Amazon SQL, or Structured Query Language, is a programming language used to store and process information in relational databases. Users use SQL statements to update, remove, search, and retrieve information from individual databases. This article will cover some of the common questions you will likely encounter in these interviews to help you ace your upcoming interview. Let’s get started.
1. Differentiate SQL From Nosql
Structured Query Language (SQL) is used to store information in a structured manner, while NoSQL databases store data in a semi-structured or unstructured way. Additionally, SQL databases are used for complex transactions and queries, while NoSQL databases work for applications that require scalable and fast data access. Lastly, SQL databases define the data structure using a schema, while NoSQL databases do not.
2. Define What A Subquery Is
A subquery in Amazon SQL refers to a query within another. Subqueries are generally used to retrieve data to be used by the main queries. Some common clauses where subqueries are used in Amazon SQL include WHERE, SELECT, HAVING, and FROM.
3. Define What A Join Is In Amazon SQL
As the name suggests, a join is used to sum up, data from two or more tables on a database column. There are four main types of joins in Amazon SQL, namely inner join, right join, left join, and full outer join. Left joins return the rows from the left table and matching rows from the right table, while inner joins return the rows with matching values in both tables. The right join returns records from the right table and matching records from the left table, while the full outer join returns the records that match the left or right table records.
4. Differentiate A Primary Key From A Foreign Key In SQL
A primary key is a single or combination of fields and columns used to identify table rows uniquely. Primary keys accept unique values from every row. On the other hand, a foreign key is a column or group of columns in a given table that refer to the primary key of another table. Foreign keys establish a relationship between the two existing tables.
5. What Do You Understand By Clustered And Non-Clustered Index?
Clustered indexes determine how data physically appear in a table, while non-clustered indexes create separate structures to store index data. Clustered indexes are used for frequently searched or sorted columns, while non-clustered indexes work for those frequently used in queries. In simple terms, clustered indexes define the order of data in a table, generally arranging it in alphabetical order like a dictionary, while non-clustered indexes collect data at one place and record it at the other.
6. Define Normalization In SQL
Normalization is a unique process in SQL used to organize data in databases. Its main aim is to reduce data redundancy and improve data integrity. As a distinct process, normalization breaks down tables into smaller components/tables and creates relationships between the tables to reduce instances of data duplication. It eliminates unwanted characteristics such as update, deletion, and insertion anomalies. Thanks to normalization, some of the normal forms in SQL include 1NF, 2NF, 3NF, BCNF( Boyce-Codd Normal Form), 4NF, 5NF, and 6NF.
7. Define What A Key Is In SQL
Keys in SQL are values used to uniquely identify records in a given table. In Amazon SQL, a key refers to a single column or a combination of columns that uniquely identify table rows or tuples. It can identify duplicated information and create a relationship between several tables in a database. A common key in Amazon SQL is the primary key which is a single-column value that uniquely identifies a database record. It cannot be NULL, its value must be unique and rarely be changed, and the key must be given a value with the insertion of a new record.
8. What Is The Difference Between A Composite And A Foreign Key?
A composite key is a primary key composed of several columns uniquely identifying a record. On the other hand, a foreign key is used to reference the primary key of a different table and is generally used to connect the two tables. Foreign keys generally have different names from the primary keys and ensure that all the rows in a table have corresponding rows in others. They don’t have to be unique like the primary keys but can be NULL, unlike them.
9. Differentiate A Stored Procedure From A Function In SQL
A stored procedure comprises several SQL statements stored in a database. These SQL statements can be called by scripts or other programs. Stored procedures perform several operations, such as generating reports or updating a database. On the other hand, a function is a stored procedure that returns a value. There are three main functions used in SQL servers, i.e., scalar functions, which return a single value; inline table value functions, which have single TSQL statements and return a table set and multi-statement table value functions, which have several TSQL statements and return table sets.
10. Differentiate A View From A Materialized View
Views are virtual tables that are generated by queries. On the other hand, materialized views are physical tables that reveal query results. Materialized views precompute and store the results of complex queries, allowing them to improve query performance. The main difference between views and materialized views is that views are not materialized and are often dynamically computed whenever they are accessed.
11. Define A Transaction In Amazon SQL
A transaction in Amazon SQL refers to a set of SQL statements executed as one unit of work. They offer atomicity, isolation, consistency, and durability (ACID) properties that ensure data integrity. By atomicity, transactions ensure that all the statements are executed, or none gets executed. Durability ensures the permanence of all changes made by a transaction, while isolation ensures the independent execution of transactions. Lastly, consistency ensures that all the data is in a valid state before and after transactions.
12. Differentiate A Unique Key From A Primary Key In Amazon SQL
A unique key is single or several columns in a table that requires unique values and are generally used to enforce data integrity. Unique keys also offer a unique identifier for rows. However, they can’t be used to establish relationships between tables. On the other hand, primary keys are unique ones generally used to establish relationships between different tables. They enforce referential integrity and are often used as foreign keys in other tables. All in all, both keys are required for the proper functioning of Amazon SQL.
13. Define What A Schema And Deadlock Is In Amazon SQL
A schema is a group of database objects such as tables, procedures, functions, and views combined for security and logical reasons. It is generally used to organize data or objects into logical groups either by application or department. Schemas can also be used to control database object access by granting specific users or roles permission. On the other hand, a deadlock occurs when two or more transactions wait upon one another for resource release. Deadlocks, therefore, result in an unresolved circular dependency. It’s also possible that deadlocks can occur when several transactions are modifying the same data simultaneously. The Amazon SQL server automatically resolved deadlocks by stepping back one of the transactions thanks to its deadlock detection mechanism.
