Most frequently asked SQL interview questions in 2021
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If you are reading this article then, I guess you are preparing for SQL interview questions!
All the best for your interview!
Here I have made a small attempt to bring down a consolidated list of the most frequently asked sql interview questions
Hope it helps you!
Here we go…
- What is SQL?
- SQL stands for Structured Query Language.
- Its main aim to store, access, and manipulate the data in relational databases.
- SQL is an American National Standards Institute ( ANSI ) standard, developed by Raymond F.Boyce and Donald D.chamberlin at IBM.
- All relational database management systems like MySQL, MSaccess, oracle, and SQL Server use SQL a standard database language.
- What SQL can do?
- To execute queries against a database.
- To retrieve data from a database.
- To inserts records in a database.
- To updates records in a database.
- To delete records from a database.
- To create new databases.
- To create new tables in a database.
- To create views in a database.
- To perform complex operations on the database.
- What is join in SQL?
SQL Joins – Combining rows from two or more tables, based on the related column between them.
Types of Joins:
- Inner Join − When there is a match in both tables- “Returns rows”
INNER JOIN table2
ON table1.column_name = table2.column_name;
- Left Join − Returns all records/rows from the left table, even if there are no matches in the right table.
LEFT JOIN table2
ON table1.column_name = table2.column_name;
- Right Join− Returns all records/rows from the right table, even if there are no matches in the left table.
RIGHT JOIN table2
ON table1.column_name = table2.column_name;
- Full outer Join − returns records/rows when there is a match in either left or right table
FULL OUTER JOIN table2
ON table1.column_name = table2.column_name
- Self Join − Table is joined itself this is Self Join ( Regular Join)SELECT column_name(s)
FROM table1 T1, table1 T2
- CARTESIAN JOIN (CROSS JOIN) − Sets of rows/records from the two or more joined tables will be returned, this is cartesian join.SELECT *
CROSS JOIN table2;
- What are the different types of SQL commands?
SQL commands are segregated into the following types:
- DDL – Data Definition Language
- DML – Data Manipulation Language
- DQL – Data Query Language
- DCL – Data Control Language
- TCL – Transaction Control Language
- What are the subsets of SQL?
There are three significant subsets of the SQL:
- Data definition language (DDL): Data structure can be defined by DDL and it also consists of commands like CREATE, ALTER, DROP, etc.
- Data manipulation language (DML): DML helps in manipulating existing data in the database and it also consists of commands like SELECT, UPDATE, INSERT, etc.
- Data control language (DCL): DCL helps in controlling the data access and commands such as GRANT, REVOKE are useful for controlling the data.
- What are the different DDL commands in SQL?
DDL commands help in defining or altering the structure of the database.
- CREATE: Helps in creating databases and database objects.
- ALTER: Helps in altering existing database objects.
- DROP: To drop databases and database objects.
- TRUNCATE: Helps in removing all records from a table but not its database structure.
- RENAME: To rename database objects.
- What are the different DML commands in SQL?
DML commands help in managing data present in the database.
- SELECT: Can select specific data from a database.
- INSERT: Can insert new records into a table.
- UPDATE: Can update existing records.
- DELETE: Can delete existing records from a table.
- What are the different DCL commands in SQL?
DCL commands are helping in creating roles, granting permission, and control access to the database objects.
- GRANT: Can provide user access.
- DENY: Can deny permissions to users.
- REVOKE: Can remove user access.
- What are the different TCL commands in SQL?
TCL commands help in managing the changes made by DML statements.
- COMMIT: Helps in writing and storing the changes to the database.
- ROLLBACK: Helps in restoring the database since the last commit.
- What is an Index?
An index is an on-disk structure used to speed up the performance of queries which helps in the retrieval of data from the table.
- What are all the different types of indexes?
There are three types of indexes
- Unique Index: Unique Indexes keep up data integrity by ensuring that no two rows of data in a table have identical key values.
- Clustered Index: Clustered Index is only one per table which helps in reordering the physical order of the table and searches based on the key values.
- Non-Clustered Index: Unlike the Clustered index, this has many non-clustered indexes in each table and it doesn’t alter the physical order of the table and maintains a logical order of the data.
- What are the differences between Cluster and Non-Cluster Index?
|Clustered Index||Non-Clustered Index|
|Data from the database can be retrieved faster||Data from the database can be retrieved slower|
|One table can only have one clustered index||One table can have multiple non clustered index|
|Type of index which helps in sorting the data rows in the table||Type of index which helps in sorting data at one location and indices at another location|
- What is a View in SQL?
