Top 20 SQL Interview Questions and Answers you must know

SQL is widely used by various programmers. If you are attending an interview and don’t know which SQL interview questions to study, don’t worry! We have you covered. This blog covers the top SQL interview questions required for you to ace your interview. 

What is SQL?

Structured command language, more commonly referred to as SQL, is seen as the standard for managing data kept in electronic database management systems. For instance, SQL statements are to perform tasks including updating the info and also retrieving specific data from a database.

In SQL, What is Join and what are the different types of Join available in SQL?

Join is used to accumulate data or rows from two or more tables or dataset based on a common field between them.

Four types of Joins are available in SQL. Details are mentioned below.

  • Inner Join

It only returns the matching rows.

  • Left outer join

Return the full left result set along with the matching right result set.

  • Right Outer join

Return the full right result set along with the matching left result set.

  • Full outer Join

Return both the full result set with matching and non-matching result set.

What is an index and why is it useful to have?

An index is a powerful utility to retrieve the records from tables faster. The index creates an entry for each value entered into the tables and stores the location of the keyword, which in turn makes it much quicker to fetch the required data. An index is basically a pointer to data contained within a table. There are various types of indexes available in different RDBMS; usually, B-Tree and BITMAP are commonly used indexes.

What are the more common types of SQL constraint and what do they mean?

There are several common SQL constraints. These are 

  • PRIMARY KEY, which identifies UNIQUE not null values in the columns. It can be single or composite columns also. 
  • FOREIGN KEY: provides a unique identification of a row or record within another table. It used to represent the parent-child relationship between tables. 
  • DEFAULT, which replaces the null values with some default values.
  • CHECK, which used to verify that all values meet specific criteria or conditions in Columns.
  • UNIQUE, which restricts unique values, is always inserted into a column. This can contain NULL values also.
  • Finally, INDEX is used to create a pointer of the data for faster retrieval of data from tables.

What is database Denormalization?

Denormalization is a method that is used to improve the performance of a database, allowing the retrieval of data in the quickest time possible. Essentially, the process of denormalization to store the redundant data into different tables and fields for faster retrieval of data and improve query performance.

What is database normalization?

Normalization reduces the redundancy of data within a database. Normalization is a design technique that divides larger database tables into smaller tables and then uses various relationship types to link between them. In other words, normalization is the process of putting each piece of data in the appropriate place, using multiple tables. It allows to maintain the data integrity and makes the updating faster, but retrieval of data can then, in turn, be slower.

What are the advantages of Normalization?

  • IT offers better database organization.
  • More tables with smaller rows.
  • Efficient data access
  • Greater flexibility for queries.
  • Quickly finds the information.
  • Easier to implement security.
  • Allows easy modification.
  • Reduction of redundant data and duplicate data.
  • More compact database. 
  • Ensures consistent data after modification.

In SQL, what’s a subquery?

A subquery is usually mentioned as an inner query or inner select. As per the naming convention, it’s an SQL query nested inside a bigger query, like a get, UPDATE, or INSERT statement, or indeed inside another subquery. A subquery is often wont to retrieve data that will be utilized in most queries. This successively provides a condition to limit the retrieval of knowledge even further.

What happens to the data rows in the table when the table contains a clustered index?

The data rows will be stored in order. Cluster indexes sort data rows that supported their key values. When a table doesn’t use the clustered index, this is often called a heap – in other words, the info rows are going to be unordered.

What is NULL value?

NULL means absence of information, Null value is not at all the same as that of zero of blank space that is because null value represents a value which is unavailable, unknown unassign and not applicable whereas zero is a number and a blank space is a character, so all these three terms are completely different. Zero is a number. Blank space is a character and now represents a value that is unavailable, unknown, unassigned or not applicable right.

What is the difference between the ‘HAVING’ clause and the ‘WHERE’ clause?

The main difference between having clauses and where Clauses basically is that the ‘HAVING’ Clause can be only used with the select statement. it’s usually employed by the group by Clause, and whenever group by isn’t used, ‘HAVING’ basically behaves sort of a ‘WHERE’ Clause coming to the where Clause is applied—each row before they’re a part of the group by function during a query. So ‘HAVING’ Clause is essentially used with the select statement, and it’s usually utilized in the group by Clause when the ‘HAVING’ Clause isn’t used with the group by Clause, the ‘HAVING’ Clause behaves sort of a ‘WHERE’ clause.

What is the difference between Delete, Truncate and Drop command?

Delete is a DML statement, the selected number of records or whole table data can be deleted i.e. it allows delete of records through conditions.

Truncate is a DDL statement, it deletes all the records in a table But the table structure exists in the database.

Drop is DDL statement. Drop command removes the table structure along with all the data in it.

What are the differences between a cross joint and a natural join?

The cross joint produces the cross product or Cartesian product of two tables. This may or may not have the joining conditions. On the other hand, the natural join is based on all the columns having the same name and data types in both tables. The main difference between both of them is that the cross joint basically produces the cross product of both the tables, and the natural join is completely based on all the columns having the same name data types in both the teams.

What is a sub query in SQL?

A subquery is basically a query inside under the query where a query is defined to retrieve data or information back from the database. Sub queries are always executed first and the result of it is passed on to the main query.

What are the different types of sub-query?

There are mainly two types of sub queries i.e. correlated and non-correlated subquery. The correlated subqueries are used to select data from a table referenced in the outer query. It is an independent query as it refers to another table. The non-correlated subquery is an independent query where the output of the subquery is substituted in the main query. 

What is a relationship and what are the different types of relationship?

The connection between entities with each other is termed relationships. So relationships can also be defined as the relationship or links between the tables in a database. The different types of relationships are the one to one relationship, the one to many relationships many to one relationship and the self-sensing the relationship when a relationship between two tables at his table a and table be so when the one-to-one relationship between both the tables table a and table be a single recording table a will be related to a single record in the table be coming to one-to-many relationship a single recording table a can be related to many records and in many to one relationship, many records in the table is linked to one record in the table. The self-referencing relationship basically means that it can have a single table out of which two columns are related to each other with the relationship.

What is the difference between rollback and rollback to?

The simple rollback statement the transaction is completely eliminated basically you can completely undo a transaction and release all of the locks that are present on the data values and on the other hand when use rollback to, we can specify up until which save point we want to undo that particular transaction moving.

What is the difference between CHAR & VARCHAR?

CHAR blocks the fixed spaces in the memory. CHAR is basically used to minimize the block chaining and reduces the IO whereas it tends to waste more space. Suitable where more frequent updates occur with variables character length or size.

VARCHAR blocks the variable memory space as per actual size of the inserted character. There is less wastage of spaces, but I/O and block chaining will increase if updates occur with more character size.

What is MySQL?

MySQL is an open-source relational database management system which is based on SQL, most commonly used for web databases. The remainder is obviously an abbreviation for Structured Query Language. Some of the renowned websites, including YouTube, Facebook and Twitter have used MySQL. This is also used by database-driven web applications like WordPress, Joomla and others. It is Written using C and C++ languages. It supports the major platforms you would expect such as macOS, Linux, and Windows

What is the difference between SQL and MySQL?

SQL is usually to access, manipulate and update the info within a database, whereas MySQL is a electronic database management system that keeps the info within the database organized.SQL is a language that manages relational databases, and MySQL is an open-source electronic database management system, supported SQL. MySQL has the programming language part.

This brings us to the end of the blog on the top SQL interview questions which you must learn to ace your upcoming interviews! Good luck and happy learning.