Preparing for the SQL interview questions to become more proficient in data science with example questions and a framework for answering them.
Amongst the different tools that data scientists must have in their arsenal, SQL holds a special significance. Its tabular format for data storage makes it easier to comprehend the info. In addition, there are several other features of SQL that make it one of the popular tools for data scientists. The blog includes a variety of tips and tactics for responding to SQL screening interviews confidently. It also covers a variety of SQL query-related questions along with an example coding question and the step-by-step process for writing SQL code during the interview.
Structured Query Language or SQL for Data Science is one of the most important skills that every Data Science aspirant must master. As you prepare for your interview for Data Science roles, you should also start grasping the concepts of SQL to come out with flying colors.
General SQL Interview Composition
SQL is essential for information retrieval, analysis, and interpretation. Anyone working in the field of Data Science uses SQL to interface with relational databases.
As an application of SQL in Data Science is indispensable, hence, every aspiring data scientist who is preparing for this interview should be well-versed in it. Most of the data scientist or data analyst interviews have a SQL technical evaluation section. This phase of the interview demands you to address real-world problems using SQL.
Usually, you may be given some typological issues, but the ultimate purpose is for the interviewer to check if you are adept in utilizing SQL rather than merely talking about it. So, not only knowing the terms but also solving problems in SQL is a must for cracking any interview. Most commonly, such exams take one of three forms:
- Whiteboard – SQL whiteboard exams are a regular component of interviews. A whiteboard test requires you to construct SQL queries by hand, allowing firms to check your comprehension of SQL principles and problem-solving abilities.
- Coding tests – During live interviews, many employers may ask you to develop code and execute queries. You may check for syntax problems while working with live code displays, and it allows firms to monitor your coding effectiveness.
- SQL case studies – In the case of interviews, you are given a real-world problem and asked to answer it using your SQL expertise. These are often open-ended inquiries that allow for analysis and problem-solving creativity.
Some Common SQL Questions
Q. What is a Primary Key
In SQL, a primary key is a distinct identifier for each record (row) in a table. It is used to guarantee that no two rows have identical values in the main key fields and to enforce the integrity of the data in the table.
A primary key in a table can be one or many columns. A composite primary key is created when numerous columns are utilized. Primary key column values must be unique across all rows in the table and cannot be NULL. Foreign keys in linked tables are derived from primary keys.
Q. What are the different subsets of SQL?
SQL (Structured Query Language) is divided into numerous subsets that serve distinct purposes:
- DDL (Data Definition Language): This SQL subset comprises instructions required to define the database structure, such as CREATE, ALTER, and DROP statements.
- Data Manipulation Language (DML): This subset of SQL comprises instructions for managing and manipulating data stored in a database, such as SELECT, INSERT, UPDATE, and DELETE statements.
- Data Govern Language (DCL): This SQL subset comprises commands used to control database and data access, such as the GRANT and REVOKE statements.
- Transaction Control Language (TCL): This SQL subset comprises transaction-management commands such as the COMMIT, ROLLBACK, and SAVEPOINT statements.
- Dynamic SQL is a subset of SQL that contains instructions for dynamically generating and executing SQL statements at runtime.
Q. What do you mean by DBMS? What are its different types?
DBMS is an abbreviation for Database Management System. It is a software system that allows users to save, retrieve, and change data by providing an interface for dealing with a database.
There are several types of DBMS, including
- Relational DBMS (RDBMS): A type of DBMS that organizes data into tables, with each table consisting of rows (records) and columns (fields). Relationships between tables can be established using keys.
- Object-Relational DBMS (ORDBMS): A type of DBMS that extends the relational model by including support for objects and object-oriented programming concepts.
- Hierarchical DBMS: A type of DBMS that organizes data into a tree-like structure, with each record having one parent and zero or more children.
- Network DBMS: A type of DBMS that organizes data into a network of records, where records can have multiple parent and child relationships.
- NoSQL DBMS: A type of DBMS that does not use the relational model and instead uses alternative data storage and retrieval methods, such as a key-value, document-based, graph, or column-based.
- Distributed DBMS: A type of DBMS that distributes data and processing across multiple physical and/or logical servers, allowing for scalability and improved performance.