- The view is a subset of a table that is stored logically in a database.
- It is a virtual table.
- A view contains rows and columns.
- Views do not contain data of their own.
- What are the advantages of Views?
Some of the advantages of Views are
- Views occupy no space.
- Views help in retrieving complicated queries that need to be executed often.
- The view helps in hiding data complexity and restricting access to the database
- What is a Database?
A Database is defined as a systematic collection of data that is stored in a computer in an organised manner and can be accessed in various ways.
Database helps in
– Collection of schemas.
– Accessing and manipulation of data.
- Does SQL support programming language features?
- SQL is a command language, not programming knowledge!
- SQL has commands like query, update, delete, etc. not conditional statements like for loops or if..else, etc…
- SQL allows us to manipulate data in a database.
So, SQL does not support programming languages.
- What is a Data warehouse?
The data warehouse is a central consolidated repository of data from multiple sources of information and these data are helpful in data mining as well as online processing.
- What is a Table in a Database?
A table is a database object that contains all the data in a database in the form of columns and rows that holds data.
- What is a Field in a Database?
A field in a database is a single piece of information from a record.
- What is a Record in a Database?
A row is generally called a record in a database which helps in the collection of fields of different data types which is typically in a fixed number and sequence.
21.What is a column in a Table?
A column is an “attribute” that we keep track of all information associated with a specific field in a table.
22. What is an SQL server?
- SQL Server is a relational database management system, or RDBMS, developed (By Microsoft ) which has worked on windows for more than 20 years and Microsoft made it available on Linux in October 2016
- SQL Server helps in interacting with relational databases.
- What is DBMS?
Database Management System is software that enables the user to store, retrieve, update, and delete information from a database.
- What are the types of DBMS?
There are two types of DBMS
- Relational Database Management System (RDBMS)
- Non-Relational Database Management System
- What is RDBMS?
RDBMS stands for Relational Database Management System that is based on the relational model which can be accessed using Structured Query Language (SQL).
- What are the popular Database Management Systems in the IT Industry?
- Microsoft SQL Server
- Microsoft Access
- What is a query?
A database query is either a “ SELECT” or “ ACTION” query that helps in requesting data or information from a database table or combination of tables.
- What is a Subquery?
A Subquery is a SQL query, nested inside another subquery that can be used anywhere where “expression” is allowed.
A subquery may occur in :
- – A SELECT clause
- – A FROM clause
- – A WHERE clause
- What are the types of subquery?
- Single-row subquery: Returns zero or one row.
- Multiple row subquery: Returns one or more rows.
- Multiple column subqueries: Returns one or more columns.
- Correlated subqueries: subquery is related to the outer SQL statement.
- Nested subqueries: Subqueries are placed within another subquery.
- What is SQL Injection?
SQL Injection is a type of code injection technique where malicious SQL statements are inserted into an entry field of a database which helps in accessing sensitive data and perform administrative tasks on databases.
- What is the difference between Local Variables and Global Variables?
Local Variables: Local variables exist only inside the function and are not referred to by any other functions.
Global Variables: Global variables exist throughout the program and can access globally.
- What is Data Integrity?
Data integrity is used to maintain the accuracy and consistency of the data stored in a database.
- What is Auto Increment in SQL?
Auto increment keyword allows creating a unique number when a new record is inserted into a table.
- What is a temp table?
A temp table helps in storing the data temporarily.
- Difference between SQL Vs NoSQL?
|Type of DataBase||Relational database||Non Relational database/Distributed database|
|Standardization||Standard Query Language exists||No proper standards defined|
|Reporting Tools||Various tools available to analyze the performance||Unavailability of tools to analyze data and performance|
|Development model||Fine-grained database model||Architects can create new DB models|
|Price||Expensive compared to NoSQL||Low Cost – Mostly Open Source|
|Schema||Predefined Schema available||Unstructured data with dynamic schema|
|Database Examples||Postgres, SQLite, Oracle, etc.,||BigTable, Cassandra, MongoDB, etc.,|
|Type of Data storage||Not suitable for hierarchical data storage||Best suitable for hierarchical data.|
- What is Normalization?
Normalization database design technique to avoid duplication and redundancy.
- What are the advantages of Normalization?
Some of the advantages are:
- Better Database organization.
- Efficient data access.
- Greater Flexibility for Queries.
- Quickly find the information.
