Common SQL Joins Interview Questions and Answers

Here is a list of popular SQL joins interview questions and answers to help you prepare and boost your candidacy:

What is a join clause in SQL?

Example: Join clauses are used to combine the records from two or more separate tables. Most times, we may link data from two separate tables in SQL. We use join clauses to link them together. Joins can also help determine the relationship between different tables. You can also use joins to select data points from another table or multiple tables. 

Tell me about the basic types of joins.

Example: There are two types of joins: joins using concept and joins using operators. Examples of joins using concept include Inner Join, Outer Join, Self Join, and Cross Join. Joins using operators include Equi Join and Non Equi Join. We can further classify Outer Joins into Left Outer Joins, Right Outer Joins, and Full Outer Joins. 

An inner join shows matching records transferred from one table to another, while a full outer join is the combination of right and left outer joins. 

What is an equi join?

Example: An equi join is a category in SQL formed by joining two or more tables using the equal to operator. 

What is outer join?

Example: An outer join is one where rows are returned despite having no matches through the join criteria on the second table. 

What is a right join?

Example: A right join clause is used to return all results from the right table in an SQL statement. Where possible, the results are matched against rows in the left table. However, when there are no corresponding matches in the left and right tables, the result will be listed as null values for the left table.

What is full join used for?

Example: The full join is used to return results from the right and left tables in a statement. 

Explain non equi join.

Example: A non equi join occurs when you join two or more tables without the equal to condition. Any operator can be used for a non equi join, but it is rarely used in real-life situations. 

What is a full outer join?

Example: A full outer join combines the results from left and right outer join. The result will include corresponding matches from either table. Where there are no matches, the results will be displayed with a NULL. 

Explain a left outer join.

Example: A left outer join is used to highlight the results from a table mentioned on the left of a join statement, regardless of whether there is a corresponding match on the right table. Hence, the clause returns records from the right table with NULL in each column that do not match the left table. 

What are the properties of the relational tables?

Example: The relational tables have several properties, including:

• Each row is unique
• The sequence of rows and column are insignificant
• Each column requires a distinct name
• Values are atomic
• Column values are the same

What is RBDMS?

Example: RBDMS is an acronym for Relational Data Base Management Systems, which is a type of database management system that uses tables to maintain data records and indices. Relationships between data items in a relational database system are expressed by tables, and the interconnectedness between the tables are expressed via data values instead of pointers. Using tables offers a high level of data independence and allows one to recombine data items from various files, which increases data utility. 

Explain de-normalization.

Example: De-normalization is used to boost the performance of a database by introducing redundant data. It is used to move from a higher to lower form of database modeling to increase access to database materials. The process is vital for getting the best out of database management systems that cannot run a relational model effectively. 

What is a stored procedure?

Example: A stored procedure is the name given to a group of SQL statements that you created and stored in a server database. Since stored procedures can accept input parameters, they allow several clients using different input data to use a single procedure over the network. The primary function of stored procedures is to reduce network traffic, boosting performance. They can also increase database integrity, since users can access the network via a single procedure rather than multiple entry points. 

What is index in SQL server?

Example: An index is a physical structure used to find rows of data in an existing table quickly. They point to data and speed up queries, making them a powerful, yet invisible tool for improving the performance of database applications. 

What are triggers in SQL?

Example: A trigger is an SQL procedure that starts an action when an event such as INSERT or DELETE occurs. They are used to maintain referential integrity of data and are stored and managed by the database management system. Whenever you modify data associated with a specific table related to a trigger, the DBMS automatically initiates an action. Unlike stored procedures, a trigger is event driven and starts a specific action when a table it is directly related to is modified.  

What is a nested trigger?

Example: A nested trigger is one that initiates another trigger whenever its table is modified. In essense, this type of trigger contains a data modification logic, so it can activate a data modification when it is fired by another data modification. 

What is view in SQL server?

Example: In SQL server, a view is a subset of a table used to modify data, including deleting or updating rows and retrieving data items. When one updates or deletes data in the view, the changes will reflect in the table where you created the view. Note that the results from the use of a view will not be stored permanently in the database. 

What is a linked server in SQL?

Example: Linked server is a concept that allows clients to add other SQL servers to a group so they can use T-SQL Statements to query both SQL server database systems. Linked servers make it easier to create clean SQL statements that allow the retrieval, combination, and joining of remote and local data. 

Who do you understand as collation in SQL?

Example: Collation is a set of rules that specifies how to sort and compare data. It allows you to determine rules such as those for specifying accent marks, character sequence, case sensitivity, and character width. 

What is the difference between joins and union?

Example: The SQL join allows users to find records of data on other tables based on the relationship between two tables. Meanwhile, an SQL union operation allows users to combine two similar data sets to form a new data set that contains all the data items from the original data sets. While a join requires the data sets to have a relationship, union require not conditions to add two similar data sets. 

Explain an execution plan and its uses.

Example: An execution plan is a graphical or textual road map that shows how an SQL server’s query optimizer retrieves data for a stored procedure or query. They can help a developer understand and analyze the performance metrics of a stored procedure or ad hoc query. You can obtain an execution plan with the EXPLAIN keyword in most SQL systems. 

What is the difference between primary keys and foreign keys?

Example: Primary keys refer to the unique identifiers for each row of data in a table. They are the most important keys, so primary keys cannot be null and must have unique values. Foreign keys help determine the relationship between tables and can also be used to ensure data integrity. 

What are sub-queries?

Example: A sub-query allows you to execute a select statement within the body of another SQL statement arbitrarily, which is why they are also called sub-selects. You can use sub-queries to compare values against multiple rows using the IN keyword or return a single row as an atomic value. To execute a sub-query, enclose it in a set of parentheses.

As a developer specialized in database management systems, it is important to have an in-depth understanding of SQL joins interview questions. To increase your chances of getting your hired, use these list of questions and answers to prepare for interview so you can provide convincing responses.