SQL Server Interview Questions

During last Calendar Week, I've Conducted Various Polls on SQL Server Interview Questions on this LinkedIn Group "Big Data, Analytics, Business Intelligence & Visualization Experts Community"

Answers for those polls intrigued me to end up writing this week's newsletter "SQL Server Interview Questions" for you all.

When we are talking about SQL, we'd also revisit the rich history of the Microsoft SQL Server and T-SQL Stored Procedures , The Heart of the SQL as well.

Along with that at the Bottom Line, I've also compiled the Free SQL Cookbook and Free SQL Course for you all throughout this Year!

Key Takeaways:

1. The Rich History of Microsoft SQL
2. T-SQL: The Heart of the SQL
3. SQL Server Interview Questions and Answers for Beginners
4. SQL Server Interview Questions and Answers for 5 Years of Experience
5. Advanced SQL Server Interview Questions and Answers
6. The Bottom Line: Free SQL Cookbook & Free SQL Course

1) The Rich History of Microsoft SQL

Microsoft SQL Server has a storied history, dating back to its initial release in 1989.

Over the years, it has evolved into one of the most widely used database engines in the data industry.

In fact, in the last decade, it has consistently ranked as the third most popular database engine.

Database administrators play a crucial role in ensuring data accessibility and security. They manage access permissions, perform backups, monitor performance, and handle updates and recoveries as needed.

2) T-SQL: The Heart of the SQL

Regardless of the specific role, a common thread among data professionals is the need for proficiency in T-SQL, which is SQL Server’s dialect of SQL.

Being fluent in T-SQL is vital for success, whether it’s for day-to-day tasks or to ace an?SQL Server interview.

Here are some key aspects of T-SQL to focus on:

Interactive Learning:?

Gain hands-on experience through interactive?SQL courses. Platforms like ours offer comprehensive courses covering everything from T-SQL basics to advanced concepts like GROUP BY extensions, recursive queries, and Common Table Expressions.

Exploring Beyond T-SQL:?

While T-SQL is crucial, don’t limit your knowledge to SQL Server alone. Familiarize yourself with other programming languages (e.g., Python), statistical analysis, ETL processes, database design, and even machine learning if relevant to your role.

Before We dive into the SQL Server Interview Questions and Answers, Let's Check your SQL Level here…

3) SQL Server Interview Questions and Answers for Beginners

1. What Is SQL Server?

SQL Server is Microsoft’s relational database management system (RDBMS) that facilitates data storage, integrity, security, and access. It enables tasks like creating, storing, updating, deleting, and retrieving data.

2. How can you Differentiate the Editions of SQL Server?

SQL Server comes in five editions, catering to different user needs Express, Developer, Web, Standard, and Enterprise. Each edition offers varying levels of functionality and is designed for specific use cases.

3. What Are the Main Components of SQL Server?

SQL Server consists of several components.

• The database engine takes care of storing, securing, and changing data.
• The SQL Server program starts, stops, and pauses the tasks of other parts. Its main file is called sqlservr.exe
• SQL Server Agent is like a task scheduler that gets going when something happens. Its main file is called sqlagent.exe
• SQL Server Browser handles requests coming in for SQL Server and gives information about the SQL Server versions on the computer. Its main file is called ssqlbrowser.exe
• SQL Server Full-Text Search lets you search through text in the database, even if it’s written in letters and words. Its main file is called fdlauncher.exe
• SQL Server VSS Writer helps make copies of the database so you can restore it later, even if the SQL server isn’t running. Its main file is called sqlwriter.exe
• SQL Server Analysis Services (SSAS) is a tool for exploring and understanding data, and for making reports and visuals with tools like PowerBI and Excel. You can also use Python and R for more advanced analyses. Its main file is called smsmdsrv.exe
• SQL Server Reporting Services (SSRS) is a tool for creating, sending out, and managing visual, mobile, and paper reports. Its main file is called ReportingServicesService.exe
• SQL Server Integration Services (SSIS) is a tool for moving, changing, and combining data for storing and using later. Its main file is called MsDtsSrvr.exe

4. What Is T-SQL? What Is the Difference Between SQL and T-SQL?

T-SQL is the dialect of SQL specific to SQL Server. Unlike standard SQL, T-SQL is procedural, allowing for the execution of blocks of code in a structured order. It also includes statements and functions unique to SQL Server.

5. What Are the Different Types of Joins in SQL Server?

SQL Server supports five types of joins:

• (INNER) JOIN
• LEFT (OUTER) JOIN
• RIGHT (OUTER) JOIN
• FULL (OUTER) JOIN
• CROSS JOIN

Each serves specific purposes in combining data from different tables.

