SQL, or Structured Query Language, is used for managing and manipulating databases. It’s something every data engineer should have a solid understanding of, given the importance of structured data in any organization. Let’s delve deeper into the SQL queries for data source queries and data transformations.
Understanding SQL Queries
SQL queries are the means by which we interact with a database. You can use SQL commands to create, modify, delete, and retrieve data from databases. In the context of AWS, you might use SQL queries when working with AWS RDS, Aurora, or Redshift. Some of the most common SQL operations include:
- SELECT: Retrieves data from a database. It is the most commonly used command in SQL to query data.
- INSERT: Adds new data to a database.
- UPDATE: Modifies existing data in a database.
- DELETE: Removes existing data from a database.
Data Source Queries
Data source queries are SQL queries used to fetch data from a single or multiple sources. An example of a simple data source query could be fetching employee information from a specific department. Here is an illustrative example,
SELECT * FROM Employees
WHERE Department = 'IT';
In this query, “*” represents all columns in the “Employees” table. If you instead wanted to select specific columns, you could replace “*” with the column names, separated by commas. “WHERE Department = ‘IT’” is the condition that filters the data.
Data Transformations
Data transformation is the process of converting data from one format or structure into another. In the context of SQL, this might involve changing data types, parsing strings, calculations, or combining data from multiple sources. For example, consider an Employees table with separate ‘firstName’ and ‘lastName’ columns. If we want to create a query that transforms these two separate entities into one entity ‘fullName’, we could use the CONCAT function:
SELECT CONCAT(firstName, ' ', lastName) AS 'fullName'
FROM Employees;
In this query, the CONCAT function is used to combine the ‘firstName’ and ‘lastName’ columns, with a space in between. The as ‘fullName’ portion of the query renames the newly created column.
SQL in AWS
AWS offers a variety of services and tools you can use to run SQL queries. Amazon RDS is a service that makes it easy to set up, operate, and scale a relational database in the cloud. Amazon Redshift, on the other hand, is a fully managed petabyte-scale data warehouse service that enables you to run quick and cost-efficient analytics.
Getting comfortable with SQL queries and how they are used in data source queries and data transformations is essential for the AWS Certified Data Engineer – Associate (DEA-C01) exam. By understanding these areas and practicing SQL in the context of AWS services, you’ll be well-equipped to handle the database and data manipulation aspects of the exam.
Remember, the concept of SQL queries goes hand in hand with the end game of data engineering – to make data more insightful and actionable for business needs. As you refine your SQL skills, you’re refining your prowess as a data engineer.
Practice Test
The SQL “SELECT DISTINCT” statement queries only unique values from the database.
- A) True
- B) False
Answer: A) True
Explanation: The “SELECT DISTINCT” statement in SQL ensures that the fetched data points are all unique and do not duplicate any existing values.
Use of the SQL “AND” command can fetch results based on two or more conditions at the same time.
- A) True
- B) False
Answer: A) True
Explanation: The SQL “AND” operator is used to display records if both (or all) of the conditions separated by AND are TRUE. It combines multiple conditions.
The SQL “ORDER BY” statement is used for sorting data in descending order only.
- A) True
- B) False
Answer: B) False
Explanation: The SQL “ORDER BY” statement sorts the data in ascending order by default. If you want to sort the results in descending order, you can use the DESC keyword.
A subquery in SQL can only be used inside the WHERE clause.
- A) True
- B) False
Answer: B) False
Explanation: A subquery can be used in the SELECT, INSERT, UPDATE, or DELETE statement or inside another subquery.
SQL “BETWEEN” operator selects values within a given range. The values can be numbers, text, or dates.
- A) True
- B) False
Answer: A) True
Explanation: The “BETWEEN” operator in SQL is used to filter the result set within a certain range. The values can be numbers, text, or dates.
SQL language contains three types of SQL statements.
- A) True
- B) False
Answer: B) False
Explanation: SQL contains four types of SQL statements: DDL, DML, DCL, and TCL.