Q. What are the most common aggregate functions in SQL? What do they do?
An aggregate function calculates a set of values and returns a single result that sums the set. The most common aggregate functions in SQL are COUNT, SUM, and AVG.
- COUNT – It is used for returning the number of items of a group.
- SUM – This function returns the sum of ALL or DISTINCT values in an expression.
- AVG – It returns the average of values in a group (and ignores NULL values).
Q. What is the difference between an Inner, Outer, Right, and Left join?
Here are the different types of JOINs in SQL:
- INNER JOIN: It returns records that have matching values in both tables
- LEFT JOIN: It returns all records from the left table and the matched records from the right table
- RIGHT JOIN: It returns all records from the right table and the matched records from the left table
- FULL JOIN: It returns all records when there is a match in either the left or right table
Q. What command would you use to update data in a table?
In SQL, the UPDATE command can be used to update an existing table. It is used in conjunction with SET (which contains the changed information) and WHERE to choose a specific instance.
Example: In the table ‘Customer’, one wants to change the emergency contact,
ContactName, for a customer with CustomerID 1.
SET ContactName = “Ashutosh Jindal”
WHERE CustomerID = 1;
Q. Which operator is used to select values within a range?
To choose values inside a range, use the BETWEEN operator. BETWEEN can be used with numbers, texts, or dates.
The BETWEEN operator includes both the start and end dates.
WHERE CustomerID BETWEEN 378 AND 492
Q. When would you use the GROUP BY statement?
The GROUP BY statement in SQL is used to group together rows with the same values in specified columns into summary rows, typically for calculating aggregate values such as the sum, average, count, etc., for each group.
It is commonly used in combination with aggregate functions such as SUM(), AVG(), COUNT(), MIN(), MAX(), etc., to perform data analysis. For example, to find the total sales for each product in a sales table, one might write a query like this:
SELECT product_id, SUM(sales)
GROUP BY product_id;
This query would return a table showing the total sales for each unique product in the sales table.
Q. What is a Self-Join?
A self-join in SQL is the table is joined to itself. In other words, a self-join allows you to compare rows within a single table. A self-join is useful in cases where you want to find relationships between rows within the same table,
Eg. finding all employees who are managers of other employees in an ‘EMPLOYEES’ table. To do this, you would join the ‘EMPLOYEES’ table with itself, using a ‘WHERE’ clause to specify the relationship between the two instances of the table.
Q. What is schema in SQL Server?
A schema in SQL Server is a container for database objects such as tables, views, stored procedures, and functions. Schemas provide a way to logically group these objects within a database, making it easier to manage and organize the database structure.
A schema can be thought of as a namespace for database objects, similar to how a folder is a namespace for files in a file system. Each database object within a schema has a unique name within the schema but can have the same name as an object in another schema.
In SQL Server, each database user can own one or more schemas. By default, each database user is assigned a default schema that has the same name as the user.
Example Coding Question (Real industry based)
Question: Analyse the best days for displaying ads on different platforms for Diwali Sales Analysis
To analyze the best days for displaying ads for Diwali sales on different platforms using SQL, you would need to have data about the sales performance on each platform for previous Diwali seasons, as well as data about the sales performance on each platform for the current Diwali season.
Assuming you have this data in a table named sales, you could perform the following analysis:
In this example, the ‘WITH’ clause creates a derived table ‘diwali_sales’ that aggregates the sales data for each platform and the date for the Diwali season. The main ‘SELECT’ statement then selects the platform, date, total sales, and a ranking based on the total sales for each platform. The ‘ROW_NUMBER()’ function is used to generate the ranking, with the ‘PARTITION BY’ clause specifying that the ranking should be separate for each platform and the ‘ORDER BY’ clause specifying that the ranking should be based on the total sales.
The result of this query will be a table with columns for the platform, date, total sales, and rank. You can then use this table to identify the best days for displaying ads on each platform by selecting the rows with the highest rank for each platform.
Wrapping it up !!!
This brief guide on SQL will help you prepare to excel in Data Science interviews. Even if you have just started with SQL, regular practice and persistence will help you excel in it. Moreover, try to answer as many SQL interview questions as possible. In Pickl.AI, we assist you in mastering all such integral concepts of Data Science. From case studies to projects and internships, Pickl.AI assures you that by the end of its Data Science Program, you will excel in all the conceptual and practical implications of Data Science.