6. What Is the Purpose of the GROUP BY and HAVING Clauses in T-SQL?

The GROUP BY organizes clause organizes rows with identical values in specified columns into groups, often used with aggregate functions. The HAVING clause filters these groups based on specified conditions.

7. What Is a Subquery in T-SQL?

A subquery is a query embedded within another query (the outer query). In T-SQL, subqueries can be used in various statements like SELECT, INSERT, UPDATE, and DELETE often placed in the FROM, WHERE, or HAVING clauses.

8. What Are the Different Types of Subqueries?

Subqueries come in various types, including scalar subqueries (returning one value), It is used in the FROM, WHERE, and HAVING clauses.

Multiple-row subqueries (returning multiple rows), are used in the same clauses as scalar subqueries but with the IN, NOT IN, ANY, ALL, EXISTS, or NOT EXISTS operators.

Correlated subqueries (referencing outer query values), and non-correlated subqueries (independent of the outer query).

9. What Is the Difference Between UNION and UNION ALL?

Both UNION and UNION ALL merge the results of two or more queries into one result set. The difference is that UNION doesn’t include duplicates, while UNION ALL does.

10. List the Most Common Functions in SQL Server.

Here are the most commonly used function categories and functions in SQL Server:

String functions:

• CONCAT() – Merges two or more strings.
• LEFT(), RIGHT() & SUBSTRING() – Returns the specified number of characters from the left or right end of a string or a stated position in the string.
• LEN() – Returns the number of characters in the string.
• LTRIM(), RTRIM() & TRIM() – Removes spaces or other characters from the left, right, or a specified position in the string.
• REPLACE() – Replaces all the occurrences of a specified substring with another string.
• LOWER() and UPPER() – Turns all the characters lowercase or uppercase, respectively.

Numeric functions:

• ABS() – Returns the absolute value.
• ROUND() – Rounds a number to a specified decimal place.

Date and time functions:

• DATEDIFF() – Returns the difference between two specified dates.
• CURRENT_TIMESTAMP() – Returns the current date and time.
• DATEADD() – Adds a date or time interval to a date and returns a new date.
• DAY(), MONTH(), YEAR() – Returns the day, month, or year of a date.

Aggregate functions:

• COUNT() – Counts the number of rows returned by a query.
• SUM() – Returns the sum of values.
• AVG() – Returns the average value.
• MIN() – Returns the minimum value.
• MAX() – Returns the maximum value.

Other functions:

• CAST() & CONVERT() – Convert a value from one data type to a specified data type.
• COALESCE() – Returns the first non-NULL
• ISNULL() – Returns the specified value if the expression is NULL or the expression is otherwise.
• NULLIF() – Returns NULL if two expressions are the same.

11. What Is the Purpose of the COALESCE() and NULLIF() Functions?

The COALESCE() function gives back the initial non-NULL value, while the NULLIF() function yields NULL when two expressions are identical.

12. What Are Window Functions in SQL Server?

Window functions in SQL Server perform calculations within the rows linked to the current one, resembling aggregate functions but without consolidating results into a single row.

They facilitate the simultaneous display of both aggregated and individual row values, commonly used for tasks like ranking and analytics.

13. Find the Top N Rows in SQL Server.

If we want to return the top 5 rows from the album table, we could do it like this:

SELECT TOP 5 *
FROM albums;        

14. What Do ROLLUP, GROUPING SETS, and CUBE Do in T-SQL?

These functions are expansions of GROUP BY, ROLLUP lets you make multiple grouping sets, adding subtotals and grand totals. GROUPING SETS, allows you to specify multiple grouping sets, similar to combining multiple GROUP BY clauses in a single query. CUBE enables you to create groups for all possible combinations of columns and include subtotals.

15. What Is a CTE in SQL Server?

A Common Table Expression (CTE) is a named temporary outcome of a SELECT statement. It’s akin to subqueries, but unlike them, CTEs can be employed multiple times in the same query.

They share similarities with temporary tables, but the CTE result isn’t stored in the database memory; it needs to be executed each time its result is utilized in a query. CTE results can be referred to in SELECT, INSERT, UPDATE, DELETE, and MERGE statements.

16. Explain What a Recursive CTE Is and How to Write One in SQL Server.

A recursive Common Table Expression (CTE) refers to itself and, like a regular CTE in SQL Server, is written using the WITH clause.

The initial section of a recursive CTE is an anchor member that generates the CTE’s base result.

This result combines with the outcome of another query, termed a recursive member because it points back to the anchor member. This combination is achieved using UNION ALL. Using UNION, unlike in some other databases, wouldn’t suffice.

