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Designing Database Queries
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Microsoft SQL Server 2005 is designed to handle small to large enterprise-level databases. However, it does little good to have data in a database unless you know how to extract portions of that data. It is critical for every database developer and administrator to know how to query a SQL Server 2005 database. Lesson 1 will begin with writing database queries, which covers everything from query basics to writing functions and remote queries. Lesson 2 will move on to writing specialized queries, which involves using query hints and writing full-text queries. Lesson 3 will feature retrieving data from Extensible Markup Language (XML) sources and will cover how to write an XQuery (an XML querying language) expression.
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Exam objectives in this chapter:
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Write and modify queries.
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Write queries. Modify queries to improve query performance. Select the correct attributes. Select the correct nodes. Filter by values of attributes and values of elements. Include relational data, such as columns and variables, in the result of an XQuery expression. Include XML attribute or node values in a tabular result set. Update, insert, or delete relational data based on XML parameters to stored procedures. Debug and troubleshoot queries against XML data sources.
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Design queries for retrieving data from XML sources.
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Designing Database Queries
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Before You Begin
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To complete the lessons in this chapter, you must have:
A computer that meets or exceeds the minimum hardware requirements listed in the Introduction at the beginning of this book. SQL Server 2005 installed, as well as the SQL Server 2005 AdventureWorks sample database. Experience designing and executing queries in SQL Server Management Studio. Experience creating and working with XML data.
Lesson 1: Writing Database Queries
Lesson 1: Writing Database Queries
Estimated lesson time: 90 minutes
SQL Server 2005 provides several methods for extracting data from a database. You can use special operators to aggregate and combine data. In addition to the data available within the database, you can query remote or heterogeneous data using linked servers. This lesson will cover the different ways you can write database queries.
Write SELECT Queries
The SELECT statement is the primary means of querying a relational database. This is a Transact-SQL statement that can be simple or quite complex depending on which clauses are applied to the SELECT statement. The basic structure of this statement is as follows, in which those statements surrounded by square brackets are optional:
SELECT <comma-delimited list of expressions or column names> [INTO <table name>] FROM <tables, views, or linked servers> [WHERE <search condition(s)>] [GROUP BY <comma-delimited list of columns>] [HAVING <search condition(s)>] [ORDER BY <comma-delimited list of columns> <ASC or DESC>]
Notice that the only portions of the statement that are required are the SELECT and FROM clauses. The simplest SELECT statement would look something like the following:
SELECT * FROM Person.Contact
The previous query requested that all rows, due to the wildcard symbol (*), should be retrieved from the Person.Contact table. Most queries that you write will not be this simple, and there are many considerations that need to be made when designing complex queries.
Performing Joins
One of the most frequently used operations in a SELECT statement is a join. Because most databases are normalized, it is often necessary to retrieve data from multiple tables. Normalization is the process in which data is separated into multiple related tables. Joins enable you to create a result set that is derived from one or more tables. A join relates tables based on a key column, such as a primary key or a foreign key. You want the column specified in your join clause to contain values common to both tables. For the AdventureWorks database, you can join the Person.Contact table to the HumanResources.Employee table to retrieve an employee s title. For
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Designing Database Queries
example, the following query returns the first name and last name from the Person.Contact table and then the title from the HumanResources.Employee table:
SELECT con.FirstName, con.LastName, emp.Title FROM Person.Contact con JOIN HumanResources.Employee emp ON con.ContactID = emp.ContactID
There are several types of joins, and which one is used depends on what data needs to be returned. Which join type is used can affect the number of rows that are returned. The different join types are as follows:
INNER This is the default join type and is used if you do not specify a join type. It indicates that all matching rows from both tables should be returned. When this join type is used for the previous query 290 rows are returned. FULL In this case, you could have rows returned from either the right or left table that do not meet the join condition. When this happens, the table that does not meet the condition will return a null value for output columns. When this join type is used for the previous quer y 19,972 rows are retur ned, which is the number of rows in the Person.Contact table. LEFT In this case, you could have rows returned from the left table that do not meet the join condition. The rows from the left table will return null values. When this join type is used for the previous query, 19,972 rows are returned, which is the number of rows in the Person.Contact table. RIGHT In this case, you could have rows returned from the right table that do not meet the join condition. The rows from the right table will return null values. When this join type is used for the previous query, 290 rows are returned.
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