Get Started with Transact-SQL

In this exercise, you will use some basic SELECT queries to retrieve data from the AdventureWorks database.

Note: This exercise assumes you have created the sample AdventureWorks database.

Use SELECT queries to retrieve data

The SELECT statement is the primary Transact-SQL statement used to query tables in a database.

Open a query editor for your AdventureWorks database, and create a new query.
In the query editor, enter the following code:
```
 SELECT * FROM SalesLT.Product;
```
Run the query, and and after a few seconds, review the results, which includes all columns for all products.

In the query editor, modify the query as follows:

 SELECT Name, StandardCost, ListPrice
 FROM SalesLT.Product;

Re-run the query, and and after a few seconds, review the results, which this time include only the Name, StandardCost, and ListPrice columns for all products.
Modify the query as shown below to include an expression that results in a calculated column, and then re-run the query:
```
 SELECT Name, ListPrice - StandardCost
 FROM SalesLT.Product;
```
Note that the results this time include the Name column and an unnamed numeric column containing the result of subtracting the StandardCost from the ListPrice.
Modify the query as shown below to assign names to the columns in the results, and then re-run the query.
```
 SELECT Name AS ProductName, ListPrice - StandardCost AS Markup
 FROM SalesLT.Product;
```
Note that the results now include columns named ProductName and Markup. The AS keyword has been used to assign an alias for each column in the results.
Replace the existing query with the following code, which also includes an expression that produces a calculated column in the results:
```
 SELECT ProductNumber, Color, Size, Color + ', ' + Size AS ProductDetails
 FROM SalesLT.Product;
```
Run the query, and note that the + operator in the calculated ProductDetails column is used to concatenate the Color and Size column values (with a literal comma between them). The behavior of this operator is determined by the data types of the columns - had they been numeric values, the + operator would have added them. Note also that some results are NULL - we’ll explore NULL values later in this lab.

Work with data types

As you just saw, columns in a table are defined as specific data types, which affects the operations you can perform on them.

Replace the existing query with the following code, and run it:

 SELECT ProductID + ': ' + Name AS ProductName
 FROM SalesLT.Product;

Note that this query returns an error. The + operator can be used to concatenate text-based values, or add numeric values; but in this case there’s one numeric value (ProductID) and one text-based value (Name), so it’s unclear how the operator should be applied.

Modify the query as follows, and re-run it:

 SELECT CAST(ProductID AS varchar(5)) + ': ' + Name AS ProductName
 FROM SalesLT.Product;

Note that the effect of the CAST function is to change the numeric ProductID column into a varchar (variable-length character data) value that can be concatenated with other text-based values.
Modify the query to replace the CAST function with a CONVERT function as shown below, and then re-run it:
```
 SELECT CONVERT(varchar(5), ProductID) + ': ' + Name AS ProductName
 FROM SalesLT.Product; 
```
Note that the results of using CONVERT are the same as for CAST. The CAST function is an ANSI standard part of the SQL language that is available in most database systems, while CONVERT is a SQL Server specific function.
Another key difference between the two functions is that CONVERT includes an additional parameter that can be useful for formatting date and time values when converting them to text-based data. For example, replace the existing query with the following code and run it.
```
 SELECT SellStartDate,
    CONVERT(nvarchar(30), SellStartDate) AS ConvertedDate,
     CONVERT(nvarchar(30), SellStartDate, 126) AS ISO8601FormatDate
 FROM SalesLT.Product; 
```

Replace the existing query with the following code, and run it.

 SELECT Name, CAST(Size AS Integer) AS NumericSize
 FROM SalesLT.Product;

Note that an error is returned because some Size values are not numeric (for example, some item sizes are indicated as S, M, or L).

Modify the query to use a TRY_CAST function, as shown here.

SELECT Name, TRY_CAST(Size AS Integer) AS NumericSize
FROM SalesLT.Product;

Run the query and note that the numeric Size values are converted successfully to integers, but that non-numeric sizes are returned as NULL.

Handle NULL values

We’ve seen some examples of queries that return NULL values. NULL is generally used to denote a value that is unknown. Note that this is not the same as saying the value is none - that would imply that you know that the value is zero or an empty string!

Modify the existing query as shown here:

 SELECT Name, ISNULL(TRY_CAST(Size AS Integer),0) AS NumericSize
 FROM SalesLT.Product;

Run the query and view the results. Note that the ISNULL function replaces NULL values with the specified value, so in this case, sizes that are not numeric (and therefore can’t be converted to integers) are returned as 0.

