10 RDBMS Interview Questions and Answers in 2023
1. Describe the process of normalizing a database and explain the benefits of doing so.
Normalizing a database is the process of organizing data into tables in such a way that the results of using the database are always unambiguous and as intended. This is done by eliminating redundant data and ensuring data dependencies make sense.
The process of normalizing a database involves breaking down larger tables into smaller, more focused tables. This is done by identifying related data and then separating them into their own tables. For example, if a table contains data about customers and their orders, the customer data and order data should be separated into two different tables.
The benefits of normalizing a database are numerous. Normalization reduces data redundancy, which in turn reduces the amount of storage space required to store the data. It also reduces the chances of data inconsistency, as related data is stored in the same table. Normalization also makes it easier to add, modify, and delete data, as related data is stored in the same table. Finally, normalization makes it easier to query the database, as related data is stored in the same table.
2. What is the difference between a primary key and a foreign key?
A primary key is a column or set of columns in a table that uniquely identifies each row in the table. It is used to ensure data integrity and to prevent duplicate records. A primary key is usually a single column, but it can also be a combination of multiple columns.
A foreign key is a column or set of columns in a table that references the primary key of another table. It is used to establish and maintain relationships between tables. A foreign key is used to link two tables together, and it ensures that the data in the related tables is consistent. For example, if a customer table has a foreign key that references the primary key of an address table, then the customer table will always have the correct address information for each customer.
3. How do you ensure data integrity in a relational database?
Data integrity in a relational database is essential for the accuracy and consistency of data. To ensure data integrity, I use a combination of database design techniques, database constraints, and database triggers.
Database Design:
When designing a relational database, I use normalization techniques to ensure that data is stored in the most efficient and logical way. This includes breaking down data into smaller, related tables and establishing relationships between them. This helps to reduce data redundancy and ensure that data is stored in the most efficient way.
Database Constraints:
I also use database constraints to ensure data integrity. Database constraints are rules that are applied to the data in the database. These rules can include primary keys, foreign keys, unique constraints, and check constraints. Primary keys are used to uniquely identify each record in a table. Foreign keys are used to establish relationships between tables. Unique constraints are used to ensure that no two records in a table have the same value. Check constraints are used to ensure that data meets certain criteria.
Database Triggers:
Finally, I use database triggers to ensure data integrity. Database triggers are stored procedures that are executed when certain events occur in the database. These triggers can be used to enforce data integrity by performing checks on data before it is inserted or updated in the database. For example, a trigger can be used to check that a value is within a certain range before it is inserted into the database.
4. What is the purpose of a transaction log in a relational database?
The purpose of a transaction log in a relational database is to provide a record of all changes made to the database. This includes all data manipulation language (DML) statements such as INSERT, UPDATE, and DELETE, as well as data definition language (DDL) statements such as CREATE, ALTER, and DROP. The transaction log also records any changes made to the database structure, such as adding or removing columns or tables.
The transaction log is used to ensure the integrity of the database by providing a record of all changes made to the database. This allows the database to be restored to a previous state in the event of a system failure or data corruption. The transaction log also allows for the recovery of lost data, as it stores a record of all changes made to the database.
The transaction log is also used to provide a record of all transactions that have been executed in the database. This allows for the tracking of changes made to the database, as well as the ability to audit the database for any suspicious activity.
Finally, the transaction log is used to improve the performance of the database. By recording all changes made to the database, the database engine can quickly identify which changes need to be applied to the database in order to maintain consistency. This allows the database engine to quickly apply the necessary changes, resulting in improved performance.
5. Explain the concept of ACID properties in a relational database.
ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee the reliability and consistency of data in a relational database.
Atomicity ensures that all operations within a transaction are either completed or none of them are. This means that if one part of the transaction fails, the entire transaction will be rolled back and the database will remain in a consistent state.
Consistency ensures that all data in the database is valid according to the defined rules and constraints. This means that any data entered into the database must adhere to the predefined rules and constraints.
