Good knowledge of Structured Query Language, or SQL for short, is considered one of the most crucial skills for data analysts. So if you plan to get a job in this field, you would better be prepared for the questions you may be asked.
To help you with your SQL interview preparation, we created a list of tricky SQL interview questions and offered our answers. For your convenience, we divided these questions into several categories based on the skill level required to answer them.
We hope that our expert knowledge will help you get a dream job.
SQL interview questions for freshers
Rookies can expect to be asked the following basic SQL interview questions:
1. What is Database?
The term database describes an organized collection of logically related information stored, accessed, modified, and updated electronically.
2. What is a database management system?
As the name suggests, a database management system or DBMS is a software system designed to store, extract, and manage data in a database.
3. What is a Relational database management system? What is the difference between an RDBMS and DBMS?
A relational database management system is a software system where data are stored in tables and can be used in relation to other datasets. Several users can access RDBMSs. At the same time, in DBMSs, data are stored in files that one user can only manage.
4. What is SQL?
SQL or Structured Query Language is a domain-specific language developed to store, retrieve, and manipulate data kept in a relational database.
5. What is a SQL query?
A query can be seen as a request for data results or for a certain action on data (combine data from multiple tables, add, modify the data or remove them from the database).
7. What are the main types of SQL queries?
There are four types of SQL queries:
- Data Definition Language (DDL) – to create objects;
- Data Manipulation Language (DML) – to manipulate the data;
- Data Control Language (DCL) – to assign and remove permissions;
- Transaction Control Language (TCL) – to save and restore changes to a database.
7. What are tables and fields in SQL?
A table is an arrangement of data in a database. Traditionally it consists of rows (or records) and fields (or columns). Records are collections of values of a certain entity, whereas the term “field” denotes an area within a record meant for a particular piece of information.
8. How to create a database in SQL?
To create a database in SQL, use the following command:
CREATE DATABASE database_name.
9. How to create a table in SQL?
The following command is used to create a table:
CREATE TABLE table_name ( column1 datatype, column2 datatype, column3 datatype, .... );
Other basic SQL interview questions about tables may concern changing a table name, deleting a row, or removing the whole table.
10. What is Normalization in SQL?
Normalization is the multi-step process of organizing the data in the database to eliminate data redundancy and ensure data integrity.
SQL interview questions for experienced specialists
Below you can see a list of complex SQL interview questions for specialists with a more profound technical background.
1. What is SQL injection?
SQL injection is a code injection technique that implies the usage of malicious code to access sensitive data that should not be displayed.
2. What is a trigger in SQL?
A trigger is a certain type of procedure running automatically when some event takes place in the database servicer. For example, a DML trigger runs when a user tries to edit certain data through a Data Manipulation Language event.
3. Can we disable a trigger?
Yes, it’s possible to disable a trigger. For this purpose, use “DISABLE TRIGGER triggerName ON<>. If you need to disable all the triggers, use DISABLE TRIGGER ALL ON ALL SERVER.
4. How to use LIKE in SQL?
We use LIKE operator in WHERE clause if we need to look for a particular pattern in a column. For example:
SELECT * FROM eployees WHERE first_name like ‘Steven’
5. How is a non-clustered index different from a clustered index?
A clustered index determines the order in which the data are stored in a table. A non-clustered index, in turn, does not sort the data inside the table. Actually, non-clustered index and table data are stored in two separate places.
6. What is ISAM?
ISAM, also known as the Indexed Sequential Access Method, was invented by IBM for storing and extracting data from secondary storage systems.
7. What is Database Black Box Testing?
BlackBox testing implies testing interfaces and database integration. It consists of data mapping, the verification of the incoming data, and the verification of outgoing data from query functions.
8. What is the COMMIT in SQL?
The COMMIT statement is used when we need to finish the current transaction and make all changes in it permanent. A transaction, in turn, is the sequence of SQL statements seen as a single unit by the Oracle Database.
9. What is the difference between TRUNCATE and DROP statements?
TRUNCATE statement removes all rows from the table while the DROP command deletes a table from a database. In both cases, the operation cannot be rolled back.
10. What is a collation?
The term collation refers to a set of rules that specify how the database engine should sort and compare the character data.
Advanced SQL Interview Questions
Now we are getting to the advanced SQL technical interview questions that cover the topics of stored procedures, functions, data modeling, and hierarchical queries.
1. What is ALIAS in SQL?
We use the ALIAS command to give a column in a table or a table itself a temporary name with the purpose of making a column header easier to read.
2. What are the SQL constraints?
SQL constraints determine rules for the data in a table. To be more specific, they can limit the type of data for a table to ensure its reliability and accuracy.
