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Top 35 SQL Interview Questions for Data Engineer

  • November 18, 2023
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Meet the Author : Mr. Sharat Chandra

Sharat Chandra is the head of analytics at 360DigiTMG as well as one of the founders and directors of Innodatatics Private Limited. With more than 17 years of work experience in the IT sector and Worked as a Data scientist for 14+ years across several industry domains, Sharat Chandra has a wide range of expertise in areas like retail, manufacturing, medical care, etc. With over ten years of expertise as the head trainer at 360DigiTMG, Sharat Chandra has been assisting his pupils in making the move to the IT industry simple. Along with the Oncology team, he made a contribution to the field of LSHC, especially to the field of cancer therapy, which was published in the British magazine of Cancer research magazine.

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Table of Content

  • What is SQL and why is it important in data engineering?

    Relational databases are managed and altered using the language SQL (Structured Query Language). For the purpose of maintaining, querying, and updating data kept in relational database systems, it is essential in data engineering.

  • Explain the difference between INNER JOIN, LEFT JOIN, and RIGHT JOIN in SQL.

    INNER JOIN returns records with matching values in both tables. LEFT JOIN returns all records from the left table and matched records from the right table. RIGHT JOIN does the opposite, including all records from the right table.

  • What are primary keys and foreign keys in SQL?

    A column (or group of columns) that is used to identify each row in a table in a unique way is called a primary key. A foreign key is a column that establishes a relationship among two columns by referencing the primary key in another table.

  • What is normalization and why is it important in databases?

    The practise of organising data to minimise duplication and enhance data integrity is known as normalisation. It entails creating linkages between the tables in a database by splitting it up into two or more. It's crucial for effective database performance and design.

  • Explain the use of GROUP BY and HAVING in SQL.

    GROUP BY groups rows that have the same values in specified columns into summary rows. HAVING is used to filter records that work on aggregated results, much like WHERE but for groups.

  • What is a SQL injection and how can it be prevented?

    Data-driven systems can be attacked using the code injection technique known as SQL injection. Prepared statements, input validation, and parameterized queries can all be used to stop it.

  • What are indexes in SQL and why are they used?

    Indexes are used to speed up the retrieval of rows from a database table by providing quick access to rows. They work similarly to an index in a book and are particularly useful on large tables.

  • Explain the concept of transactions in SQL.

    In SQL, a transaction is a series of actions carried out as a single logical work unit. To maintain the integrity in the database, it must either be finished completely or not at all.

  • What is a VIEW in SQL and how is it used?

    A VIEW is a virtual table based on the result-set of an SQL statement. It is used to simplify complex queries, encapsulate queries, and present a different view of the data.

  • What are the differences between DELETE and TRUNCATE commands?

    DELETE allows for reversal by removing rows one at a time & logging each row deletion. TRUNCATE is a quicker but irreversible way to delete all rows from a table without recording the deletion of individual rows.

  • Explain the concept of a subquery in SQL.

    A SQL query nestled inside another SQL query is called a subquery. As a need to further limit the data that may be obtained, it is used to return information that will be utilised in the primary query.

  • What is a stored procedure in SQL and its advantages?

    A collection of SQL statements which may be kept in a database is called a stored procedure. Improved security, less network traffic, and increased performance are among the benefits.

  • How do you handle errors in SQL stored procedures?

    Errors in SQL stored procedures are handled using TRY...CATCH blocks, which allow you to catch and respond to errors in a controlled way.

  • What is a data warehouse and how does SQL fit into it?

    A central store for integrated data of several sources is called a data warehouse. data warehousing employs SQL to manage and query the data kept in these warehouses.

  • What are window functions in SQL and give an example?

    Window functions compute values over a range of table rows that are connected to the current row. The ROW_NUMBER() method, which gives each row a distinct integer, serves as an illustration.

  • What is the difference between UNION and UNION ALL?

    UNION combines the results of two or more queries and removes duplicate rows. UNION ALL also combines results but includes all rows, including duplicates.