The final part of a recursive CTE is the termination condition, specifying the conditions that halt recursion to prevent it from continuing indefinitely.

4) SQL Server Interview Questions and Answers For 5 Years of Experience

Now, let’s explore SQL Server Interview Questions and Answers For 5 Years of Experience.

17. How Do You Create a Table in SQL Server?

In SQL Server, tables are formed using the CREATE TABLE command. In this command, you define the table name, column names, and the data type accepted by each column.

The syntax is as follows:

CREATE TABLE table_name (
   column_1 datatype,
   column_2 datatype,
   column_3 datatype,
   …
);        

18. What Is a Primary Key? What Is a Foreign Key?

The primary key, whether a single column or a combination of columns, guarantees the unique identification of rows in a table.

A foreign key, on the other hand, is a column or a combination of columns in one table that refers to the primary key in another table.

19. What Is the Difference Between DELETE and TRUNCATE Commands?

Although they might appear similar, DELETE and TRUNCATE belong to different categories of SQL commands. DELETE is a Data Manipulation Language (DML) command employed to delete all or specific rows (using the WHERE clause) from a table.

On the other hand, TRUNCATE is a Data Definition Language (DDL) command. It removes all rows from the table, and you can’t use it to delete only certain rows. Another distinction is that TRUNCATE doesn’t log changes for each row, making it faster than DELETE. DELETE removes rows one by one, with each deletion recorded in the log.

20. What Is the Difference Between a Table and a View in SQL Server?

A table serves as the main data storage in relational databases, comprising rows and columns, and is stored in memory.

In contrast, a view is a stored query that produces a virtual table with rows and columns of data. A view doesn’t hold any data itself; it generates output only when executed. This enhances usability, and data security, and conserves memory since the view result is not stored in memory.

21. Can You Explain the Different Types of Indexes in SQL Server?

These indexes are supported in SQL Server:

• Hash
• Memory-Optimized Nonclustered
• Clustered
• Nonclustered
• Unique
• Columnstore
• Index with Included Columns
• Index on Computed Columns
• Filtered
• Spatial
• XML
• Full-text

Clustered indexes are integral to the table structure, determining the physical order of storage and sorting based on the index key value. Each table can have only one clustered index. When searching for data, the query scans the table row by row until it locates the specified index.

Non-clustered indexes contain key values, each with a pointer to the data row holding that key value. This allows queries to directly access the data row with the specified index without traversing all the data. Non-clustered indexes are organized and stored separately from the indexed tables.

22. What Is the Difference Between a Function and a Stored Procedure?

Functions in SQL Server are routines that compute values and cannot alter database objects. They require at least one parameter and yield a result.

There are two types of functions: built-in and user-defined. Built-in functions are inherent to SQL Server and include aggregate, analytic, bit manipulation, ranking, row set, and scalar functions. They operate on groups of rows, analyze sets related to the current row, manipulate binary data, rank rows, work with sets of rows, or return single values.

User-defined functions, created by users, share characteristics with built-in functions and can be stored and called as needed.

Stored procedures, unlike functions, are pre-compiled SQL statements that perform a set of actions repeatedly. They don’t need parameters, aren’t obliged to return results, and can modify database objects. Stored procedures permit INSERT, UPDATE, and DELETE commands, while functions work with SELECT, WHERE, and HAVING clauses.

Stored procedures can utilize transactions and call functions, but functions can’t use transactions or call stored procedures.

23. What Is the Difference Between a Temporary Table and a Table Variable?

Temporary tables provide temporary storage for query results and are only accessible within the session they are created.

Table variables, on the other hand, are local variables that also store data temporarily and are utilized in functions, stored procedures, and batches.

While temporary tables are actual tables, making them more suitable for handling larger data volumes, table variables have limitations. Temporary tables support operations like SELECT INTO, ALTER TABLE, and TRUNCATE, and allow for index creation. They can be employed in transactions and stored procedures but are not applicable in functions.

24. What Is a Trigger? When Would You Use One?

A trigger is a specific type of stored procedure that automatically executes (or is “triggered”) when a defined event occurs.

DML triggers activate when a user attempts to modify table data through INSERT, UPDATE, or DELETE operations. DDL triggers run in response to CREATE, ALTER, or DROP statements. Additionally, logon triggers are activated when a user establishes a connection with SQL Server.

For example, you might use a DML trigger on a table containing employee data, including salary information. This trigger could store changed data in a separate table every time someone modifies employee information. This approach ensures a historical record of each employee’s information changes.

25. What Is the Difference Between Commit and Rollback?

COMMIT TRANSACTION permanently preserves all the data modifications made during a transaction.

ROLLBACK TRANSACTION allows you to revert or undo all changes that occurred since the last commit of transaction modifications.

