Concepts

Transact-SQL (T-SQL) is a powerful programming language used to interact with and manipulate data in Microsoft SQL Server. As a Data Engineer working with Microsoft Azure, it is essential to have a thorough understanding of T-SQL for transforming data to meet your business requirements. In this article, we will explore some key T-SQL techniques for data transformation.

1. SELECT Statement

The SELECT statement is the most basic form of retrieving data from a database. It allows you to specify the data you want to retrieve and how it should be presented. Here’s an example:

SELECT column1, column2
FROM table
WHERE condition;

You can use the SELECT statement to extract specific columns, apply aggregate functions (like SUM, AVG, COUNT), and filter data based on conditions.

2. JOINs

JOINs are used to combine rows from two or more tables based on related columns. They enable you to retrieve data from multiple tables with a single query. Here are some commonly used JOIN types:

  • INNER JOIN: Returns only the matched records from both tables.
  • LEFT JOIN: Returns all records from the left (first) table and the matched records from the right (second) table.
  • RIGHT JOIN: Returns all records from the right (second) table and the matched records from the left (first) table.
  • FULL JOIN: Returns all records when there is a match in either the left or right table records.

Here’s an example of an INNER JOIN:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
INNER JOIN Customers ON Orders.CustomerID = Customers.CustomerID;

3. Aggregation Functions

T-SQL provides a variety of aggregate functions to perform calculations on a set of values. Some commonly used aggregate functions include COUNT, SUM, AVG, MAX, and MIN. Here’s an example:

SELECT COUNT(OrderID) as TotalOrders, SUM(Amount) as TotalAmount
FROM Orders;

This query will return the total number of orders and the sum of the order amounts from the Orders table.

4. CASE Statement

The CASE statement allows you to perform conditional logic in your T-SQL queries. It is useful for transforming data based on certain conditions. Here’s an example:

SELECT OrderID, Quantity,
   CASE
        WHEN Quantity > 10 THEN 'High'
        WHEN Quantity <= 10 THEN 'Low'
        ELSE 'N/A'
   END AS QuantityCategory
FROM OrderDetails;

This query assigns a category (‘High’ or ‘Low’) to each order based on the quantity.

5. Common Table Expressions (CTEs)

CTEs are temporary result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. They provide a way to simplify complex queries and improve query readability. Here’s an example:

WITH CTE_TotalAmount AS (
   SELECT CustomerID, SUM(Amount) AS TotalAmount
   FROM Orders
   GROUP BY CustomerID
)
SELECT Customers.CustomerName, CTE_TotalAmount.TotalAmount
FROM Customers
JOIN CTE_TotalAmount ON Customers.CustomerID = CTE_TotalAmount.CustomerID;

In this query, the CTE_TotalAmount calculates the total order amount for each customer, which is then joined with the Customers table.

These are just a few examples of how you can use T-SQL to transform data in Microsoft Azure for the Data Engineering exam. By mastering T-SQL and its various features, you’ll be equipped to handle complex data transformation tasks efficiently.

Remember, practice is key to becoming proficient in T-SQL. The more you explore different scenarios and experiment with the language, the better you’ll become at transforming data to meet your specific requirements.

Answer the Questions in Comment Section

Which T-SQL statement is used to modify table structure in SQL Server?

a) ALTER INDEX
b) ALTER TABLE
c) ALTER PROCEDURE
d) ALTER DATABASE

Correct answer: b) ALTER TABLE

Which T-SQL statement is used to create a temporary table in SQL Server?

a) CREATE
b) INSERT
c) SELECT
d) DECLARE

Correct answer: d) DECLARE

What does the TRUNCATE TABLE statement do in T-SQL?

a) Deletes all data from a table
b) Removes a table from the database
c) Updates data in a table
d) Inserts new data into a table

Correct answer: a) Deletes all data from a table

Which T-SQL statement is used to retrieve data from multiple tables based on a condition in SQL Server?

a) SELECT
b) INSERT
c) UPDATE
d) DELETE

Correct answer: a) SELECT

What is the purpose of the GROUP BY clause in a T-SQL query?

a) Sorts the results in ascending order
b) Limits the number of rows returned
c) Combines rows into summary results
d) Filters the results based on a condition

Correct answer: c) Combines rows into summary results

Which T-SQL operator is used to combine the results of two queries into a single result set?

a) UNION
b) JOIN
c) INTERSECT
d) EXCEPT

Correct answer: a) UNION

Which T-SQL statement is used to add a new column to an existing table in SQL Server?

a) ALTER INDEX
b) ALTER TABLE
c) ALTER VIEW
d) ALTER PROCEDURE

Correct answer: b) ALTER TABLE

What is the purpose of the HAVING clause in a T-SQL query?

a) Specifies the columns to be included in the result set
b) Filters the results based on a condition
c) Orders the results in ascending or descending order
d) Groups the results based on a column

Correct answer: b) Filters the results based on a condition

Which T-SQL function is used to return the current date and time in SQL Server?

a) GETDATE()
b) CURDATE()
c) NOW()
d) SYSDATETIME()

Correct answer: a) GETDATE()

Which T-SQL statement is used to permanently remove a table from the database in SQL Server?

a) DROP INDEX
b) DROP TABLE
c) DROP VIEW
d) DROP PROCEDURE

Correct answer: b) DROP TABLE

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Scarlett Sullivan
6 months ago

Great article on transforming data using T-SQL! Very helpful for the DP-203 exam preparation.

Liposlav Magura
11 months ago

I found that using the APPLY operator has significantly improved my queries.

Patricia Mora
1 year ago

Can someone explain the difference between CROSS APPLY and OUTER APPLY?

Garance Mathieu
6 months ago

I wish there was more focus on window functions in the blog.

Tasso da Costa
1 year ago

Thanks for the article! It really helped me understand the MERGE statement better.

Gavrilo Dokić
1 year ago

How does one optimize T-SQL queries for better performance?

Juanita Day
11 months ago

The section on CTEs was concise and informative.

Rohan Prabhakaran
11 months ago

I would recommend the blog to anyone preparing for DP-203.

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