In the field of software development and data management, SQL (Structured Query Language) is an essential skill that many companies seek. Knowing this programming language is extremely crucial for both freshers and experienced professionals looking for jobs in the IT field. The pay package and growth prospects in SQL-related jobs are lucrative. If you are looking forward to giving interviews, you must prepare the right SQL interview questions beforehand. It will increase your chance of cracking the interview and landing the job.
What are Some SQL Interview Questions?
There are so many SQL interview questions that might be asked in interviews. But it is not practically possible to prepare for each and every possible question out there. This is why it would be most beneficial for you to know and prepare some of the most important SQL interview questions. Let’s explore them below one by one.
What Does SQL Stand for? Explain Its Purpose.
This is one of the most basic SQL interview questions. The full form of SQL is Structured Query Language. This is a standardized programming language that is used for relational database interactions. With the help of SQL, users can effectively perform different operations, such as updating, querying, and managing data. This language is widely used in database management systems (DBMS), such as SQL servers, Oracle, and MySQL.
What are the Types of SQL Statements?
This is among the important SQL interview questions. There are five types of SQL statements:
DML (Data Manipulation Language)
It uses ‘INSERT’, ‘SELECT’, ‘DELETE’, and ‘UPDATE’ commands to perform different functions in database.
DDL (Data Definition Language)
It uses various commands like ‘ALTER’, ‘CREATE’ and ‘DROP’ to modify and define database structures.
DCL (Data Control Language)
Commands like ‘REVOKE’ and ‘GRANT’ are used to control access to data. Users can use these commands as and when needed.
DQL (Data Query Language)
This type of SQL statement commands like ‘SELECT’ to fetch data from databases.
TCL (Transaction Control Language)
It includes different commands such as ‘SAVEPOINT,’ ‘ROLLBACK,’ and ‘COMMIT’ to effectively manage different transactions.
What is the Difference between SQL and MySQL – SQL Interview Questions?
SQL is a query language that users utilize to manipulate and manage databases. On the other hand, MySQL is an open-source RDBMS (Relational Database Management System) that uses SQL as a standard language for querying databases. SQL is a standardized language, but MySQL is a software used to manage databases and implement SQL queries. It is surely one of the best SQL interview questions.
Explain Primary Keys and Foreign Keys: Essential SQL Interview Questions
This question comes under common SQL interview questions.
Primary Keys
It is a unique identifier that identifies records in database tables. Primary keys denote that every record is unique and cannot be null. For instance, the ‘user_id’ is the primary key in the ‘users’ table.
Foreign Key
This is a field in one specific table that links to the primary key in another table. This is how Foreign Key establishes a relationship between two different tables to ensure referential integrity. For instance, the foreign key ‘order_id’ in the ‘orders’ table could be a foreign key linking to the ‘order_id’ in the ‘products’ table.
What is Normalization in SQL? Describe Its Types.
Organizing data effectively in the database is known as the normalization process. It helps to improve data integrity and minimize redundancy. In this process, large tables are divided into smaller ones, and relationships are defined between them. You must know these SQL interview questions. There are several forms (or types) of normalization processes:
1NF (First Normal Form)
In this type, every column in a table has atomic values, where every record is unique.
2NF (Second Normal Form)
2NF is built on 1NF by making all non-key columns dependent on the primary key.
3NF (Third Normal Form)
This is the next refined form, or 2NF. This is done by effectively ensuring that non-key columns would not depend on any other non-key column.
BCNF (Boyce-Codd Normal Form)
This is the stricter form of 3NF. BCNF makes every determinant a candidate key. Knowing these SQL interview questions will be useful for you.
What is the Definition of JOIN in SQL? Described Its Types
A JOIN in SQL combines rows from two or more tables if they have related columns between them. Understand these SQL interview questions. There are different types of JOIN:
INNER JOIN
It returns records having matching values in two tables. It is among the good SQL interview questions.
RIGHT (OUTER) JOIN
It is used to match records from the left table and return records from the right table. ‘NULL’ will be the result for all the unmatched records in the left table.
LEFT (OUTER) JOIN
This matches records from the right table and returns records from the right table. ‘NULL’ will be the result for all the unmatched records in the right table.
CROSS JOIN
The Cartesian product of two different tables is returned, i.e., all the possible combinations of the rows. Understand these SQL interview questions properly.
FULL (OUTER) JOIN
If there is a match in the right or left table, then it returns all the records. It also includes unmatched records from both sides.
Explain Subquery in SQL
A subquery is also called a nested query or inner query, which is nothing but an SQL query within another query. The main query uses the result of the subquery. One has the option to use the subqueries in different SQL clauses like ‘INSERT,’ ‘SELECT,’ ‘UPDATE,’ and ‘DELETE.’ This also comes under common SQL interview questions.
