1. Writing SELECT Queries
20761B
3: Writing SELECT Queries
Module 3
The 20761B-MIA-SQL VM used in the lab for this module includes software services that can take a while to start. For the best experience, have students start the MSL-TMG1, 20761B-MIA-DC and 20761B-MIA- SQL VMs at the beginning of the module so that the services are ready before they start the lab.
Microsoft Azure Pass
There are demonstrations in this course that require access to Microsoft® Azure®. Make enough time for the setup and configuration of a Microsoft Azure pass.
Details of how to acquire Microsoft Azure passes are available here:
The Azure demonstration in this module requires a copy of the AdventureWorksLT database. For detailed steps on creating a copy of the AdventureWorksLT database in Azure, see D:\Creating an AdventureWorks Database on Azure.docx.
Writing SELECT Queries

2. Writing Simple CASE Expressions
20761B
Module Overview
3: Writing SELECT Queries
Writing Simple CASE Expressions

3. Lesson 1: Writing Simple SELECT Statements
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Lesson 1: Writing Simple SELECT Statements
3: Writing SELECT Queries
Demonstration: Writing Simple SELECT Statements
Question
You have a table named Sales with the following columns: Country, NumberOfReps, TotalSales.
You want to find out the average amount of sales a sales representative makes in each country. What SELECT query could you use?
Answer
SELECT Country, (TotalSales / NumberOfReps) AS AverageSalesPerRep
FROM Sales;

4. Elements of the SELECT Statement
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Elements of the SELECT Statement
3: Writing SELECT Queries
Clause
Expression
SELECT
<select list>
FROM
<table or view>
WHERE
<search condition>
GROUP BY
<group by list>
ORDER BY
<order by list>

5. Retrieving Columns from a Table or View
3: Writing SELECT Queries
Use SELECT with column list to show columns
Use FROM to specify the source table or view
Specify both schema and object names
Delimit names if necessary
End all statements with a semicolon
Keyword
Expression
SELECT
<select list>
FROM
<table or view>
SELECT companyname, country
FROM Sales.Customers;

6. Displaying all columns
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Displaying Columns
3: Writing SELECT Queries
Displaying all columns
This is not best practice in production code!
Displaying only specified columns
SELECT *
FROM Sales.Customers;
SELECT companyname, country
FROM Sales.Customers;

7. Using Calculations in the SELECT Clause
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Using Calculations in the SELECT Clause
3: Writing SELECT Queries
Calculations are scalar, returning one value per row
Using scalar expressions in the SELECT clause
Operator
Description +
Add or concatenate -
Subtract *
Multiply /
Divide %
Modulo
SELECT unitprice, qty, (qty * unitprice)
FROM Sales.OrderDetails;

8. Lesson 2: Eliminating Duplicates with DISTINCT
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Lesson 2: Eliminating Duplicates with DISTINCT
3: Writing SELECT Queries
Demonstration: Eliminating Duplicates with DISTINCT
Question
You have company departments in five countries. You have the following query for the Human Resources database:
SELECT DeptName, Country
FROM HumanResources.Departments
This returns:
DeptName Country
Sales UK
Sales USA
Sales France
Sales Japan
Marketing USA
Marketing Japan
Research USA
You add a DISTINCT keyword to the SELECT query. How many rows are returned?
Answer 7

9. SQL Sets and Duplicate Rows
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SQL Sets and Duplicate Rows
3: Writing SELECT Queries
SQL query results are not truly relational:
Rows are not guaranteed to be unique
No guaranteed order
Even unique rows in a source table can return duplicate values for some columns
SELECT country
FROM Sales.Customers;
country
Argentina
Belgium
Austria

10. Understanding DISTINCT
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Understanding DISTINCT
3: Writing SELECT Queries
DISTINCT specifies that only unique rows can appear in the result set
Removes duplicates based on column list results, not source table
Provides uniqueness across set of selected columns
Removes rows already operated on by WHERE, HAVING, and GROUP BY clauses
Some queries may improve performance by filtering out duplicates before execution of SELECT clause

