SQL Interview Questions And Answers

In this post we are going to share the interview questions or the information which you must know as a programmer or a developer, especially if you are a Dot Net developer. I hope you will like this article.

Background

I am a dot net developer. As a dot net developer, there are so many things that I must be aware of. I am sharing those in the form of articles, you can always read my other interview questions here in the following links.

So shall we now discuss about SQL interview questions.

SQL Interview Questions

Question: What are the types of Joins in SQL. Explain?

  • INNER JOIN: Returns all rows when there is at least one match in BOTH the tables.
  • LEFT JOIN: Returns all rows from the left table, and the matched rows from the right table.
  • RIGHT JOIN: Returns all rows from the right table, and the matched rows from the left table.
  • FULL JOIN: Returns all rows when there is a match in ONE of the table.

Question: What is the default join in SQL? Give an example query?

The default join is INNER JOIN.

Example

  1. SELECT column_name(s)  
  2. FROM table1  
  3. INNER JOIN table2  
  4. ON table1.column_name=table2.column_name;  
Question: Describe all the joins with examples in SQL?

 

  • SQL LEFT JOIN

    The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match.
    1. SQL LEFT JOIN Syntax  
    2. SELECT column_name(s)  
    3. FROM table1  
    4. LEFT JOIN table2  
    5. ON table1.column_name=table2.column_name;  
  • SQL RIGHT JOIN

    The right join returns all the rows in the right table i.e. table 2 with the matching ones on the left table (table1).
    1. SELECT column_name(s)  
    2. FROM table1  
    3. RIGHT JOIN table2  
    4. ON table1.column_name=table2.column_name;  
  • SQL FULL OUTER

    The full join returns all rows from the left table (table1) and from the right table (table2).
    1. SELECT column_name(s)  
    2. FROM table1  
    3. FULL OUTER JOIN table2  
    4. ON table1.column_name=table2.column_name;  

Question: What is Union and Union All ? Explain the differences?

  • SQL UNION

    The UNION operator is used to combine the result-set of two or more SELECT statements.

    Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

    Note: The UNION operator selects only distinct values by default.
    1. SELECT column_name(s) FROM table1  
    2. UNION  
    3. SELECT column_name(s) FROM table2;  
  • SQL UNION ALL
    1. SQL UNION ALL Syntax  
    2. SELECT column_name(s) FROM table1  
    3. UNION ALL  
    4. SELECT column_name(s) FROM table2;  
    Allows duplicate values.

Question: Differentiate Clustered and Non clustered Index in SQL?

A clustered index is one in which the index’s order is arranged according to the physical order of rows in the table. Due to this reason there can only be one clustered index per table, usually this is the primary key.

A non clustered index is one in which the order of index is not in accordance with the physical order of rows in the table.

Create Index Syntax

    CREATE INDEX [ CLUSTERED | NONCLUSTERED ] PIndex ON Persons (LastName,FirstName)

Question: Explain the difference between Stored Procedure and User Defined Function?

  • Stored Procedure

    Stored procedures are reusable code in database which is compiled for first time and its execution plan saved. The compiled code is executed when every time it is called.

  • Function

    Function is a database object in SQL Server. Basically it is a set of SQL statements that accepts only input parameters, perform actions and return the result. Function can return only a single value or a table. We can’t use functions  to Insert, Update, Delete records in the database table(s). It is compiled every time it is invoked.

Basic Difference

Function must return a value but in Stored Procedure it is optional (Procedure can return zero or n values).
Functions can have only input parameters for it whereas procedures can have input/output parameters.

Functions can be called from Procedure whereas Procedures cannot be called from Function.

Advanced Differences

  • Procedure allows SELECT as well as DML(INSERT/UPDATE/DELETE) statement in it,  whereas Function allows only SELECT statement in it.

  • Procedures cannot be utilized in a SELECT statement, whereas function can be embedded in a SELECT statement.

  • Stored Procedures cannot be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section, whereas function can be.

  • The most important feature of stored procedures over function is retention and reusing the execution plan while in case of function it will be compiled every time.

  • Functions that return tables can be treated as another rowset. This can be used in JOINS with other tables.

  • Inline Function can be though of as views that take parameters and can be used in JOINS and other Rowset operations.

  • Exception can be handled by try-catch block in a procedure, whereas try-catch block cannot be used in a Function.

  • We can use transactions in stored procedure but not in functions.

Conclusion

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