  • Explain ACID properties in database systems.

    The letters ACID stand for Atomicity, Consistency, Durability, and Isolation. These characteristics guarantee dependable operation in database systems regardless of the event of mistakes, blackouts, and other calamities.

  • What is a cursor in SQL and its use?

    A cursor is a database object used to retrieve data from a result set one row at a time. It's useful for processing each row returned by a SQL query individually.

  • Describe the role of a Data Engineer in managing SQL databases.

    A Data Engineer is responsible for designing, building, and maintaining the architecture of SQL databases. This includes setting up database systems, integrating data from various sources, ensuring data consistency, and optimizing queries for performance.

  • How do you ensure the security of a SQL database?

    Security can be ensured by implementing proper access controls, using SSL connections, regular patching of the database software, encrypting sensitive data, and monitoring for unusual access patterns.

  • What is the difference between a clustered and a non-clustered index?

    A clustered index sorts and stores the data rows of the table based on the index key. A non-clustered index creates a separate structure to hold the index and a pointer to the location of the data in the table.

  • Explain partitioning in databases and its benefits.

    Partitioning is the process of dividing a large database table into smaller pieces. It improves performance and manageability by enabling more efficient data access and maintenance.

  • What is data redundancy, and how does SQL help in managing it?

    Data redundancy refers to the unnecessary repetition of data. SQL helps manage it through normalization, which organizes data efficiently by eliminating redundant data and ensuring data dependencies make sense.

  • How do you optimize SQL queries for performance?

    SQL query optimization includes using indexes effectively, avoiding unnecessary columns in SELECT statements, using joins instead of subqueries where appropriate, and minimizing the use of functions in predicates.

  • Explain the concept of database sharding.

    Database sharding involves dividing a large database into smaller, more manageable pieces called shards. Each shard is independent and can be hosted on separate hardware, improving performance and scalability.

  • What is a self-join and when would you use it in SQL?

    A self-join is a regular join, but the table is joined with itself. It's useful for comparing rows within the same table or querying hierarchical data stored in a single table.

  • Explain the concept of a composite key in a database.

    A composite key was a primary key that consists of two or more columns that work together to provide each row in a table a unique identity. When no one column can uniquely identify every row, it is utilised.

  • How do you perform pagination in SQL queries?

    To achieve pagination in SQL, using the OFFSET and LIMIT clauses. The maximum number of rows that can be returned is indicated by LIMIT, and the number of rows to skip is indicated by OFFSET.

  • What is the difference between a correlated and a non-correlated subquery?

    A correlated subquery depends on data from the outer query and is evaluated repeatedly, once for each row. A non-correlated subquery is independent and can be run on its own.

  • Describe the use of the COALESCE function in SQL.

    COALESCE returns the first non-null value in a list of arguments. It's often used to provide a default value when querying columns that might contain null values.

  • How can you update a value in a table based on a value in another table?

    A JOIN with the UPDATE statement can be used to accomplish this. A common column is used to connect the columns, and the data from the joined table are used to apply the UPDATE to the targeted table.

  • What is a CTE (Common Table Expression) and when would you use it?

    You may use a CTE as a temporary result set in SELECT, INSERT, UPDATE, and DELETE statements. It is employed to make complicated queries more readable and organised.

  • Explain the difference between WHERE and HAVING clauses in SQL.

    WHERE filters rows before grouping, while HAVING filters after grouping. HAVING is used to apply conditions on groups created by GROUP BY or on aggregate functions.

  • What are the best practices for ensuring data integrity in SQL databases?

    Best practices include using primary and foreign keys, implementing constraints (like CHECK, UNIQUE), normalizing the database, and using transactions to maintain a consistent state.

  • How would you handle performance tuning in SQL databases?

    Performance tuning can be approached by optimizing queries (using EXPLAIN plans), indexing strategically, normalizing or de-normalizing data where appropriate, and monitoring and tuning the database configuration.

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