SQL Position Interview Questions: Top 10 Questions and Answers [Updated 2024]

Sample Answer
SQL Joins are used to combine rows from two or more tables, based on a related column between them. There are four basic types of Joins: Inner Join, Left Join, Right Join, and Full Join. An Inner Join returns records that have matching values in both tables. A Left Join returns all records from the left table, and the matched records from the right table. A Right Join returns all records from the right table, and the matched records from the left table. Lastly, a Full Join returns all records when there is a match in either the left or the right table.

Sample Answer
SQL Injection is a code injection technique that attackers can use to insert malicious SQL statements into input fields for execution. This can lead to unauthorized access to data, data theft, and even data loss. To prevent it, we can use several methods. First, it’s important to always use parameterized queries. This ensures that parameters are passed to queries in a safe manner, thereby preventing execution of harmful queries. Second, we should always validate and sanitize user input to ensure that it does not contain harmful content. Third, using a Web Application Firewall can also help to detect and prevent SQL Injection attacks. Finally, regular updates and patches should be applied to ensure that the system does not contain vulnerabilities that can be exploited through SQL Injection.

Sample Answer
A primary key is a column or a set of columns that uniquely identifies each row in the table. It enforces the entity integrity of the table by ensuring that each row has a unique identity and that it is not null. On the other hand, a unique key is a constraint that enforces the uniqueness of the values in a column or set of columns, similar to the primary key. However, the unique key allows one null value. An important difference is that a table can have multiple unique keys but only one primary key.

Sample Answer
In SQL, a clustered index determines the physical order of data in a table, which means a table can have only one clustered index. The leaf nodes of a clustered index contain the data pages. On the other hand, a non-clustered index doesn’t sort the physical data inside the table. Instead, it creates a logical order that resides in a separate object within a table. A table can have multiple non-clustered indexes. The leaf node of a non-clustered index does not consist of data pages. Instead, it contains index rows. If we compare it to a library, a book on a shelf (the data in a table) is analogous to a clustered index, whereas the catalog at the front desk (a separate object with a reference) is like a non-clustered index.

Sample Answer
GROUP BY statement is used in SQL to group rows that have the same values in specified columns into aggregated data. It is often used with aggregate functions like COUNT, MAX, MIN, SUM, and AVG to group the result-set by one or more columns. For example, if we have a sales data of a company and we want to know the total sales made in each region, we can use the GROUP BY statement to group the sales data based on the region and then use the SUM function to calculate the total sales for each region.

Sample Answer
Normalization is a database design technique that organizes tables in a manner that reduces redundancy and dependency of data. It divides larger tables into smaller tables and links them using relationships. The benefits of normalization include minimizing duplicate data, reducing the complexity of data, ensuring relationships between tables, and improving the integrity and consistency of the database. The process of normalization includes several stages called normal forms: first (1NF), second (2NF), third (3NF), Boyce-Codd normal form (BCNF), fourth (4NF), and fifth (5NF). Each normal form has a set of rules or conditions that a database must meet.

Sample Answer
A stored procedure in SQL is a prepared SQL code that you can save, so the code can be reused over and over again. The main benefit of stored procedures is that they encapsulate logic for data manipulation, which can reduce the amount of information sent between a client and a server, and thus improve performance. For example, you could create a stored procedure to update a table with new information. This procedure could look something like this: CREATE PROCEDURE UpdateEmployee @Name varchar(100), @EmpId int AS UPDATE Employees SET Name = @Name WHERE EmployeeId = @EmpId.

Sample Answer
SQL Views are virtual tables based on the result-set of a SQL statement. They are used in several scenarios. One common use of views is to provide a simplified version of a complex schema. They can also be used to restrict access to the data such that a user can only see limited data and not the whole table. Additionally, they can aggregate data for quick analysis and can be used for security purposes so that a user can only see specific rows and columns.

Sample Answer
A SQL Trigger is a procedural code that is automatically executed in response to certain events on a particular table or view in a database. Triggers can be defined to execute before or after INSERT, UPDATE, and DELETE operations. They are used to maintain the integrity of the data in the database and to implement business rules. For example, you might set up a trigger on a ‘Orders’ table that automatically updates the ‘Inventory’ table whenever a new order is placed. This ensures that your inventory count always reflects the most recent sales.

Sample Answer
In SQL, DELETE, TRUNCATE, and DROP are all commands used to eliminate data, but they do so in different ways and have different effects. The DELETE command is used to delete specific rows from a table based on a condition. It’s a DML command and it does not free the space containing the table. The TRUNCATE command is used to delete all rows from a table and free the space containing the table. It is a DDL command and it is faster than DELETE because it does not generate any undo logs. The DROP command is used to delete the table structure along with the data. It is also a DDL command and it removes the table schema from the database permanently.