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02 | Advanced SELECT Statements

Brian Alderman | MCT, CEO / Founder of MicroTechPointTobias Ternstrom | Microsoft SQL Server Program Manager

Course Topics

Querying Microsoft SQL Server 2012 Jump Start01 | Introducing SQL Server 2012 SQL Server types of statements; other SQL statement elements; basic SELECT statements

02 | Advanced SELECT Statements DISTINCT, Aliases, scalar functions and CASE, using JOIN and MERGE; Filtering and sorting data, NULL values

03 | SQL Server Data Types Introduce data types, data type usage, converting data types, understanding SQL Server function types

04 | Grouping and Aggregating Data Aggregate functions, GROUP BY and HAVING clauses, subqueries; self-contained, correlated, and EXISTS; Views, inline-table valued functions, and derived tables

| Lunch Break Eat, drink, and recharge for the afternoon session

Advanced SELECT clauses (DISTINCT, aliases, CASE, and scalar functions)Query multiple tables using JOIN statementsFiltering and sorting data

Module Overview

Advanced SELECT Clauses

Understanding DISTINCTSpecifies that only unique rows can appear in the result setRemoves duplicates based on column list results, not source tableProvides uniqueness across set of selected columnsRemoves rows already operated on by WHERE, HAVING, and GROUP BY clausesSome queries may improve performance by filtering out duplicates prior to execution of SELECT clause

SELECT DISTINCT syntaxSELECT DISTINCT <column list>

FROM <table or view>

SELECT DISTINCT StoreID FROM Sales.Customer;

StoreID-------12345709021898710

Using aliases to refer to columnsColumn aliases using AS

Column aliases using =

Accidental column aliases

SELECT SalesOrderID, UnitPrice, OrderQty AS Quantity FROM Sales.SalesOrderDetail;

SELECT SalesOrderID, UnitPrice, Quantity = OrderQty FROM Sales.SalesOrderDetail;

SELECT SalesOrderID, UnitPrice Quantity FROM Sales.SalesOrderDetail;

Using aliases to refer to tablesCreate table aliases in the FROM clause using AS

Table aliases without AS

Using table aliases in the SELECT clause

SELECT SalesOrderID, ProductID FROM Sales.SalesOrderDetail AS SalesOrders;

SELECT SalesOrderID, ProductID FROM Sales.SalesOrderDetail SalesOrders;

SELECT SalesOrders.SalesOrderID, SalesOrders.ProductID FROM Sales.SalesOrderDetail AS SalesOrders;

T-SQL CASE expressionsSimple CASE

Compares one value to a list of possible values and returns first matchIf no match, returns value found in optional ELSE clauseIf no match and no ELSE, returns NULL

Searched CASEEvaluates a set of predicates, or logical expressionsReturns value found in THEN clause matching first expression that evaluates to TRUE

T-SQL CASE expressions return a single (scalar) valueCASE expressions may be used in:

SELECT column list (behaves as calculated column requiring an alias)WHERE or HAVING clausesORDER BY clause

Writing simple CASE expressions

SELECT ProductID, Name, ProductSubCategoryID, CASE ProductSubCategoryID WHEN 1 THEN 'Beverages' ELSE 'Unknown Category' ENDFROM Production.Product

Keyword Expression component

SELECT <select list>

CASE <value to compare>

WHEN <value to match>

THEN <result>

END N/A

FROM <table source>

Using basic SELECT clausesDemo

JOIN Statements

Overview of JOIN typesJOIN types in FROM clause specify the operations performed on the virtual table:

Join Type Description

Cross Combines all rows in both tables (creates Cartesian product).

Inner Starts with Cartesian product; applies filter to match rows between tables based on predicate.

Outer Starts with Cartesian product; all rows from designated table preserved, matching rows from other table retrieved. Additional NULLs inserted as placeholders.

Understanding INNER JOINSReturns only rows where a match is found in both tablesMatches rows based on attributes supplied in predicate

ON clause in SQL-92 syntax Why filter in ON clause?

Logical separation between filtering for purposes of JOIN and filtering results in WHERETypically no difference to query optimizer

If JOIN predicate operator is =, also known as equi-join

INNER JOIN SyntaxList tables in FROM Clause separated by JOIN operatorTable order does not matter, and aliases are preferred

SELECT SOH.SalesOrderID, SOH.OrderDate, SOD.ProductID, SOD.UnitPrice, SOD.OrderQtyFROM Sales.SalesOrderHeader AS SOH JOIN Sales.SalesOrderDetail AS SOD ON SOH.SalesOrderID = SOD.SalesOrderID;

FROM t1 JOIN t2 ON t1.column = t2.column

Understanding OUTER JOINSReturns all rows from one table and any matching rows from second tableOne table’s rows are “preserved”

Designated with LEFT, RIGHT, FULL keywordAll rows from preserved table output to result set

Matches from other table retrievedAdditional rows added to results for non-matched rows

NULLs added in place where attributes do not matchExample: Return all customers and for those who have placed orders, return order information. Customers without matching orders will display NULL for order details.

OUTER JOIN examplesCustomers that did not place orders:

SELECT CUST.CustomerID, CUST.StoreID, ORD.SalesOrderID, ORD.OrderDateFROM Sales.Customer AS CUSTLEFT OUTER JOIN Sales.SalesOrderHeader AS ORDON CUST.CustomerID = ORD.CustomerIDWHERE ORD.SalesOrderID IS NULL;

Understanding CROSS JOINSCombine each row from first table with each row from second tableAll possible combinations are displayed Logical foundation for inner and outer joins

INNER JOIN starts with Cartesian product, adds filterOUTER JOIN takes Cartesian output, filtered, adds back non-matching rows (with NULL placeholders)

Due to Cartesian product output, not typically a desired form of JOINSome useful exceptions:

Generating a table of numbers for testing

CROSS JOIN ExampleCreate test data by returning all combinations of two inputs:

SELECT EMP1.BusinessEntityID, EMP2.JobTitleFROM HumanResources.Employee AS EMP1 CROSS JOIN HumanResources.Employee AS EMP2;

Understanding Self-JoinsWhy use self-joins?