Which of the following is not a set operator?
- A) UNION
- B) INTERSECT
- C) JOIN
- D) MINUS
Answer: C) JOIN
Explanation: UNION, INTERSECT, and MINUS are all set operators. JOIN is used to combine rows from two or more tables based on a related column.
What does the SQL “HAVING” statement do?
- A) Selects from a database
- B) Updates a database
- C) Acts like a WHERE clause but on groups
- D) None of the above
Answer: C) Acts like a WHERE clause but on groups
Explanation: HAVING is used in combination with GROUP BY to filter groups based on a certain condition.
In SQL, _________ Clause is used for sorting.
- A) SELECT
- B) UPDATE
- C) ORDER BY
- D) FROM
Answer: C) ORDER BY
Explanation: ORDER BY in SQL is used to sort the data in ascending or descending order, based on one or more columns.
What does the SQL “LIKE” operator do?
- A) Compares two strings for equality
- B) Returns TRUE if the operand is within the range
- C) Searches for a specified pattern in a column
- D) Compares a column to a specified value
Answer: C) Searches for a specified pattern in a column
Explanation: The “LIKE” operator in SQL is used within a WHERE clause to search for a specified pattern in a column.
Interview Questions
What SQL command is typically used to extract data from a database in AWS?
The SELECT command is typically used to extract data from a database in AWS.
What does the SQL COUNT function do?
The SQL COUNT function returns the number of rows that matches a specified criterion.
Can you write a SQL query to create a new table in AWS RDS?
Yes, you can create a new table in AWS RDS using the CREATE TABLE statement. Here is an example:
CREATE TABLE Employees (
ID int,
Name varchar (255),
Age int,
Address varchar (255),
Salary decimal (18, 2));How can you alter an existing AWS RDS table using SQL?
The ALTER TABLE statement is used to add, delete/drop or modify columns in an existing RDS table. For example, to add a column you can use the following SQL command:
ALTER TABLE table_name ADD column_name datatype;What is the purpose of the SQL JOIN statement?
The SQL JOIN statement is used to combine rows from two or more tables, based on a related column between them, essentially allowing you to query data from multiple tables as one source.
Can you mention a SQL command to delete a table from AWS RDS?
Yes, the SQL command to delete a table from AWS RDS is DROP TABLE followed by the name of the table:
DROP TABLE table_name;What is a subquery in SQL and where can it be used?
A subquery is a query that is embedded within the WHERE or HAVING clause of another SQL query. It can be used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.
Can you rename a column in an existing AWS RDS table using SQL? If yes, how?
Yes, you can rename a column in an AWS RDS table using SQL. You would use the ALTER TABLE statement followed by the RENAME COLUMN command. Here’s an example:
ALTER TABLE table_name RENAME COLUMN old_column_name TO new_column_name;How can you delete data from an AWS RDS table using SQL?
You can delete data from an AWS RDS table using the DELETE statement. For example:
DELETE FROM table_name WHERE condition;What is the purpose of the SQL GROUP BY statement?
The SQL GROUP BY statement groups rows that have the same values in specified columns into aggregated data, like sum, average or count.
How would you insert data into an AWS RDS table using SQL?
You can insert data into an AWS RDS table by using the INSERT INTO statement. Here is an example:
INSERT INTO table_name (column1, column2, column3) VALUES (value1, value2, value3);What is the purpose of the SQL UPDATE statement?
The SQL UPDATE statement is used to modify the existing records in a table.
What is the ORDER BY keyword used for in SQL?
The ORDER BY keyword is used in SQL to sort the result-set in ascending or descending order based on one or more columns.
How can you select distinct values from a column in AWS RDS using SQL?
You can select distinct values from a column in AWS RDS using the DISTINCT keyword. For example:
SELECT DISTINCT column_name FROM table_name;What is the purpose of the WHERE clause in SQL?
The WHERE clause in SQL is used to filter records that fulfill a specified condition.
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