Are you on a job search in the tech industry? Then you are at the right place, we are here to help you guide with the most commonly used SQL questions. These questions can be applied in all mainstream RDBMS, including MySQL, Oracle, Microsoft SQL Server, PostgreSQL, and any others that you may use.
Though learning SQL (Structured Query Language) can seem to be challenging, with the right guidance and preparation you will increase your possibility of getting selected. You will explore the Top 25 Essential Questions of SQL, that are expected to be asked in the interview.
Master SQL with Top 25 interview questions
Let’s begin with the important SQL questions that will help you achieve your dream job!
What are SQL and its uses?
SQL (Structured Query language) is one of the standard languages which is based on the English language. These are mainly used for accessing and managing database and is the core of the relational database. Data can be updated or retrieved from the database by using SQL.
What is a relational database (RDBMS)?
RDBMS stands for Relational Database that keeps and provides access to data points that are related to one another. Each row in the data represents a unique record or instance of a given entity. The data is organized in rows and columns.
What are the different types of SQL commands?
Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), and Transaction Control Language (TCL) are the many categories of SQL commands.
What is the difference between a primary key and a foreign key?
| Primary Key | Foreign Key |
| A primary key is a column or a set of columns in a table that uniquely identifies each row in that table. | A foreign key is a column or a set of columns in one table that refers to the primary key in another table. |
| Each row in the table will have a unique primary key. | The same foreign key value can be present in multiple rows. |
| Primary keys cannot have null values. | Foreign keys can have null values. |
| The main purpose of a primary key is to implement a relationship within a table, ensuring each record within the table is unique. | The main purpose of a foreign key is to implement a relationship between two tables. |
| A table can have only one primary key. | A table can have multiple foreign keys. |
| Primary keys ensure referential integrity within their own table. | Foreign keys ensure referential integrity between two tables. |
What is a JOIN, and what are the different types of JOIN?
A procedure called a JOIN merges two or more tables based on a shared column. The various forms of joins are FULL OUTER JOIN, LEFT JOIN, RIGHT JOIN, and INNER JOIN.
What is normalization, and what are the different levels of normalization?
Data in a database are organized by a process called normalization that lessens dependencies and redundant data.
- First Normal Form (1NF),
- Second Normal Form (2NF),
- Third Normal Form (3NF),
- Boyce-Codd Normal Form
- Fourth Normal Form
What is an index, and how does it improve performance?
A data structure called an index accelerates data retrieval operations on a database table. It operates by building a new data structure with the values of a specified column or collection of columns and a pointer to the position of the relevant row in the table.
What is a stored procedure, and what are its benefits?
A precompiled set of SQL statements that can be frequently executed in a database are known as stored procedures.
The benefits of stored procedure are given as follows
- Improved performance,
- code reuse,
- security
What is a trigger, and how does it work?
An exclusive class of stored procedure known as a trigger is carried out automatically in response to a certain event or action in the database. Triggers can be used to uphold data integrity, enforce business rules, and enhance performance.
What is a view, and what are its benefits?
An SQL statement’s result set serves as the foundation for a view, which is a virtual table. Views are used to add an additional layer of protection, simplify complex searches, and allow or limit access to specific columns or rows. The advantages of employing views include increased performance, reused code, and simplicity.
What is a subquery, and how does it work?
A query that is nested inside another query is known as a subquery. In order to use the data as a condition or criterion in the main query, subqueries are needed to retrieve the data from one or more tables. WHERE, HAVING, and FROM clauses can all make use of subqueries.
What is a transaction, and what are its properties?
The logical unit of work known as a transaction consists of one or more database actions that must be carried out simultaneously. A transaction’s atomicity, consistency, isolation, and durability are among its features (ACID).
What is a cursor, and how does it work?
A cursor is a database object that pulls data one row at a time from a result set. When a single query cannot complete the data retrieval operation or when data manipulation is necessary, cursors are employed.
What is a constraint, and what are the different types of constraints?
To ensure data integrity, a constraint is a rule that is specified on a column or set of columns. NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK are the various categories of constraints.
What is the difference between UNION and UNION ALL?
| UNION | UNION ALL |
| UNION is used to combine the result sets of two or more SELECT statements, but it removes any duplicate rows. | UNION ALL also combines the result sets of two or more SELECT statements, but it does not remove duplicate rows. |
| UNION eliminates duplicate rows from the result set. | UNION ALL keeps all duplicate rows in the result set. |
| UNION operation involves additional work of removing duplicates, so it can be slower. | UNION ALL is faster because it doesn’t have to check for duplicate rows. |
| Use UNION when you want to avoid duplicate rows in the result set. | Use UNION ALL when you want to include all rows, even if there are duplicates. |
What is the difference between a clustered and a non-clustered index?
An index that controls the physical order of data in a table is called a clustered index, whereas an index that does not control the physical order of data in a database is called a non-clustered index. A table can have numerous non-clustered indexes but only one clustered index.
What is a temporary table, and how does it work?
A table that is generated and utilized only during a session or a transaction is referred to as a temporary table. Temporary tables are used to simplify difficult queries or to store intermediate results.
What is the difference between a view and a materialized view?
A materialized view is a physical replica of a SQL statement’s result set, whereas a view is a virtual table based on the result set of a SQL statement. Using materialized views, which precompute the result set, helps queries run more quickly.
What is a backup, and how do you perform a backup in SQL?
In the event of data loss or corruption, a backup is a copy of a database or a subset of a database that can be used to restore the original data. Several methods, including complete backups, differential backups, and transaction log backups, can be used to perform backups.
What is a deadlock, and how do you prevent it?
When two or more transactions are waiting for each other to release locks on resources they need to access, the condition is known as a deadlock. By adopting effective transaction design, locking techniques, and concurrency control mechanisms, deadlocks can be avoided.
What is a stored function, and what are its benefits?
An SQL statement or another stored procedure can invoke a user-defined function that is saved in a database as a stored function. Improved security, code reuse, and efficiency are all advantages of employing saved functions.
What is a stored trigger, and what are its benefits?
A user-defined trigger that is saved in a database and can be automatically triggered in response to a certain event or action in the database is known as a stored trigger. Using stored triggers has advantages for data integrity, performance, and application code simplification.
What is normalization, and why is it important in database design?
A database’s data is organized through the normalization process to eliminate data redundancy and increase data integrity. Because it assures that every piece of data is stored in a single location, gets rid of inconsistent data, and boosts database speed, normalization is crucial in database architecture.
What is denormalization, and why is it sometimes used?
Denormalization is the deliberate duplication of data in one or more tables, which adds redundancy to a database. By lowering the quantity of database joins and queries necessary to retrieve data, denormalization is occasionally used to boost performance.
What are the best practices for writing efficient SQL code?
Using indexes, avoiding correlated subqueries, reducing data conversions, leveraging bind variables, avoiding wildcards in SELECT statements, and using stored procedures and functions are some of the best practices for designing efficient SQL code.
Conclusion
We really hope that our list of the top 25 SQL interview questions was useful to you as you prepared for your upcoming job interview. We now eagerly await your feedback!
- What SQL query do you find the most difficult, and why?
- Would you mind sharing any more SQL interview questions you’ve come across?
- Do you have any suggestions for SQL-related subjects we should cover in upcoming blog posts?
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