16 SQL Interview Questions and Answers

This article is intended to help you get familiar with the top 16 SQL interview questions and answers that you may meet in your quest for an SQL-related job. The questions asked are usually random and can get you unawares. Therefore, make sure that you read this reliable compilation to have a rough idea of what to expect and the most appropriate answers to give. We will now get straight to the 15 SQL interview questions.

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Question 1: What is a Union and how is it different from Union All?

Union combines the contents of two tables that are compatible based on their structures into one final table. The significant difference between the Union and the Union All functions is that former will omit similar records while union all will include even the duplicated information. Therefore, when using Union, the server is prompted to perform the extra work of removing matching fields or duplicates. Therefore, for conditions where duplicates don?t pose a significant threat, then the Union All is the better option.

Question 2: what is SQL?

SQL is an acronym for Structured Query Language and is made for the sole purpose of communicating with databases. It is also a language or tool for accessing and manipulating databases and is standardized by the American National Standards Institute (ANSI).

Question 3: Explain the different types of SQL?s Statements?

The SQL statements are divided into the following 3 categories:
i) Data Definition Language. This?is used for the definition of the data holding structures. They include: create, drop, truncate and alter table.
ii) Data Manipulation Language. It is involved in the actual data manipulation process. The operations contained here are: Insert, Delete, Update and Retrieve functions. Select is also sometimes categorized in this criterion since it can change the form of data within the database.
iii) Data Control Language: It performs actions like controlling the access of data in a database in addition to setting the privileges for table creation. Some of the functions here include Grant, Revoke access permission.

Question 4: Is there a difference between SQL, MySQL or SQL Server?

SQL is a language which relays information to and from a relational database thus providing ways through which they can be created and manipulated. MySQL and SQL Server, on the other hand, are management systems for the relational databases which use the SQL language as their standard relational database language.

Question 5: What is DBMS?

It stands for database management system and is a consortium of programs that enables users to store, retrieve update and remove information from databases.

Question 6: What is an RDBMS?

RDBMS or a relational database management system is a DBMS that is built on a relational blueprint. The data in an RDBMS can be manipulated in a varied number of ways without having to reorganize information in the individual database tables. Therefore, the data they contain is usually accessed via an Application program interface like SQL.

Question 7: What is a database?

A database is an organization of data in such a way that they can be easily accessed, stored, manipulated and managed. It may also be defined as a structured form of data that can be reached in several ways. Usually, the data is organized into tables and fields. In tables, the data is arranged in rows and columns. Fields are a specified number of columns within a table.

Question 8: Differentiate between Join and Union operations.

The Join operation in SQL allows us to reference or relate records in other tables based on the conditions existing between them. The structures joining the tables are referred to as relationships. Union allows the user to merge two tables or similar data sets to a new dataset that contains all the data from the two datasets. It doesn?t need any conditions in the merging process.

Question 9: Give the difference between the inner and outer joins.

The inner join is a type of join that combines the records from two tables hence creating a set of data where just the records found in both the tables are present. Therefore it will only take place in rows where at least a match is present in both the tables. When this condition is not satisfied the result of the operation is a null set.

In the outer join, matching rows as well as unmatched rows from either or both of the merged tables is displayed on the resultant data set. It can be further categorized into the full outer or single outer joins.

Question 10: Are there advantages in using SQL? Briefly, discuss them.

Firstly, it is not a database-specific proprietary language. Standard DBMS are supported by SQL. Therefore, its programmers can interact with other database platforms like ORACLE, MySQL among others.
Secondly, it is simple to learn. SQL language is basically English and isn’t many in number.
Thirdly, despite being simple, it can be used to accomplish sophisticated database operations.

Question 11: Define a database transaction

It is a process that transfers a database from a particular consistent state to the next. In the case of a failure, the system should be prompted to stay in the original state. Otherwise, the system will show a successful completion if the process goes through.

Question 12: What are the various properties of a database transaction?

This is one of the SQL interview questions that cut across all the stages of SQL qualifications. They can be summed up into the simple acronym ACID. They are listed below:
I. Atomicity. Upon the termination of all the transaction steps, the results will be displayed in the database or else the process is rolled back.
II. Consistency. The database is shifted from one consistent state to the other in the event of a successful transaction but left in the prior state if it fails.
III. Isolation. Each transaction is carried out independently like it is the only operation in the system.
IV. Durability. Updates in the dataset after a transaction are made available permanently for other transactions.

Question 13: Outline the specific tasks of the SQL functions.

The SQL functions play the following roles:

  • Taking care of calculations involved in data manipulation.
  • Modification of each data item individually.
  • Manipulation of the output or information.
  • Formatting data types such as dates and numbers.
  • Conversion of data types.

Question 14: What is the role of the group functions in SQL?

The group operations in SQL such as AVG, COUNT, MAX, and VARIANCE to name a few, work on records and return a single result for every group of data.

Question 15: What is a primary key?

This is a unique identifier of each record in a database. It provides uniqueness to a column hence there can only be one primary key in a table.

Question 16: List the types of indexes in SQL

They are: unique, clustered and non-clustered indexes.

Note: You can read more about this matter in our articles about the clustered index and about the main differences between clustered vs. nonclustered index.

Conclusion

We hope that after reading the compilation you will be equipped with SQL knowledge to help you excel in your interview. You can share the information or give us feedback on what you feel about the questions and how they are answered here.

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