What is "Transaction"?

What is the "Database Schema"?

What is "SQL Injection"?

What is the purpose of the <code>COALESCE()</code> function?

What is the <code>GROUP BY</code> clause used for?

What is the <code>ORDER BY</code> clause?

What is a "Composite Key"?

What is a "Unique Constraint"?

What is a "Check Constraint"?

What is the <code>NULLIF()</code> function?

What is "Data Integrity"?

What is the difference between <code>NULL</code> and <code>Blank</code>?

What is the difference between <code>Inner</code> and <code>Outer</code> join?

What is a "Non-Clustered Index"?

What is a "Clustered Index"?

What is the <code>CASE</code> statement in SQL?

What is the <code>DISTINCT</code> keyword?

What is the <code>EXISTS</code> operator?

What is "Database Denormalization"?

What is a "Database Transaction Log"?