14. Explain What A Cursor And Trigger Are In Amazon SQL
A trigger is a unique stored procedure that automatically executes as a response to specific database events. The available triggers are generally used to audit data changes, enforce business rules or synchronize data across different tables. Triggers are used with constraints to ensure data integrity. On the other hand, cursors are database objects used to go through the results of a SELECT statement one row at a time. They generally process large data sets in stored procedures and triggers. Cursors can also be used to delete, insert or update data in tables.
15. Define What A Common Table Expression In SQL Is
Common Table Expression, popularly known as CTE, is a temporary result defined with a one-time execution of the commands SELECT, DELETE, UPDATE, INSERT, and CREATE VIEW. Common Table Expressions simplify complex queries, increasing their readability. They are also similar to views but only exist within the scope of the definitive statements.
16. Differentiate Between A Subquery And Correlated Subquery In SQL
A subquery is a select statement found within another SELECT statement used to retrieve data from single or multiple tables. They are also used in different parts of SQL statements, such as the FROM, WHERE and HAVING clauses. Another subquery function is performing aggregate functions on data subsets. On the other hand, a correlated subquery is a subquery that references columns from outside queries. Related subqueries filter results based on outer query values are generally used in the WHERE or HAVING clauses of SQL statements.
17. Differentiate Between A Self And Cross Join In Amazon SQL
Self and cross-joins are the most common joins in SQL. Cross joins are also referred to as Cartesian products. They have joined operations that return the sum value of all rows from two or multiple tables. Cross joins also generate test data for performing a join devoid of a specific join condition. A self-join is a join operation that joins a table with itself. It determines the relationships between tabular rows, such as several subjects with a common variable.
18. Define What A Window Function And Recursive Cte Are?
A window function is a function operating in a subset of the result set. It is defined by a window specification and is used to perform different calculations over several tabular rows, such as moving averages and running totals. On the other hand, a recursive common table expression references itself in the SELECT statement. They are generally used to perform hierarchical queries, for example, establishing the descendants of a given node in a tree structure.
19. Define A Pivot Table And Materialized View Log On Amazon SQL
A pivot table allows the transformation of rows into columns and performs functional aggregation on a given dataset, given that it is a data summarization tool. Thanks to their ability to analyze large datasets, common applications of pivot tables include reporting applications and business intelligence. On the other hand, a materialized view log records changes to the base table of a given materialized view. Such logs perform a fast refresh of materialized views, hence improving query functions. They also precompute and store the results of complex queries.
20. Differentiate A View And Table In Amazon SQL
Views are often used to simplify complex queries, enforce data security rules and simplify complex queries, given that they are virtual tables based on the results of an Amazon SQL query. Views also offer a means of joining several tables into a single view capable of getting queried like a regular table. On the other hand, a table is a physical storage structure that has data. Tables, therefore, carry all the data in databases, which are organized in column-and-row formats that resemble a spreadsheet, with each row having a unique record and every column representing a field in the record. An advantage of using views is that they offer an additional layer of abstraction, improving query performance and simplifying maintenance.
21. Differentiate Between Normalization And Denormalization In Amazon SQL
Normalization refers to the organization of data into databases in a way that promotes data integrity and reduces redundancy. It generally requires splitting data into several tables depending on their logical relationships and ensuring that the data is complete and consistent by defining constraints. All the levels of normalization have their own sets of rules and guidelines. On the other hand, decentralization refers to the improvement of query performance by adding redundancy to the database design. Data is therefore duplicated in several tables or columns, preventing expensive joins and reducing the database operations required for data retrieval. Denormalization can be used together with normalization to achieve both data integrity and query performance.
22. What Are The Uses Of Stored Procedures In SQL?
Stored procedures are a combination of SQL statements stored in databases executed as single units. They are generally used to reduce network traffic, encapsulate challenging logic and improve data security by regulating/controlling data access. Stored procedures are also used to implement business workflows and rules, including data validation and approval processes. Stored procedures are generally called from other SQL processes such as INSERT, UPDATE, SELECT, DELETE, or the application code.
23. Differentiate Between Self-Join And Regular Join
Self-joins and regular joins are common operations in SQL. As a join operation, a self-join links a table to itself, mostly through an alias. They are generally used to relate rows within single tables, such as variables with a common subject. On the other hand, regular joins are joined operations that join two or more tables based on a single common column. Regular joins combine data from several tables, making them equally important as the first type of join.
24. Differentiate Between An Index And A Deadlock In Amazon SQL
An index is a common database object in Amazon SQL that speeds up data retrieval by establishing a sorted copy of a given table’s data in a different location. Indexes are normally created on one or multiple tabular columns and are generally used to optimize data filtering, grouping, and sorting queries. They also ensure uniqueness and referential integrity hence improving joins performance. On the other hand, a deadlock is a situation that occurs when two or more transactions are blocked because of failure to release a needed common resource. This normally happens when two or more transactions fail to synchronize correctly or whenever they acquire resources in different orders. Some of the common effects of deadlocks include performance issues and data inconsistency. The best resolution to manual deadlocks is manual intervention.
25. How Would You Explain SQL To Someone Without Any Relevant Or Technical Knowledge?
I would tell them that SQL means Structured Query Language, which is a language that administrators use to communicate with databases. All the administrator has to do is input a query in the form of a string of commands and then search for different database values. I would ensure that my language is a simple as possible and even offer illustrations where necessary for better understanding. Lastly, I would ask questions to ensure that they fully understood my explanation. In short, I would do everything possible to ensure that I put my points across.
These are some of the most common questions to expect in an Amazon SQL interview. We hope you will walk confidently into your interview room by brainstorming the questions beforehand. Ensure you also groom well and work on your non-verbal cues before your interview to increase the chances. We wish you all the best and remember to do everything in your power to create a good first impression on the interviewer.