Crack the Code: The Most Common SQL Server Interview Questions

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Hello everyone and welcome to my newsletter where I discuss real-world skills needed for the top data jobs. 👏

This week I’m discussing the SQL Server technical interview questions you’ll see time and time again. 👀 

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Data is the new oil. Well, that’s what we’ve been told and it does sound great. The truth is, it’s always been a companies most precious commodity. It’s considered the new oil because we now have machine learning models that can take advantage of massive troves of data we are collecting globally. How much are we collecting? Here you go. The Y axis is in zettabytes. 😄 

SQL is the language of data. There are many vendors who use SQL. Most of them use SQL inside their relational database and data warehouse products. These are the two most common data stores in the real-world. Nothing else is close. SQL is an acronym that stands for structured query language. If you want to communicate with a database, you’ll be using SQL.

We’ve noted that vendors use SQL inside their data products. The most widely used database on earth is the relational database. These database are for optimized for real-time transaction processing (inserts, updates, deletes). Relational databases commonly used in day-to-day business operations (e.g., banking systems, CRM tools). The top databases in the real-world are SQL Server, Oracle and MySQL.

In this article I’ll be focusing on SQL Server. This is Microsoft’s relational database. It’s one of the most ubiquitous and it’s used inside of Fabric, the data ecosystem that 70% of Fortune 500 companies now use. The flavor other type of SQL used for SQL Server is called Transact-SQL.

Interviewers will often use interview question they find on the internet. You’ll see the same technical interview questions time and time again. The technical interview is where you get the job or lose the job. Companies are hiring you for a certain set of technical skills they need. The need to make sure you have those skills. These questions are in no specific order.

QUESTION ONE

What is the difference between the DELETE statement and the TRUNCATE statement?

Answer: There is more to this than meets the eye but most don’t know it. You don’t need to know it, but I’m going to explain it. First, this is how you answer the technical interviewer. Both of these statements are used to remove data from a table. The TRUNCATE statement is all or nothing. It deletes all the data or none of it. The DELETE statement allows the use of the WHERE clause. This restricts the data you can DELETE. The DELETE statement is much more flexible.

Deeper Insight: Very few people know the true technical minutia behind relational databases due to their complexity. Those who aren’t technically astute will say that TRUNCATE isn’t a logged operation. However, that’s false, it’s completely logged and creates about the same amount of logs that the full DELETE statement does. Why would this matter? Because non-logged operations are much faster. They don’t need to write to the log file before completion.

QUESTION TWO

What is a view?

Answer: A view in SQL Server is a virtual table based on the result of a SQL query. It doesn't store data itself but provides a way to simplify complex queries, abstract underlying table structures, and enhance security by controlling data access. You can use the SELECT statement to query a view. The CREATE VIEW statement is used to create a view.

QUESTION THREE

What is the WHERE clause?

Answer: The WHERE clause in SQL is used to filter rows from a query result based on specific conditions. It tells the database to return only those rows that meet the condition(s) you specify. The WHERE clause works with the conditions below.

• = (equal to)

• <> or != (not equal to)

• <, >, <=, >=

• BETWEEN, IN, LIKE, IS NULL

• Logical operators: AND, OR, NOT

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QUESTION FOUR

What is the benefit of a stored procedure?

Answer: A stored procedure in SQL Server is a precompiled collection of one or more SQL statements that you can save and reuse. Stored procedures are precompiled and cached by SQL Server, so they often execute faster than equivalent ad hoc queries.

Deeper Insight: When you execute a query in SQL Server, a plan of that query is created and that plan is stored in something called a cache. This happens for almost every query. When the same query is executed by another user, SQL Server can reuse that plan without having to create a new one. This speeds up data retrieval and significantly reduces resource consumption.

QUESTION FIVE

What is an INNER JOIN?

Answer: An INNER join returns the matching rows from both tables.

Deeper Insight: A JOIN in SQL Server is used to combine rows from two or more tables based on a related column between them—usually a foreign key and primary key relationship. This visual will help you understand the concept.

Here are the core joins.

• An inner join will return records that have matching values in both tables.

• A left outer join will return all records from the left table and the matched records from the right table.

• A right outer join will return all records from the right table and the matched records from the left table.

• A full outer join will return all records when there is a match from either table.

QUESTION SIX

What is a primary key?

Answer: A primary key in SQL Server (and other relational databases) is a column or set of columns that uniquely identifies each row in a table.

Deeper Insight: There are two common types of primary keys. There are surrogate and natural keys. Surrogate keys have no business meaning. Natural keys have business meaning. For example, if the client has to be born in the US, each US citizen is given a social security number. This number could be used as a natural key. Almost all keys used in the real-world are surrogate keys. 

QUESTION SEVEN

What is a clustered index?

Answer: A clustered index in SQL Server determines the physical order of data rows in a table. It’s like a phone book where the data is sorted by last name—SQL Server stores the actual rows in the same order as the clustered index. A table can have only one clustered index, because the data can be physically ordered only one way. By default, the primary key creates a clustered index.

Here are the most important points about a clustered index.

• A clustered index determines the physical order of the rows.

• A table can have only one clustered index.

• By default, the primary key creates a clustered index.

QUESTION EIGHT

What is DDL and DML?

Answer: There are two different forks in the SQL language. One is DML and it stands for data manipulation language. The other is DDL and it stands for data manipulation language. 

There are four DML statements.

• SELECT - This is for reading the data.

• INSERT - This puts data into a table.

• UPDATE - This updates the data in the table.

• DELETE - This removes the data from the table.

There are many kinds of DDL statements. Here are four of the most commonly used.

• CREATE - Creates new database objects like tables, views, or indexes

• ALTER - Modifies the structure of an existing object

• DROP - Deletes an existing object permanently

• TRUNCATE - Removes all rows from a table quickly, without logging individual row deletions 

QUESTION NINE

What are the ACID properties in SQL?

Answer: The ACID properties in SQL (and relational databases like SQL Server) are a set of guarantees that ensure data reliability, consistency, and integrity when performing database transactions.

ACID = Atomicity, Consistency, Isolation, Durability

Here’s what each mean:

• Atomicity - All or nothing. Either every operation in a transaction succeeds, or none do.

• Consistency - A transaction must leave the database in a valid state. If a transaction violates a constraint, it will fail and roll back.

• Isolation - Transactions don’t interfere with each other. Two people can’t update the same order at the same time and cause conflicts.

• Durability - Once a transaction is committed, it won’t be lost, even after a crash. SQL Server writes committed transactions to disk (transaction log), so they're safe from power failures or system crashes.

QUESTION TEN

What is a data type?

Answer: In SQL, a data type defines the kind of data that can be stored in a column, variable, or parameter. It tells SQL Server how much space to allocate, how to interpret the data, and what operations are valid.

Here are a few examples:

• Int - Stores numeric values.

• char - Fixed length strings.

• varchar - Variable length strings.

• Date - Dates

• Time - Time

If you want to work in a data role, you can’t get away from SQL. It is the language of data. You must know the basics and the basics are more than you think.

Thanks for reading and have a great day. 👏

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