3. What are Group Functions?
Group functions show results based on sets or groups of rows. For instance, users can get sums/totals, averages, minimums, and maximums by utilizing group functions.
4. How can we execute dynamic SQL?
Dynamic SQL can be executed in three different ways. First, we can write a query with parameters. Second, we can use the EXEC command. Finally, we can use sp_executesql.
5. What is the First Normal Form and what are their main rules?
The first normal form is a property with two primary rules for an organized database. The first one is to remove the identical columns for the same table. The second rule implies creating a separate table for each set of related data. The third rule says we should identify each table with a unique primary key column.
6. What are the main case manipulation functions in SQL?
The primary case manipulation functions are as follows:
LOWER/LCASE – converts the specific argument into lower case
UPPER/UCASE – converts the particular argument into lower case
INITCAP – converts the first letter of a word into uppercase while other letters are converted into lowercase
7. What ACID properties ensure that the database transactions are processed?
The ACID acronym defines the set of properties of database transactions that ensures the validity of the data regardless of power failures, errors, or other issues. These properties are atomicity, consistency, isolation, and durability.
8. What is SQL Grand Command used for?
We use Grand Command to offer users privileges to database objects. Also, we can grant permissions to other users with the help of this command.
9. What is the BCP and when do we use it?
The BCP or the bulk copy program is a command-line tool used for exporting or importing the data into a data file or vice versa. Additionally, this utility can generate format files and export certain data from a query.
10. What are the three primary closes of SQL statements?
Three main clauses that enable us to restrict and manage the data using valid constraints are the Where clause, Union Clause, and Order By clause.
SQL Server Interview Questions
Finally, we would recommend looking at the most complex SQL interview questions related to the server.
1. What is an SQL Server?
SQL Server is a relational database management system created by Microsoft to store and extract information as requested by other software apps.
2. How to install SQL Server?
The process of SQL Server installation looks the following way:
1. Get the latest version of the SQL Server official release there
2. Choose the type of SQL Server that needs to be installed. You can use it on a Cloud Platform or as an open-source edition.
3. The next step is to click on the download button
4. Save the .exe file on your computer and click on Open with the right mouse button.
5. Click Yes to allow necessary changes and install SQL Server.
6. As soon as the SQL Server is installed, restart the system if it is necessary and launch the SQL Server Management Studio app from the START menu.
3. How to uninstall SQL Server?
If you use Windows 10, go to the START menu and locate the SQL Server. Click the right mouse button and choose to uninstall to start the uninstallation process.
4. How can you find the server name in SQL Server?
If you are looking for the server name, you need to run the query SELECT @@version and you will be shown the name and the latest version of the SQL Server.
5. How to restore the database in SQL Server?
First, launch the SQL Server Management Studio app. From the window pane called Object Explorer, click the right mouse button on databases and choose Restore. Your database will be automatically restored.
6. What is SQL Profiler?
SQL profiler is an instrument used by system administrators for monitoring the events in the SQL Server. With the help of this tool, administrators capture and save data about each event of a file or table for further analysis.
7. What is SQL Server Agent?
SQL Server Agent is a significant part of Microsoft SQL Server used for executing scheduled administrative tasks commonly known as jobs.
8. What is the ISNULL() operator?
ISNULL() operator is a function that returns a specified value if the expression is NULL. In case the expression is NOT NULL, it is returned by the function.
9. What is replication in SQL Server?
When it comes to SQL Server, replication is a process that implies copying and distributing the data from one database to another one and synchronizing the data between the two databases to ensure data integrity and consistency.
10. What is a function in SQL Server?
A function is a piece of pre-written code executed on a SQL Server that helps you complete a certain task regarding the viewing, managing, and processing of the data.
Experience of Syndicode team in dealing with databases
Syndicode technical experts created these lists of tricky SQL queries for an interview based on their extensive knowledge of SQL. Thanks to their skills, we can build complex web applications that store a great amount of data.
SQL is the most widely used database language. So you should definitely master it if you want to pursue your career in data science. We hope that SQL tricky questions discussed above will help you to expand your knowledge of SQL and get a prestigious and well-paid job.
Advanced SQL covers more complex SQL tops such as functions, stored procedures, triggers, and packages. Additionally, advanced SQL skills require a profound knowledge of query optimization, Dynamic SQL, hierarchical queries, and experience in data modeling.
The most crucial SQL commands are as follows: UPDATE – update the data in a database, CREATE DATABASE – create a new database, DELETE – remove certain data from a database, INSERT INTO – enter new data into a database, and ALTER DATABASE – make changes to a database.
Professionals use PostgreSQL as the main data store or data warehouse for multiple webs, mobile, and analytical software solutions.