Job Interview Questions on SQL
I am going to present the most common and practical Job Interview Questions on SQL with answers. These questions are pretty adequate to reiterate the already understand the basic concept of SQL. The article will not only help beginners but experienced candidates as well because the fundamentals and complex concepts of SQL are covered in this article. One can always refer to these questions for a quick revision of major SQL concepts before appearing for an interview.
Top 40 Job Interview Questions on SQL
Apex job interview questions on SQL with answers
Are you ready? Let’s start.
1. What is SQL?
Answer: SQL is the standard language for dealing with relational databases.
2. What does SQL stands for?
Answer: Structured Query Language.
3. What is data?
Answer: Data is every single item stored in the database.
4. What is database?
Answer: Database is a systematic collection of data that supports the management of data.
5. What is DBMS?
Answer: Database Management System is a collection of programs which enables its users to access database, control data, create reports and representation of data.
6. How many types DBMS have?
Answer: There are four major types of DBMS:
- Hierarchical DBMS: It’s an obsolete type of DBMS which is rarely used nowadays. The window registry used in Windows XP is the best example of this type of DBMS.
- Network DBMS: This type of DBMS supports many to many relations. Its a model where multiple member records or files can be linked to multiple owner files and vice versa.
- Relational DBMS: These are tabulated relational databases. It is the most popular DBMS in the market. MySQL, Oracle and Microsoft SQL Server database are some examples of it.
- Object-Oriented Relation DBMS: This type of DBMS support storage of new data types in the form of objects. The objects have attributes and methods to decide what to do with data. PostgreSQL is an example of this type of DBMS.
7. What is NoSQL?
Answer: NoSQL means not only SQL. It is an upcoming category of DBMS. It is non-adherence to relational database concept. NoSQL is designed to overcome the problem of slow system response time over massive volumes of data.
8. What can SQL do?
- Can execute queries against a database.
- Can retrieve data from a database.
- Can insert, update and delete records from the database.
- Can create new databases.
- Can create new tables in a database.
- Can create stored procedures in a database.
- Can create views in a database.
- Can set permissions on tables, procedures, and views.
9. Does SQL support programming language features?
Answer: Although SQL is a language it does not support programming as it is not a programming language.
10. What do you mean by data definition language?
Answer: Data definition language or DDL allows to execute queries like create, drop, and alter (the queries which define the data).
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11. What is difference between primary key and unique constraints?
Answer: Primary key cannot have a NULL value, however; unique constraints can have NULL values. There is only one primary key in a table but there can be multiple unique constraints. The primary key creates cluster index automatically but the unique key does not.
12. What do you mean by foreign key?
Answer: A foreign key is a field which can uniquely identify each row in another table. This constraint is used to specify a field as a foreign key. This field points to the primary key of another table. It’s a kind of link between two tables.
13. What is a join in SQL?
Answer: An SQL join is a statement used to combine data or rows from two or more tables based on a common field between them.
14. Describe types of joins?
Answer: There four types of join. Inner Join, Left Join, Right Join, and Full Join.
15. What is the most common type of join?
Answer: I think its inner join.
16. Inner join is a MySQL or simple SQL?
Answer: it is MySQL join.
17. How would you define entities?
Answer: An entity is an object that exists but does not have to do anything. It just has to exist. It can be a single thing, place, person, or an object.
18. What is the relationship?
Answer: It is a situation that exists between two relational database tables when one table has a foreign key, referencing the primary key of another table.
19. What are the indexes?
Answer: Indexes are used to quickly locate data without having to search every row in a database table every time database table is accessed.
20. How many methods of SQL indexes are there?
Answer: There are primarily three methods of indexing: Clustered Indexing, Non Clustered indexing, and Multilevel Indexing.
21. What is the drop command?
Answer: It is used to remove an object from the database. Once an object is dropped, it cannot be undone.
22. What is the truncate command?
Answer: This command removes all the rows from the table. It is a faster command than delete.
23. What is the trigger in SQL?
Answer: A trigger is a stored procedure in a database which automatically invokes whenever a special event in the database occurs.
24. What is a query in SQL?
Answer: It is a question or set of questions to retrieve data from database.
25. What is Sub query in SQL?
Answer: It is a query within another (SQL) query and embedded within the ‘Where’ clause. It is used to return data that will be used in main query as a condition to further restrict the data to be retrieved. It is also called the Inner Query or Nested Query.
26. What are the types of sub query?
Answer: There are five types of subquery:
- Single-row subquery.
- Multiple row subquery.
- Multiple column subquery.
- Correlated subquery.
- Nested subquery.
27. What are SQL clauses?
Answer: SQL clause helps to limit the result set by providing conditions to quarry.
28. How many types of SQL clauses are there?
Answer: There are five types of clauses in SQL:
- Order By clause.
- Top clause.
- Where clause.
- Group By clause.
- Having clause.
29. What is the ‘Having’ clause?
Answer: It is used to select the statement.
30. What you think about ‘Where’ clause?
Answer: Where clause is used to restrict number of records returned by select statement.
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31. How can you fetch common record from tables?
Answer: Intersect key word is used to fetch the common records from tables.
32. What are the set operations available in SQL?
Answer: There are four set operations (functions) available in SQL:
- Union All.
33. How can you select a unique record from the table?
Answer: It can be done by using ‘distinct’ keyword.
34. What is ‘view’ in SQL?
Answer: It is a composition of a table in the form of a predefined SQL query.
35. Why is ‘view’ used for in SQL?
Answer: Views are used for security purpose in SQL by restricting the users from viewing certain rows and columns.
36. What is data warehouse?
Answer: Data warehouse acts as a central repository for various data. It is one of the primary components in SQL.
37. What is the auto-increment in SQL?
Answer: Autoincrement allows a unique number to be generated automatically when a new record is inserted into a table.
38. What are transactions?
Answer: A transaction can be defined as the sequence task that is performed on the database in a logical manner to gain certain results.
39. How many transaction controls are there?
Answer: There are four transaction controls:
- Set Transaction.
40. What are the properties of the transaction?
Answer: There are four properties of the transaction. These are known as an ACID.
SQL is a vital part of the database system. Having a comprehensive knowledge of database and thorough information about SQL concepts will truly be advantageous to rank crown amongst the candidates and embrace the career opportunity.
It is to be understood that besides the foreseen major perceptions, there are some concealed facts that affect your performance in the interview. Hope, this perspective will help you in preparing for the interview as I have collected most common and frequently asked job interview questions on SQL and presented to you in simple and understandable terms.