- Easier to implement Security.
- Allows easy modification.
- Reduction of redundant and duplicate data.
- Ensure Consistent data after modification.
- What is the difference between DROP and TRUNCATE commands?
- DROP command is used to remove the table and it cannot be rolled back from the database.
- TRUNCATE command helps in deleting all the rows from the table.
- Explain different types of Normalization?
Normalization helps in organising the data in databases in many successive levels of normalization.
The best three normal forms are :
- First Normal Form (1NF)
Second Normal Form (2NF)
- Third Normal Form (3NF)
- What is an ACID property in a database?
ACID stands for Atomicity, Consistency, Isolation, Durability.
- Atomicity: Atomicity is a process when one part of any transaction fails, the entire transaction fails and the database state is left unchanged.
- Consistency: Consistency ensures that the data never leaves the database without completing its state, it should complete all the validation process.
- Isolation: Isolation helps in concurrency control.
- Durability: Durability has no obstacle, it will occur in any condition such as power loss, crash, or any sort of error.
- What do you mean by “Trigger” in SQL?
- Triggers in SQL are a special type of stored procedure that allows you to execute a batch of code when an insert, update, or any other query is executed against a specific table automatically.
- What are the different operators available in SQL?
- Arithmetic Operators
- Logical Operators
- Comparison Operators
- Are NULL values the same as that of zero or a blank space?
- A NULL value represents a value that is unavailable, unknown, assigned, or not applicable whereas zero is a number and blank space is a character.
- What are Constraints in SQL?
Constraints are a set of rules concerning data in the table. Using ALTER and TABLE command constraints can be applied for single or multiple fields in SQL.
The constraints are:
- Not Null
- Primary Key
- Foreign Key
- What is a UNIQUE constraint?
- A UNIQUE constraint helps in determining whether all the values in a column are different. This helps in identifying each row uniquely.
- What are UNION, MINUS, and INTERSECT commands?
- The UNION is a binary set operator that combines the result set of two select queries
SELECT (coloumn_names) from table1 [WHERE condition] UNION SELECT (coloumn_names) from table2 [WHERE condition];
- The MINUS is a binary set operator in SQL that performs minus operation between two selections returns the rows that are present in the first selection but not in the second selection
SELECT (coloumn_names) from table1 [WHERE condition] MINUS SELECT (coloumn_names) from table2 [WHERE condition];
- The INTERSECT clause in SQL that helps in intersection operation between two selections
SELECT (coloumn_names) from table1[WHERE condition] INTERSECT SELECT (coloumn_names) from table2 [WHERE condition];
- What are Entities and Relationships?
- Entity: An entity can be either tangible or intangible and this is a real-world object
For example- In a college database, students, professors, departments are referred to as entities.
- Relationships: Relations are the entities that have something to do with each other.
For example – The employee table in a company’s database can be associated with the salary table in the same database.
- List the different types of relationships in SQL.
- One-to-One Relationship
Eg: Consider there is two entities ‘Person’ (Id, Name, Age, Address)and ‘Passport’(Passport_id, Passport_no). So, one passport- one person.
- One-to-Many or Many-to-One Relationship
Eg: Consider a ‘Customer’ and ‘Account’ then each ‘Customer’ can have more than one ‘Account’ but each ‘Account’ is held by only one ‘Customer’.
- Many-to-Many Relationship
Eg: Consider a ‘Customer’ and ‘Product’ then each customer can buy more than one product and a product can be bought by many different customers.
- What are transactions and their controls?
A transaction is a process that is performed logically to gain certain results. Operations like Creating, updating, deleting records performed in the database come from transactions.
There are 4 transaction controls such as
- COMMIT: Save all changes made through the transaction.
- ROLLBACK: All changes made by the transaction are reverted/rolled back but the database remains the same.
- SET TRANSACTION: Set the name of the transaction.
- SAVEPOINT: It is used to set the point where the transaction is to be rolled back.
- How many Aggregate functions are available in SQL?
SQL Aggregate functions help in calculating values from multiple columns in a table and return a single value.
There are 7 aggregate functions in SQL:
- AVG(): Returns the average value from specified columns.
- COUNT(): Returns the number of table rows.
- MAX(): Returns the largest value among the records.
- MIN(): Returns smallest value among the records.
- SUM(): Returns the sum of specified column values.
- FIRST(): Returns the first value.
- LAST(): Returns last value.
This brings us to the end of the SQL Interview questions.
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