These are some of the common SQL questions and answers that may be helpful for final round interviews. However, the questions may vary depending on the position and company, so it’s important to research the company and position to prepare for the specific interview.
- What is SQL, and what is it used for?
- SQL stands for Structured Query Language, and it is used to manage relational databases. SQL is used to insert, update, retrieve and delete data from databases. It is used by data analysts, data scientists, and developers for various purposes like data management, data analysis, and data manipulation.
- What are the different types of SQL commands?
- There are mainly four types of SQL commands:
- Data Definition Language (DDL): Used for defining the database schema, tables, indexes, and other database objects.
- Data Manipulation Language (DML): Used for manipulating the data within the database.
- Data Control Language (DCL): Used for controlling the access to the database.
- Transaction Control Language (TCL): Used for managing transactions within the database.
- What is a primary key, and why is it important?
- A primary key is a unique identifier for each row of data in a table. It is used to enforce data integrity and ensure that each row is unique. It also provides a way to link data between tables through foreign keys.
- What is a foreign key, and how does it relate to a primary key?
- A foreign key is a field in a table that refers to the primary key of another table. It is used to establish a relationship between two tables in a database. By using foreign keys, we can enforce referential integrity and maintain consistency in the data between related tables.
- What is the difference between a left join and an inner join?
- Both left join and inner join are used to combine data from two or more tables. The main difference between them is that an inner join returns only the matching rows from both tables, while a left join returns all the rows from the left table and matching rows from the right table.
- What is a subquery, and how is it used in SQL?
- A subquery is a query that is nested inside another query. It is used to retrieve data that will be used in the main query. Subqueries can be used in different parts of a SQL statement, like the SELECT, FROM, WHERE, and HAVING clauses.
- What is a stored procedure, and how is it used in SQL?
- A stored procedure is a pre-compiled SQL code that is stored in the database. It is used to encapsulate a complex SQL logic that can be reused multiple times. Stored procedures can be executed with different parameters, and they can also be used to manipulate data, perform calculations, or generate reports.
- What are the different types of indexes in SQL, and how are they used?
- There are mainly three types of indexes in SQL:
- Clustered Index: It is used to physically reorder the table data based on the index key.
- Non-Clustered Index: It is used to create a separate data structure to store the index key and the pointer to the table data.
- Unique Index: It is used to enforce uniqueness on a table column.
Indexes are used to improve the performance of the database queries by reducing the time required to search for specific data in a table.