SQL (Structured Query Language) is an essential tool for managing and manipulating relational databases. It is a critical skill for many roles in the tech industry, such as database administrators, data analysts, and developers. In this blog post, we’ll go over 100+ common SQL interview questions that you may encounter during a job interview.
What is SQL and what is it used for?
SQL is a standard language used to interact with relational databases. It is used to insert, update, and retrieve data from databases, as well as to manage the structure of the database itself (e.g., creating tables, setting relationships between tables, etc.).
What are the main commands in SQL?
The main commands in SQL include SELECT, INSERT, UPDATE, DELETE, and CREATE. SELECT is used to retrieve data from a database, INSERT is used to insert new data into a database, UPDATE is used to modify existing data in a database, DELETE is used to delete data from a database, and CREATE is used to create new database objects (e.g., tables, views, indexes, etc.).
How would you retrieve all the data in a table?
To retrieve all the data in a table, you would use the following SQL statement:
SELECT * FROM table_name;
What is a primary key and why is it important?
A primary key is a unique identifier for each record in a table. It is used to enforce the integrity of the data in the table, as well as to create relationships between tables in a database. A primary key is important because it ensures that each record in a table is unique and can be easily retrieved and updated.
What is a foreign key and how is it used?
A foreign key is a column in one table that refers to the primary key of another table. It is used to create relationships between tables in a database, ensuring that data is consistent and related information can be easily retrieved.
How would you retrieve data from multiple tables using a join operation?
To retrieve data from multiple tables using a join operation, you would use the following SQL statement:
SELECT * FROM table_1 JOIN table_2 ON table_1.column = table_2.column;
What is a subquery and how is it used in SQL?
A subquery is a query within another query. It is used to retrieve data from one table based on data in another table. For example, you could use a subquery to retrieve all customers who have made a purchase in the last year.
How would you aggregate data in a table (e.g. sum, average, count)?
To aggregate data in a table, you would use aggregate functions such as SUM, AVG, and COUNT. For example, to find the average salary of employees in a table, you would use the following SQL statement:
SELECT AVG(salary) FROM employees;
How would you sort data in a table in ascending or descending order?
To sort data in a table, you would use the ORDER BY clause. For example, to sort data in a table in ascending order by last name, you would use the following SQL statement:
SELECT * FROM employees ORDER BY last_name;
What is a transaction in SQL and how is it used to ensure data consistency?
What is a stored procedure in SQL and why is it useful?
How would you handle missing or duplicate data in a table?
What is indexing in SQL and how does it improve query performance?
How would you handle large data sets and ensure scalability in SQL?
What is a trigger in SQL and how is it used?
What is normalization and why is it important in database design?
What is denormalization and when would you use it in SQL?
What is a relationship in SQL and how is it represented in a database?
What is a relationship in SQL and how is it represented in a database?
What is a one-to-many relationship in SQL?
What is a many-to-many relationship in SQL?
What is a self-join in SQL and how is it used?
What is a union in SQL and how is it used?
What is a left join in SQL and how is it different from a right join?
What is an inner join in SQL and how is it used?
What is a full outer join in SQL and how is it used?
What is a cross join in SQL and how is it used?
What is a correlated subquery in SQL and how is it used?
What is a stored procedure in SQL?
stored procedure in SQL is a pre-written set of SQL statements that are stored in the database and can be executed as a single unit. Stored procedures are used to improve performance, provide a higher level of security, and make it easier to manage complex operations in the database.
What is a stored function in SQL and how is it used?
stored function in SQL is a pre-written piece of code that can be used to perform a specific task and return a value to the calling statement. Stored functions can be used to encapsulate complex business logic, to provide a higher level of abstraction, and to make your code more reusable and maintainable.
What is a dynamic SQL in SQL and how is it used?
Dynamic SQL is a type of SQL code that is generated and executed at runtime, rather than being written and executed in advance. Dynamic SQL is used when the SQL statements that need to be executed are not known until runtime, and is useful for building flexible and dynamic database applications.
What is a materialized view in SQL and how is it used?
What is a table variable in SQL and how is it used?
Table variable in SQL is a type of variable used to store data in a tabular format, similar to a table in a database. Table variables are used to store intermediate results in a stored procedure, function, or other database operation, and are faster and more efficient than temporary tables.