Isolation ensures that concurrent transactions do not interfere with each other. This means that each transaction is isolated from other transactions and will not be affected by any changes made by other transactions.
Durability ensures that any changes made to the database are permanent and will not be lost in the event of a system failure. This means that any changes made to the database will be persisted even if the system crashes or is shut down.
6. What is the difference between a clustered and a non-clustered index?
A clustered index is a type of index that reorders the physical order of the table and searches based on the key values. It is the most efficient type of index as it allows the database engine to both search and retrieve data in the same operation. A non-clustered index is a type of index that does not alter the physical order of the table and searches based on a different set of columns than the clustered index. Non-clustered indexes are less efficient than clustered indexes as they require two operations to search and retrieve data.
7. How do you optimize a query for better performance?
Optimizing a query for better performance involves a few different steps. First, you should analyze the query to identify any potential areas of improvement. This includes looking for redundant operations, such as unnecessary joins or subqueries, and identifying any areas where indexes can be used to improve performance.
Next, you should consider the data types used in the query. If the data types are not optimal for the query, you should consider changing them to improve performance.
You should also consider the query plan. The query plan is the set of steps the database engine takes to execute the query. By analyzing the query plan, you can identify any areas where the query can be optimized. This includes looking for inefficient joins, subqueries, or sorting operations.
Finally, you should consider the hardware and software environment in which the query is running. If the hardware or software is not optimized for the query, you should consider making changes to improve performance.
By following these steps, you can optimize a query for better performance.
8. What is the difference between a stored procedure and a trigger?
A stored procedure is a set of SQL statements that can be stored in the database and executed on demand. It is a pre-compiled set of instructions that can be used to perform a specific task. A stored procedure can accept parameters, return values, and modify data in the database.
A trigger is a special type of stored procedure that is automatically executed when a specific event occurs in the database. Triggers are used to enforce data integrity and to maintain data consistency. Triggers are usually associated with a table and are executed when a data modification operation (INSERT, UPDATE, or DELETE) is performed on the table. Triggers can also be used to perform additional tasks such as sending emails or logging data changes. Unlike stored procedures, triggers cannot accept parameters or return values.
9. How do you handle deadlocks in a relational database?
Deadlocks in a relational database can be handled in a few different ways.
The first step is to identify the deadlock. This can be done by monitoring the system for long-running queries, or by using the database's built-in deadlock detection tools. Once the deadlock has been identified, the next step is to analyze the cause of the deadlock. This can be done by examining the query plans and the locks that are being held by each transaction.
Once the cause of the deadlock has been identified, the next step is to take corrective action. This can be done by changing the query plans, or by using techniques such as lock escalation or lock timeout. Lock escalation is a technique where locks are escalated from row-level locks to table-level locks, which can reduce the number of locks held by each transaction. Lock timeout is a technique where locks are released after a certain amount of time, which can help to prevent deadlocks from occurring.
Finally, it is important to monitor the system for deadlocks and take corrective action as needed. This can be done by setting up alerts or by using the database's built-in deadlock detection tools. By taking these steps, deadlocks can be prevented and managed effectively.
10. Explain the concept of referential integrity and how it is enforced in a relational database.
Referential integrity is a concept in relational databases that ensures data integrity by maintaining the consistency of data across multiple tables. It is enforced by establishing a relationship between two tables, usually through a foreign key. A foreign key is a column in one table that references a primary key in another table. This relationship ensures that any data entered into the foreign key column must match an existing value in the primary key column.
For example, if a table contains a foreign key column that references a primary key in another table, any data entered into the foreign key column must match an existing value in the primary key column. If a value is entered that does not match an existing value, the database will reject the data and an error will be returned.
Referential integrity is enforced by the database engine and is typically implemented through constraints. Constraints are rules that are applied to the data in the database to ensure that it meets certain criteria. For example, a constraint can be used to ensure that a foreign key column must contain a value that matches an existing value in the primary key column.
Referential integrity is an important concept in relational databases as it ensures that data is consistent across multiple tables. Without it, data integrity would be compromised and the database would be vulnerable to errors and inconsistencies.