In this example, the ISNULL function is applied to the output of the inner TRY_CAST function, but you can also use it to deal with NULL values in the source table.
Replace the query with the following code to handle NULL values for Color and Size values in the source table:
```
 SELECT ProductNumber, ISNULL(Color, '') + ', ' + ISNULL(Size, '') AS ProductDetails
 FROM SalesLT.Product;
```
The ISNULL function replaces NULL values with a specified literal value. Sometimes, you may want to achieve the opposite result by replacing an explicit value with NULL. To do this, you can use the NULLIF function.
Try the following query, which replaces the Color value “Multi” to NULL.
```
 SELECT Name, NULLIF(Color, 'Multi') AS SingleColor
 FROM SalesLT.Product;
```
In some scenarios, you might want to compare multiple columns and find the first one that isn’t NULL. For example, suppose you want to track the status of a product’s availability based on the dates recorded when it was first offered for sale or removed from sale. A product that is currently available will have a SellStartDate, but the SellEndDate value will be NULL. When a product is no longer sold, a date is entered in its SellEndDate column. To find the first non-NULL column, you can use the COALESCE function.
Use the following query to find the first non-NULL date for product selling status.
```
 SELECT Name, COALESCE(SellEndDate, SellStartDate) AS StatusLastUpdated
 FROM SalesLT.Product;
```
The previous query returns the last date on which the product selling status was updated, but doesn’t actually tell us the sales status itself. To determine that, we’ll need to check the dates to see if the SellEndDate is NULL. To do this, you can use a CASE expression in the SELECT clause to check for NULL SellEndDate values. The CASE expression has two variants: a simple CASE that evaluates a specific column or value, or a searched CASE that evaluates one or more expressions.

In this example, our CASE expression must determine if the SellEndDate column is NULL. Typically, when you are trying to check the value of a column you can use the = operator; for example the predicate SellEndDate = ‘01/01/2005’ returns True if the SellEndDate value is 01/01/2005, and False otherwise. However, when dealing with NULL values, the default behavior may not be what you expect. Remember that NULL actually means unknown, so using the = operator to compare two unknown values always results in a value of NULL - semantically, it’s impossible to know if one unknown value is the same as another. To check to see if a value is NULL, you must use the IS NULL predicate; and conversely to check that a value is not NULL you can use the IS NOT NULL predicate.
Run the following query, which includes searched CASE that uses an IS NULL expression to check for NULL SellEndDate values.
```
 SELECT Name,
     CASE
         WHEN SellEndDate IS NULL THEN 'Currently for sale'
         ELSE 'No longer available'
     END AS SalesStatus
 FROM SalesLT.Product;
```
The previous query used a searched CASE expression, which begins with a CASE keyword, and includes one or more WHEN…THEN expressions with the values and predicates to be checked. An ELSE expression provides a value to use if none of the WHEN conditions are matched, and the END keyword denotes the end of the CASE expression, which is aliased to a column name for the result using an AS expression.

In some queries, it’s more appropriate to use a simple CASE expression that applies multiple WHERE…THEN predictes to the same value.

Run the following query to see an example of a simple CASE expression that produced different results depending on the Size column value.

 SELECT Name,
     CASE Size
         WHEN 'S' THEN 'Small'
         WHEN 'M' THEN 'Medium'
         WHEN 'L' THEN 'Large'
         WHEN 'XL' THEN 'Extra-Large'
         ELSE ISNULL(Size, 'n/a')
     END AS ProductSize
 FROM SalesLT.Product;

Review the query results and note that the ProductSize column contains the text-based description of the size for S, M, L, and XL sizes; the measurement value for numeric sizes, and n/a for any other sizes values.

Challenges

Now that you’ve seen some examples of SELECT statements that retrieve data from a table, it’s time to try to compose some queries of your own.

Tip: Try to determine the appropriate queries for yourself. If you get stuck, suggested answers are provided at the end of this lab.

Challenge 1: Retrieve customer data

Adventure Works Cycles sells directly to retailers, who then sell products to consumers. Each retailer that is an Adventure Works customer has provided a named contact for all communication from Adventure Works. The sales manager at Adventure Works has asked you to generate some reports containing details of the company’s customers to support a direct sales campaign.

