SQL Interview Questions: Are you getting ready for your SQL developer job interview? You’ve come to the correct place. This tutorial will assist you in brushing up on your SQL abilities, regaining confidence, and being job-ready!

SQL Interview QuestionsHere is a collection of real-world interview questions from companies such as Google, Oracle, Amazon, and Microsoft. etc.

20 Top Possible SQL Interview Questions

Below are the top possible SQL Interview Questions:

1. What is a Database?

A database is a structured collection of data that is digitally saved and accessed from a distant or local computer system.

Databases may be large and complicated, and they are created following predefined design and modeling methodologies.

2. What exactly is DBMS?

DBMS is an abbreviation for Database Management System. A database management system (DBMS) is a piece of system software that is in charge of creating, retrieving, updating, and managing databases.

By acting as an interface between the database and its end-users or application software, it guarantees that our data is consistent, organized, and easily available.

3. What exactly is RDBMS? What distinguishes it from DBMS?

RDBMS is an abbreviation for Relational Database Management System. The major distinction between RDBMS and DBMS is that RDBMS stores data in the form of a group of tables, with relations created between the common fields of these tables.

RDBMS is the foundation of most current database management systems, including MySQL, Microsoft SQL Server, Oracle, IBM DB2, and Amazon Redshift.

4. What is SQL?

SQL is an abbreviation for Structured Query Language. It’s the industry standard for relational database management systems. It is especially beneficial when dealing with structured data made up of entities (variables) and relationships between them.

5. What’s the difference between SQL and MySQL?

SQL is a language that is used to retrieve and manipulate organized databases. MySQL, on the other hand, is a relational database management system used to manage SQL databases, similar to SQL Server, Oracle, or IBM DB2.

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6. What are Tables and Fields?

A table is a structured collection of data contained in rows and columns. Columns are classified as vertical, whereas rows are classified as horizontal.

A table’s columns are known as fields, and its rows are known as records.

7. What are SQL Constraints?

Constraints are used to establish the rules that apply to the data in the table. It may be applied to single or many fields in a SQL table during or after construction using the ALTER TABLE command. The limitations are as follows:

NOT NULL – Prevents the insertion of a NULL value into a column.

CHECK – Checks if all values in a field meet a criterion.

DEFAULT – Assigns a default value to a field if no value has been supplied.

UNIQUE – Ensures that only unique values are placed into the field.

INDEX – Indexes a field to allow for quicker record retrieval.

PRIMARY KEY – Identifies each record in a table uniquely.

FOREIGN KEY – Maintains the referential integrity of a record in another table.

8. What is a primary key?

The PRIMARY KEY constraint uniquely identified each row in a table. It contains an implied NOT NULL constraint and must include UNIQUE data.

In SQL, a table can only have one primary key, which can be made up of a single or numerous fields (columns).

9. What exactly is a UNIQUE constraint?

A UNIQUE constraint assures that each value in a column is unique. This gives uniqueness to the column(s) and aids in identifying each row.

Unlike the main key, it can establish several unique constraints per table. UNIQUE’s code syntax is very similar to that of PRIMARY KEY and may be used interchangeably.

10. What is a foreign key?

A FOREIGN KEY is a single or group of fields in one table that relates to the PRIMARY KEY in another table. The foreign key constraint maintains referential integrity between two tables.

11. What is a Join?

The SQL Join clause is used to merge entries (rows) from two or more tables in a SQL database that are connected by a column.

12. What is a Self-Join?

A self-JOIN is a type of regular join in which a table joins itself based on some relationship between its own column(s). The INNER JOIN or LEFT JOIN clause is used for self-join, and a table alias is utilized to assign distinct names to the table within the query.

13. What is a cross-join?

A cross-join is defined as the cartesian product of the two tables that are joined. The table following the join has the same amount of rows as the cross-product of the two tables’ number of rows.

When a WHERE clause is used in a cross-join, the query behaves as an INNER JOIN.

14. What is an index? Describe the many categories

A database index is a data structure that allows for rapid access to data in a table’s column or columns. It speeds up processes that access data from a database table at the expense of additional writes and memory required to maintain the index data structure.

15. What is the distinction between a clustered index and a non-clustered index?

As previously stated, the discrepancies may be split down into three minor aspects. –

Based on the indexed column, a clustered index changes the way it stores entries in a database. A non-clustered index produces a new object within the table that refers to the original table.

A clustered index is used to retrieve data from a database quickly and easily, whereas a non-clustered index is used to retrieve information more slowly.

A table in SQL can have a single clustered index or numerous non-clustered indexes.

16. What is data integrity?

Data integrity is the assurance of data correctness and consistency throughout its life cycle, and it is a fundamental part of the design, implementation, and use of any system that stores, processes, or retrieves data.

It also specifies integrity constraints, which are used to impose business rules on data as it is input into an application or database.

17. What is a query?

A query is a request for information or data from a database table or databases. A database query can be of two types: select queries and action queries.

18. What is a Subquery? What are the different types?

A subquery, also known as a nested query or inner query, is a query within another query. It is used to limit or enhance the data requested by the main query, hence limiting or increasing the outcome of the main query.

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19. What is the SELECT statement?

The SQL SELECT operator is used to get data from a database. The information returned is saved in a result table known as the result set.

20. What are some frequent SQL clauses used with the SELECT query?

The following are some frequent SQL clauses used in conjunction with a SELECT query:

  • The WHERE clause in SQL is used to filter records based on certain constraints.
  • The ORDER BY clause in SQL is used to sort data in ascending (ASC) or descending (DESC) order depending on the specified field(s).
  • The SQL GROUP BY clause is used to group records with identical data and may be used with various aggregation methods to obtain summarised database results.
  • In SQL, the clause uses the HAVING clause in conjunction with the GROUP to filter records. It differs from the WHERE clause in that the WHERE clause cannot filter aggregated records.

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