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The Relational Model: Data Relations
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A relational database table is essentially a two-dimensional matrix with rows (records) and columns (fields). A primary key is a column that uniquely identifies each row, and that means you can t store the same value in a primary-key column of any two records (so it s also a unique key). The primary key is usually a single numeric or GUID column, but it can also be composed as a set of columns, in which case it s called a composite primary key. Tables can be linked or related to one another using relationships. To set up a relationship, you need foreign keys, which are columns that store primary key values in a host table. For instance, in a one-toone relationship, a record in table A can be associated with a record in table B by A containing a foreign key to B s primary key column, or vice versa, or both, depending on the navigability of the relationship. A one-to-one relationship is rare because it makes more sense to merge the two tables, unless the foreign key is allowed to be null. On the other hand, one-to-many (1..*) relationships are ubiquitous, allowing 440
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CHAPTER 15 WORKING WITH DATA
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records in table A to be associated with multiple records in table B. For instance, a Customers table is likely to be associated with an Orders table this way, allowing each customer to have multiple orders. This can be modeled by storing a foreign key in Orders that refers to a Customers primary key. By moving two (or more) foreign keys into a separate association table, you obtain a many-to-many relationship between those tables. Referential integrity refers to the fact that only valid primary key values are allowed in a foreign-key column that is, the rows these keys reference are present in the related table.
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Choosing Your Database Engine
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Table 15-1 shows some of the most common database engines, all of which can be used from F# and .NET. Table 15-1. Common Databases
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PostgreSQL SQLite
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Description
Open source database engine Small, embeddable, zeroconfiguration SQL database engine IBM s database engine
Available From
http://postgresql.org/ www.sqlite.org/
Commercial
www-01.ibm.com/software/ data/db2/ad/dotnet.html www.firebirdsql.org/ www.mysql.com/ www.mimer.com/ www.oracle.com/
Firebird MySQL Mimer SQL Oracle
Open source Open source Commercial Commercial
Based on Borland Interbase Reliable and popular database Reliable database engine One of the most popular enterprise database engines Microsoft s main database engine
SQL Server
Commercial
www.microsoft.com/sql/ default.mspx www.microsoft.com/express/ database/default.aspx www.ianywhere.com/
SQL Server Express Sybase iAnywhere
Commercial
Free and easy-to-use version of SQL Server Mobile database engine
Commercial
Applications communicate with relational databases using Structured Query Language (SQL). Each time you create tables, create relationships, insert new records, or update or delete existing ones, you re explicitly or implicitly issuing SQL statements to the database. The examples in this chapter use a dialect of Standard SQL called Transact-SQL (T-SQL), used by SQL Server and SQL Server Express.
CHAPTER 15 WORKING WITH DATA
SQL has syntax to define the structure of a database schema (loosely speaking, a collection of data tables and their relations) and also syntax to manage the data within. These subsets of SQL are called Data Definition Language (DDL) and Data Manipulation Language (DML), respectively. The most important DDL statements are the following: CREATE/ALTER/DROP TABLE CREATE/DROP VIEW
Understanding ADO.NET
ADO.NET is the central database-access machinery in the .NET Framework, and it provides full XML support, disconnected and typed datasets, scalability, and high performance. This section gives a brief overview of the ADO.NET fundamentals. With ADO.NET, data is acquired through a connection to the database via a provider. This connection serves as a medium against which to execute a command; this can be used to fetch, update, insert, or delete data from the data store. Statements and queries are articulated as SQL text (CREATE, SELECT, UPDATE, INSERT, and DELETE statements) and are passed to the command object s constructor. When you execute these statements, you obtain data (in the case of queries) or the number of affected rows (in the case of UPDATE, INSERT, and DELETE statements). The data returned can be processed via two main mechanisms: sequentially in a read-only fashion using a DataReader object or by loading it into an in-memory representation (a DataSet object) for further disconnected processing. DataSet objects store data in a set of table objects along with metadata that describes their relationships and constraints in a fully contained model. ADO.NET 2.0 comes with four data providers: SQLClient, OleDb, Oracle, and Odbc. Table 15-2 describes them and a couple of more commonly used providers. These providers act as the main gateway to the database. Table 15-2. Common Data Providers
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