15 Most Asked Interview Questions for SQL
Relational databases are commonly accessed using SQL, or Structured Query Language. Given the enormous amount of data already present, we must comprehend how to use queries to retrieve the necessary data. You can go over a few SQL queries that you should practice to become a database administrator and that will also help you ace your interviews in this blog on SQL Query Interview Questions.
Technical SQL Interview Question
1.Which subsets of SQL are present?
In addition to other database operations, it lets you CREATE, ALTER, and DELETE objects—language for Data Definition (DDL).
The Data Manipulation Language enables access to and modification of data (DML). It supports various operations such as inserting, updating, Deleting, and Retrieving the DB.
The Data Control Language can limit database access (DCL). For instance, grant or deny access permissions.
2.What do DBMS and RDBMS stand for? Explain how they differ.
Users can create, access, update, and manage databases using system software known as a database management system, or DBMS. It maintains data consistency, ensures that it is organized, and ensures that it is readily available by acting as an interface between the database and its end-users or Application Software. Here are the four different categories of DBMS:
In a database with a tree-like structure, data are maintained hierarchically. In a database, a single parent may give birth to numerous children, but there can only be one parent per child.
3.What does normalization mean?
The process of normalization involves reducing data redundancy and boosting data integrity in the database tables. It aids in cleaning up and organizing rows in tables by getting rid of duplicate rows. Relationships are used by normalization rules to divide large tables into smaller ones.
SQL normalization accomplishes two tasks at once: it gets rid of redundant data and ensures logical data storage.
4.What is the feature View in SQL?
Views are a subset of virtual tables in SQL. A view’s rows and columns match those of the actual table’s rows and columns in the database. By selecting fields from one or more database tables, views can be created. A view can either display every row in the table or it can filter the rows using user-defined criteria and only display the rows that satisfy those criteria.
A view’s security use case is its most significant one. Instead of using the table itself, it generates a searchable object that can be queried. It will only return information that was available at the time the view was created.
Here is an example of how to create a new view
CREATE VIEW view_name AS
SELECT column_lists FROM table_name
5.What is RDBMS?
Relational Database Management System is the name for this technology. RDBMS organizes the data into a set of tables connected by shared fields in the table’s columnar structure. To modify the data stored in the 6. tables, it also offers relational operators.
6.So what is SQL?
It is possible to communicate with the database using SQL, which stands for Structured Query Language. Using this standard language, you can insert, update, retrieve, and delete data from databases.
The Select command is a common SQL command.
7.Primary key is defined as what?
A field called a primary key, made up of a column or group of columns, uniquely identifies each row or record in a database table. In contrast to composite keys, which are a subset of primary keys, single-column keys are simply referred to as primary keys.
Each record has a distinct identifier thanks to the primary key. The following traits are connected to a primary key:
A primary key field’s values ought to all be distinct.
It is not permitted for primary keys to having missing or NULL values.
One primary key is required for every table.
For instance, no two customers are ever permitted to share an ID at an insurance company. Each customer is also required to have a customer ID. Customer ID can be used as a primary key as a result.
The following is a definition of a primary key:
/* Create a table Student with ID as primary key */
/*Specifying the PRIMARY KEY while creating the table*/
CREATE TABLE Students(
ID INT NOT NULL
PRIMARY KEY (ID)
/* Alter table add ID as primary key*/
/*Specifying the PRIMARY KEY when the table is already created*/
ALTER TABLE Students
ADD PRIMARY KEY (ID);
8.How do fields and tables differ?
A table is a collection of information that has been arranged using a model with columns and rows. As opposed to horizontal rows, columns can be categorized as vertical. While a record can have any number of rows, a table only has a predetermined number of columns, known as fields.
The employee is listed in the table.
Field: Emp ID, Emp Name, and Emp Birthdate.
Date: 11/15/1960; 201456; David.
9.What exactly is a unique key?
Each record in the database was uniquely identified by a unique key constraint. As a result, the column or group of columns becomes distinctive.
A primary key constraint has an automatic unique constraint set on it. But not in the case of the Unique Key.
There can be multiple unique constraints defined for each table, but there can only be one primary key constraint defined for each table.
10.What exactly is a foreign key?
A foreign key is a relation between a table and the primary key of another table. It is necessary to establish a relationship between two tables by referencing a foreign key with the primary key of another table.
11.Describe an index.
An index is a performance improvement technique that enables quicker table record retrieval. A faster way to retrieve data is through an index, which creates an entry for each value.
12.What are relationships and what do they consist of?
The relationship between the tables in a database is referred to as a database relationship. The following are some examples of databasing relationships.
- One-on-one interaction.
- Relationship of one to many.
- Relationship of many to one.
- A relationship that refers to itself.
13.Create a query that retrieves the EmpFname in the upper case from the EmployeeInfo table and uses the ALIAS name as the EmpName.
SELECT UPPER(EmpFname) AS EmpName FROM EmployeeInfo;
14.What Do Constraints Mean?
The maximum data type for the table can be specified using constraints in SQL. It can be specified when making changes to or creating a new table statement. Here is a list of some restrictions:
- NOT NULL
- PRIMARY KEY
- FOREIGN KEY
15. How to Establish a database.
A database is a set of structured data that has been organized and can be managed, stored, retrieved, and easily accessed digitally from a remote or local computer system. Databases are created using a set design and modeling approach, and they can be large and complex. Smaller databases can be kept on a file system, but larger databases are kept on computer clusters or in the cloud.