What is the Difference between HAVING and WHERE Clauses?
These two clauses come under SQL interview questions.
HAVING Clause
This clause filters records once the data gets aggregated by the ‘GROUP BY’ clause. Instead of individual rows, conditions are applied to groups.
WHERE Clause
It filters records with the help of ‘SELECT’ statements prior to groupings being made.
What is the Concept of Indexes in SQL?
The concept of index in SQL is a useful database object that ensures a higher speed of data retrieval operations on a database table. Frequently used or searched query conditions are indexed on columns. On one hand, indexes make the data retrieval process faster. On the other hand, it makes various data modification operations such as ‘UPDATE’, ‘INSERT’, and ‘DELETE’ slower due to the fact that the index requires to be updated too. These are SQL interview questions.
What are Some SQL Constraints?
SQL constraints are some specific rules applied to columns in database tables to ensure data integrity. A few SQL constraints are described below:
NOT NULL: It validates that no column can have a ‘NULL’ value.
UNIQUE: It denotes that values in a column are all unique.
CHECK: This constraint makes all the values in a column satisfy a particular condition.
PRIMARY KEY: This is nothing but an effective combination of ‘UNIQUE’ and ‘NOT NULL’, which helps to identify every record in a table.
FOREIGN KEY: It is there to promote referential integrity between two different tables in the database.
DEFAULT: This SQL constraint sets a default value to a column. This is regarded as among the most common SQL interview questions.
Optimizing SQL Queries – SQL Interview Questions
You must explore these SQL interview questions. There are various methods that can optimize different SQL queries. This helps to improve the performance of query execution. Some of the methods are described below for your convenience:
Using Indexes
You can apply indexes to columns that are frequently used in ‘ORDER BY,’ ‘JOIN,’ and ‘WHERE’ clauses.
Using Joins over Subqueries
Using Joins is known to be more useful and effective as compared to using subqueries.
WHERE Clauses Optimization
It involves filtering data in the database as quickly as possible.
Functions Avoidance in Indexed Columns
You have the option to perform operations like ‘LOWER ()’ or ‘UPPER ()’ on the indexed columns. This is how you can disable the index effectively.
Query Execution Plans
One can also optimize and analyze the query execution plan given by the DBMS.
Explain the Difference between TRUNCATE and DELETE – SQL Interview Questions
TRUNCATE
This never logs individual row deletions but removes the rows from a table.
DELETE
According to the condition of the ‘WHERE’ clause, it removes particular rows from a table in the database. This DML command can be rolled back as and when needed.
Described the Stored Procedure in SQL – SQL Interview Questions
This denotes the precompiled collection of SQL statements in the database. This can be reused several times which improves the performance by effectively lowering the need to recompile SQL code. The stored procedure can also accept different parameters for dynamic execution and maintain consistency. It is also one of the popular SQL interview questions.
What are the SQL Triggers?
SQL triggers are known to be a type of stored procedure that automatically fires or executes when some particular events like ‘UPDATE,’ ‘INSERT,’ and ‘DELETE’ operations occur in the database. The triggers maintain audit trails, validate data, and enforce business rules.
What is the Use of the ‘GROUP BY’ Clause?
As its name suggests, it effectively groups different rows with the same values in particular columns into aggregated data. This ‘GROUP BY’ clause is mainly used in conjunction with different aggregate functions like ‘MIN(),’ ‘MAX(),’ ‘AVG(),’ SUM(),’ and ‘COUNT().’ For instance, if you wish to calculate the total sales in every product category, then you have the option to utilize ‘GROUP BY’ in the ‘CATEGORY’ column.
Conclusion – SQL Interview Questions
All the above-described SQL interview questions are essential for you to know before you appear for SQL interviews. Apart from the above ones, you should also do your own research to explore other SQL interview questions and answers to be prepared. SQL is a big field. You cannot be perfectly prepared no matter how you try, but you can try to give it your best shot.
FAQs
What are Some Common SQL Interview Questions?
Ans: Questions on topics like SQL queries, normalization, subqueries, and joins come under common interview questions and answers for SQL roles.
How to Structure Your Answer during an SQL Interview?
Ans: You are required to have a clear concept of SQL to understand and structure question answers during interviews.
What is the SQL Full Form?
Ans: The full form of SQL is Structured Query Language.
Is W3Schools SQL a Good Platform for Learning SQL?
Ans: Yes, this is one of the best platforms to learn SQL or explore interview questions for freshers.
What Topics Cover DBMS Interview Questions?
Ans: Some of the covered topics in this regard are transactions, normalization, keys, and relational models.
How to Prepare MySQL Interview Questions?
Ans: You can study from some SQL book or check out different websites on the internet to know about SQL queries or other related information.
