SQL is the most popular database and many popular websites use SQL. So, here is a collection of the most asked SQL interview questions to help you prepare.
1. What is Database?
A database is an organized collection of data, stored and retrieved digitally from a remote or local computer system. Databases can be vast and complex, and such databases are developed using fixed design and modeling approaches.
2. What is DBMS?
DBMS stands for Database Management System. DBMS is a system software responsible for the creation, retrieval, updating and management of the database.
3. What is SQL?
SQL stands for Structured Query Language. It is the standard language for relational database management systems. It is especially useful in handling organized data comprised of entities and relations between different entities of the data.
4. What is the difference between SQL and MySQL?
SQL is a standard language for retrieving and manipulating structured databases. On the contrary, MySQL is a relational database management system, like SQL Server, Oracle , that is used to manage SQL databases.
5. What are Tables and Fields?
A table is an organized collection of data stored in the form of rows and columns. Columns can be categorized as vertical and rows as horizontal. The columns in a table are called fields while the rows can be referred to as records.
6. What is a Primary Key?
The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint.
A table in SQL is strictly restricted to have one and only one primary key.
7. What is a UNIQUE constraint?
A UNIQUE constraint ensures that all values in a column are different. This provides uniqueness for the column(s) and helps identify each row uniquely. Unlike the primary key, there can be multiple unique constraints defined per table.
8. What is a Foreign Key?
A FOREIGN KEY comprises of single or collection of fields in a table that essentially refers to the PRIMARY KEY in another table. Foreign key constraint ensures referential integrity in the relation between two tables.
9. What is a Join? List its different types.
The SQL Join clause is used to combine records (rows) from two or more tables in a SQL database based on a related column between the two.
LEFT OUTER JOIN
RIGHT OUTER JOIN
FULL OUTER JOIN
10. What is a Query?
A query is a request for data or information from a database table or combination of tables. A database query can be either a select query or an action query.
11. SQL create and drop database?
Create database db_name -
=> CREATE DATABASE db_name;
Drop database db_name -
=> DROP DATABASE db_name;
12. Create and drop a table in SQL?
CREATE TABLE table_name (
DROP TABLE table_name;
13. What is the difference between DROP and TRUNCATE statements?
If a table is dropped, all things associated with the tables are dropped as well. However, if a table is truncated, none of the above problems exist and the table retains its original structure.
14. How do we use the DISTINCT statement?
The DISTINCT statement is used with the SELECT statement. If a record contains duplicate values then DISTINCT will select different values among duplicate records.
SELECT DISTINCT column_name
15. What is ACID property?
In order to maintain consistency in a database, before and after the transaction, certain properties are followed. These are called ACID properties.
16. How many Aggregate functions are available in SQL?
SQL Aggregate functions calculate values from multiple columns and return a single value. There are 7 aggregate functions in SQL:
17. What is View in SQL?
A View can be defined as a virtual table that contains rows and columns with fields from one or more tables.
CREATE VIEW view_name AS
18. What is SQL Injection?
SQL Injection is a type of database attack technique where malicious SQL statements are inserted into an entry field of the database in a way that once it is executed, the database is exposed to an attacker for the attack.
19. What is Normalization? How many Normalization forms are there?
Normalization is used to organize the data tor remove redundancies .
5 type of normal form:
First Normal Form(1NF)
First Normal Form(2NF)
Third Normal Form(3NF)
Fourth Normal Form(BCNF)
20. What is a Relationship? How many types of Relationships are there?
The relationship can be defined as the connection between more than one table in the database.
Type of Relationship -
1. One to One
2. Many to One
3. Many to Many
4. One to Many
21. How to select all records from the table?
To select all the records from the table we need to use the following syntax:
SELECT * FROM table_name;
22. Difference between TRUNCATE, DELETE, and DROP commands?
DELETE removes some or all rows from a table based on the condition.
TRUNCATE removes ALL rows from a table by deallocating the memory pages.
DROP command removes a table from the database completely.
23. Define UNION, MINUS, UNION ALL?
MINUS – returns all distinct rows selected by the first query but not by the second.
UNION – returns all distinct rows selected by either query
UNION ALL – returns all rows selected by either query, including all duplicates.