5) Advanced SQL Server Interview Questions

Let’s delve into more advanced SQL Server topics and explore various complex interview questions.

26. Can You Explain the Different Types of Replication in SQL Server?

Transactional replication involves near real-time replication from the main database (publisher) to another database (subscriber).

In merge replication, changes made offline synchronize when the two databases connect. It’s akin to transactional replication, but changes occur in both the publisher and subscriber.

Snapshot replication captures the main database’s state at a specific moment and replicates it to another database.

Peer-to-peer replication, a variation of transaction replication, supports replication across multiple servers.

Bidirectional transactional replication replicates data between two servers, where both servers publish and subscribe to each other’s data.

Updatable subscription replication involves passing subscriber database changes first to the main database copy before replicating to other subscribers.

27. Explain the Different Types of Locks in SQL Server.

In database management, locks play a crucial role in maintaining transaction concurrency when multiple users access the same database simultaneously. SQL Server, when processing a query, determines which data resources will be utilized and applies a specific lock type to safeguard these resources.

Here are the lock modes in SQL Server:

• Shared (S) Lock Mode: Used for read-only statements like SELECT, making data available only for reading. It prevents changes to the data during query execution.
• Update (U) Lock Mode: Applied to data slated for updates. Unlike S lock mode, only one transaction can hold a U lock to avoid deadlocks. It transitions to X-lock mode when the transaction is ready to update data.
• Exclusive (X) Lock Mode: Ensures that only one transaction can update the same data at a time. Used with commands like INSERT, UPDATE, and DELETE.
• Intent (I) Lock Mode: Establishes a lock hierarchy and signals the intention to impose an S or X lock, preventing other transactions from imposing them. Includes intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX) locks.
• Schema (Sch) Lock Mode: Used for schema-dependent operations. Schema modification (Sch-M) lock for DDL statements (CREATE, ALTER, DROP) prevents data access during structure modification, while schema stability (Sch-S) lock allows other locks except Sch-M during schema-dependent transactions.
• Bulk Update (BU) Lock Mode: Imposed when data is copied in bulk, often with the TABLOCK hint.
• Key-Range Lock Mode: Used in serializable transaction isolation levels to prevent phantom reads, where the same query returns different results each time it runs.

28. What Is the Difference Between a Deadlock and a Livelock?

A deadlock occurs when two or more processes prevent each other from obtaining a lock. Essentially, each process holds a lock on one piece of data and attempts to acquire a lock on another piece of data already held by a different process. In such cases, SQL Server terminates one of the processes to resolve the deadlock; otherwise, it would persist indefinitely.

Conversely, a live lock happens when shared locks overlap, hindering another process from acquiring an exclusive lock.

29. What Is the Difference Between a Materialized and a Non-Materialized View?

Materialized views are logical representations of data with their output stored in a table, allowing a query to run just once. This can enhance performance, especially since the result can be indexed. However, the drawback is that the information might not be up-to-date if the materialized view hasn’t been refreshed.

Non-materialized views are also logical representations of data, but their output is not stored. The downside is that you need to execute the view each time you want to utilize its result. The advantage, though, is that you’re assured of obtaining the most recent data.

30. What Is the Difference Between Implicit and Explicit Transactions?

Explicit transactions are commenced using BEGIN TRANSACTION, and the modifications to data are confirmed with COMMIT TRANSACTION;.

In the case of implicit transactions, SQL Server automatically initiates them without the need for the BEGIN TRANSACTION command. However, the user is still required to explicitly commit or roll back these transactions.

When the implicit mode is activated, the keywords that trigger implicit transactions are:

• BEGIN TRANSACTION
• CREATE
• UPDATE
• DELETE
• DROP
• FETCH
• INSERT
• OPEN
• GRANT
• REVOKE
• SELECT
• ALTER
• TRUNCATE

6) The Bottom Line: Free Programming Cookbook & Free Course

In the Bottom Line, I hope this collective 30 SQL Interview Questions and Answers for the Beginners, Intermediate, and advanced Level Professionals will help you to practice and ace that job interview!

The T-SQL category encompasses the most questions, underscoring its significance. As mentioned earlier, the ability to write T-SQL queries is a fundamental requirement for various data-related roles dealing with relational databases.

During interview preparation, it’s crucial not to dismiss other questions as unimportant. While managing your time, prioritize mastering T-SQL. The “SQL from A to Z in MS SQL Server” offers an excellent platform for achieving this goal.

Grab this Free Programming Cookbook for SQL Language.

Throughout this Fy2024, This “Customer Behavior Analysis in PostgreSQL” Course is FREE! at LearnSQL.

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