11. SELECT DISTINCT Syntax
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SELECT DISTINCT Syntax
3: Writing SELECT Queries
SELECT DISTINCT <column list>
FROM <table or view>
SELECT DISTINCT companyname, country
FROM Sales.Customers;
companyname
country
Customer AHPOP UK
Customer AHXHT
Mexico
Customer AZJED
Germany
Customer BSVAR
France
Customer CCFIZ
Poland

12. Lesson 3: Using Column and Table Aliases
3: Writing SELECT Queries
Demonstration: Using Column and Table Aliases
Question
You have the following query:
SELECT FirstName LastName
FROM HumanResources.Employees;
You are surprised to find that the query returns the following:
LastName
Fred
Rosalind
Anil
Linda
What error have you made in the SELECT query?
Answer
You have omitted a comma between FirstName and LastName.

13. Use Aliases to Refer to Columns
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Use Aliases to Refer to Columns
3: Writing SELECT Queries
Column aliases using AS
Column aliases using =
Accidental column aliases
SELECT orderid, unitprice, qty AS quantity
FROM Sales.OrderDetails;
Question
Which of the following statements use correct column aliases?
SELECT Name AS ProductName FROM Production.Product
SELECT Name = ProductName FROM Production.Product
SELECT ProductName == Name FROM Production.Product
SELECT ProductName = Name FROM Production.Product
SELECT Name AS Product Name FROM Production.Product
Answer
Statements 1 and 4 are correct.
SELECT orderid, unitprice, quantity = qty
FROM Sales.OrderDetails;
SELECT orderid, unitprice quantity
FROM Sales.OrderDetails;

14. Use Aliases to Refer to Tables
3: Writing SELECT Queries
Create table aliases in the FROM clause
Create table aliases with AS
Create table aliases without AS
Using table aliases in the SELECT clause
SELECT custid, orderdate
FROM SalesOrders AS SO;
SELECT custid, orderdate
FROM SalesOrders SO;
SELECT SO.custid, SO.orderdate
FROM SalesOrders AS SO

15. The Impact of Logical Processing Order on Aliases
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The Impact of Logical Processing Order on Aliases
3: Writing SELECT Queries
FROM, WHERE, and HAVING clauses processed before SELECT
Aliases created in SELECT clause only visible to ORDER BY
Expressions aliased in SELECT clause may be repeated elsewhere in query
Note: The reason why an alias created in a SELECT clause may not be referenced elsewhere in the clause is due to the all-at-once processing implemented by SQL Server.

16. Lesson 4: Writing Simple CASE Expressions
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Lesson 4: Writing Simple CASE Expressions
3: Writing SELECT Queries
Demonstration: Simple CASE Expressions
Question
You have the following SELECT query:
SELECT FirstName, LastName, Sex
FROM HumanResources.Employees;
This returns:
FirstName LastName Sex
Maya Steele 1
Adam Brookes 0
Naomi Sharp 1
Pedro Fielder 0
Zachary Parsons 0
How could you make these results clearer?
Answer
Use the following query:
SELECT FirstName, LastName, Gender =
CASE Sex
WHEN 1 THEN ‘Female’
WHEN 0 THEN ‘Male’
ELSE ‘Unspecified’
END

17. Using CASE Expressions in SELECT Clauses
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Using CASE Expressions in SELECT Clauses
3: Writing SELECT Queries
T-SQL CASE expressions return a single (scalar) value
CASE expressions may be used in:
SELECT column list
WHERE or HAVING clauses
ORDER BY clause
CASE returns result of expression
Not a control-of-flow mechanism
In SELECT clause, CASE behaves as calculated column requiring an alias

18. Forms of CASE Expressions
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Forms of CASE Expressions
3: Writing SELECT Queries
Two forms of T-SQL CASE expressions:
Simple CASE
Compares one value to a list of possible values
Returns first match
If no match, returns value found in optional ELSE clause
If no match and no ELSE, returns NULL
Searched CASE
Evaluates a set of predicates, or logical expressions
Returns value found in THEN clause matching first expression that evaluates to TRUE