Retrieve customer details
- Familiarize yourself with the SalesLT.Customer table by writing a Transact-SQL query that retrieves all columns for all customers.
Retrieve customer name data
- Create a list of all customer contact names that includes the title, first name, middle name (if any), last name, and suffix (if any) of all customers.
Retrieve customer names and phone numbers
- Each customer has an assigned salesperson. You must write a query to create a call sheet that lists:
  - The salesperson
  - A column named CustomerName that displays how the customer contact should be greeted (for example, Mr Smith)
  - The customer’s phone number.

Challenge 2: Retrieve customer order data

As you continue to work with the Adventure Works customer data, you must create queries for reports that have been requested by the sales team.

Retrieve a list of customer companies
- You have been asked to provide a list of all customer companies in the format Customer ID : Company Name - for example, 78: Preferred Bikes.
Retrieve a list of sales order revisions
- The SalesLT.SalesOrderHeader table contains records of sales orders. You have been asked to retrieve data for a report that shows:
  - The purchase order number and revision number in the format * ()* – for example *PO348186287 (2)*.
  - The order date converted to ANSI standard 102 format (yyyy.mm.dd – for example 2015.01.31).

Challenge 3: Retrieve customer contact details

Some records in the database include missing or unknown values that are returned as NULL. You must create some queries that handle these NULL values appropriately.

Retrieve customer contact names with middle names if known
- You have been asked to write a query that returns a list of customer names. The list must consist of a single column in the format first last (for example Keith Harris) if the middle name is unknown, or first middle last (for example Jane M. Gates) if a middle name is known.
Retrieve primary contact details
- Customers may provide Adventure Works with an email address, a phone number, or both. If an email address is available, then it should be used as the primary contact method; if not, then the phone number should be used. You must write a query that returns a list of customer IDs in one column, and a second column named PrimaryContact that contains the email address if known, and otherwise the phone number.
  
  IMPORTANT: In the sample data provided, there are no customer records without an email address. Therefore, to verify that your query works as expected, run the following UPDATE statement to remove some existing email addresses before creating your query:
```
  UPDATE SalesLT.Customer
  SET EmailAddress = NULL
  WHERE CustomerID % 7 = 1;
```
Retrieve shipping status
- You have been asked to create a query that returns a list of sales order IDs and order dates with a column named ShippingStatus that contains the text Shipped for orders with a known ship date, and Awaiting Shipment for orders with no ship date.
  
  IMPORTANT: In the sample data provided, there are no sales order header records without a ship date. Therefore, to verify that your query works as expected, run the following UPDATE statement to remove some existing ship dates before creating your query.
```
  UPDATE SalesLT.SalesOrderHeader
  SET ShipDate = NULL
  WHERE SalesOrderID > 71899;
```

Challenge Solutions

This section contains suggested solutions for the challenge queries.

Challenge 1

Retrieve customer details:
```
 SELECT * FROM SalesLT.Customer;
```

Retrieve customer name data:

 SELECT Title, FirstName, MiddleName, LastName, Suffix
 FROM SalesLT.Customer;

Retrieve customer names and phone numbers:

 SELECT Salesperson, ISNULL(Title,'') + ' ' + LastName AS CustomerName, Phone
 FROM SalesLT.Customer;

Challenge 2

Retrieve a list of customer companies:

 SELECT CAST(CustomerID AS varchar) + ': ' + CompanyName AS CustomerCompany
 FROM SalesLT.Customer;

Retrieve a list of sales order revisions:

 SELECT PurchaseOrderNumber + ' (' + STR(RevisionNumber, 1) + ')' AS OrderRevision,
    CONVERT(nvarchar(30), OrderDate, 102) AS OrderDate
 FROM SalesLT.SalesOrderHeader;

Challenge 3

Retrieve customer contact names with middle names if known:

 SELECT FirstName + ' ' + ISNULL(MiddleName + ' ', '') + LastName AS CustomerName
 FROM SalesLT.Customer;

Retrieve primary contact details:

 SELECT CustomerID, COALESCE(EmailAddress, Phone) AS PrimaryContact
 FROM SalesLT.Customer;

Retrieve shipping status:

 SELECT SalesOrderID, OrderDate,
     CASE
         WHEN ShipDate IS NULL THEN 'Awaiting Shipment'
         ELSE 'Shipped'
     END AS ShippingStatus
 FROM SalesLT.SalesOrderHeader;