Compare rows in same table to each otherCreate two instances of same table in FROM clause

At least one alias required

Example: Return all employees and the name of the employee’s manager

Self-Join examplesReturn all employees with ID of employee’s manager when a manager exists (INNER JOIN):

Return all employees with ID of manager (OUTER JOIN). This will return NULL for the CEO:

SELECT EMP.EmpID, EMP.LastName, EMP.JobTitle, EMP.MgrID, MGR.LastNameFROM HR.Employees AS EMPLEFT OUTER JOIN HR.Employees AS MGR ON EMP.MgrID = MGR.EmpID ;

SELECT EMP.EmpID, EMP.LastName, EMP.Title, MGR.MgrID

FROM HumanResources.Employee AS EMPLEFT OUTER JOIN HumanResources.Employee AS MGRON EMP.MgrID = MGR.EmpID;

Using JOINS to view data from multiple tables

Demo

Filtering and Sorting Data

Using the ORDER BY clauseORDER BY sorts rows in results for presentation purposes

Use of ORDER BY guarantees the sort order of the resultLast clause to be logically processedSorts all NULLs together

ORDER BY can refer to:Columns by name, alias or ordinal position (not recommended)Columns not part of SELECT list unless DISTINCT clause specified

Declare sort order with ASC or DESC

ORDER BY clause examplesORDER BY with column names:

ORDER BY with column alias:

ORDER BY with descending order:

SELECT SalesOrderID, CustomerID, OrderDateFROM Sales.SalesOrderHeaderORDER BY OrderDate;

SELECT SalesOrderID, CustomerID, YEAR(OrderDate) AS OrderYearFROM Sales.SalesOrderHeaderORDER BY OrderYear;

SELECT SalesOrderID, CustomerID, OrderDateFROM Sales.SalesOrderHeaderORDER BY OrderDate DESC;

Filtering data in the WHERE clauseWHERE clauses use predicates

Must be expressed as logical conditionsOnly rows for which predicate evaluates to TRUE are acceptedValues of FALSE or UNKNOWN are filtered out

WHERE clause follows FROM, precedes other clausesCan’t see aliases declared in SELECT clause

Can be optimized by SQL Server to use indexes

WHERE clause syntaxFilter rows for customers in territory 6

Filter rows for orders in territories greater than or equal to 6

Filter orders within a range of dates

SELECT CustomerID, TerritoryIDFROM Sales.CustomerWHERE TerritoryID = 6;

SELECT CustomerID, TerritoryIDFROM Sales.CustomerWHERE TerritoryID >= 6;

SELECT CustomerID, TerritoryID, StoreIDFROM Sales.CustomerWHERE StoreID >= 1000 AND StoreID <= 1200;

Filtering data in the SELECT clauseTOP allows you to limit the number or percentage of rows returned Works with ORDER BY clause to limit rows by sort order

If ORDER BY list is not unique, results are not deterministic (no single correct result set)Modify ORDER BY list to ensure uniqueness, or use TOP WITH TIES

Added to SELECT clause:SELECT TOP (N) | TOP (N) Percent

With percent, number of rows rounded upSELECT TOP (N) WITH TIES

Retrieve duplicates where applicable (nondeterministic)TOP is proprietary to Microsoft SQL Server

Filtering using TOPFilter rows for customers to display top 20 TotalDue items

Filter rows for customers to display top 20 TotalDue items with ties

Filter rows for customers to display top 1% of TotalDue items

SELECT TOP (20) SalesOrderID, CustomerID, TotalDueFROM Sales.SalesOrderHeaderORDER BY TotalDue DESC;

SELECT TOP (20) WITH TIES SalesOrderID, CustomerID, TotalDueFROM Sales.SalesOrderHeaderORDER BY TotalDue DESC;

SELECT TOP (1) PERCENT SalesOrderID, CustomerID, TotalDueFROM Sales.SalesOrderHeaderORDER BY TotalDue DESC;

Handling NULL in queriesDifferent components of SQL Server handle NULL differently

Query filters (ON, WHERE, HAVING) filter out UNKNOWNsCHECK constraints accept UNKNOWNSORDER BY, DISTINCT treat NULLs as equals

Testing for NULLUse IS NULL or IS NOT NULL rather than = NULL or <> NULLSELECT CustomerID, StoreID, TerritoryIDFROM Sales.CustomerWHERE StoreID IS NULLORDER BY TerritoryID

Sorting and filtering dataDemo

SummaryThe SELECT statement requires columns specified (* all columns) and the FROM clause to identify what view or table the rows of data are being pulled from

Clauses like DISTINCT provide control over what items are returned in the result set.

Aliases are used to define the names of columns or tables being referenced in the SELECT statement

CASE statements are used for performing comparisons in a list of values and returns first match

SummaryJOINS are used to display content from multiple tables in a single result set. INNER, OUTER, CROSS, and SELF JOINS can all be used to create the desire result set

ORDER BY is used to sort the rows returned in the result set

WHERE is used to filter the rows returned in the result set

The TOP clause is used to define the number of rows returned in the result set by specifying the number of rows or a percentage of rows returned.

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