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Tables
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The primary unit of storage in the relational model is the table, which is a two-dimensional structure composed of rows and columns Each row corresponds to one occurrence of the entity that the table represents, and each column corresponds to one attribute for that entity The process of mapping the entities in the conceptual design to tables in the logical design is called normalization and is covered in detail in 6 Often, an entity in the conceptual model maps to exactly one table in the logical model, but this is not always the case For reasons you will learn with the normalization process, entities are commonly split into multiple tables, and in rare cases, multiple entities can be combined into one table Figure 2-6 shows a listing of part of the Northwind Orders table You must remember that a relational table is a logical storage structure and usually does not exist in tabular form in the physical layer When the DBA assigns a table to operating system files in the physical layer (called tablespaces in most RDBMSs), it is common for multiple tables to be placed in a single tablespace However, large tables can be placed in their own tablespace or split across multiple tablespaces, and this is called partitioning This flexibility typically does not exist in personal computer-based RDBMSs such as Microsoft Access Each table must be given a unique name by the DBA who creates it The maximum length for these names varies a lot among RDBMS products, from as little as 18 characters to as many as 255 Table names should be descriptive and should reflect the name of the real-world entity they represent By convention, some DBAs always name entities in the
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2: Exploring Relational Database Components
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Figure 2-6
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Northwind Orders table (partial listing)
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singular and tables in the plural, and you will see this convention used in the Northwind database (I prefer that both be named in the singular, but obviously other learned professionals have counter opinions) The point here is that you should establish naming standards at the outset so that names are not assigned in a haphazard manner, as this leads to confusion later As a case in point, Microsoft Access permits embedded spaces in table and column names, which is counter to industry standards Moreover, Microsoft Access, Sybase ASE, and Microsoft SQL Server allow mixed-case names, such as OrderDetails, whereas Oracle, DB2, MySQL on Windows, and others force all names to be uppercase letters unless they are enclosed in double quotes Because table names such as ORDERDETAILS are not very readable, the use of an underscore to separate words, per industry standards, is a much better choice You may want to set standards that forbid the use of names with embedded spaces and names in mixed case because such names are nonstandard and make any conversion between database vendors that much more difficult
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You have mentioned both files and tablespaces Are they the same thing You can think of a tablespace as a logical file that forms a layer of abstraction between the physical and logical layers, thereby providing better logical data independence A tablespace has one or more physical files assigned to it And instead of assigning tables to physical files, you assign them to tablespaces This provides great flexibility in handling the physical files that make up the database For example, when tablespaces begin to fill up, one option the DBA has is to add another file on a different device (such as a disk drive)
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Columns and Data Types
As mentioned, each column in a relational table represents an attribute from the conceptual model The column is the smallest named unit of data that can be referenced in a relational database Each column must be assigned a unique name (within the table) and a data type A data type is a category for the format of a particular column Data types provide several valuable benefits:
Restricting the data in the column to characters that make sense for the data type (for example, all numeric digits or only valid calendar dates) Providing a set of behaviors useful to the database user For example, if you subtract a number from another number, you get a number as a result; but if you subtract a date from another date, you get a number representing the elapsed days between the two dates as a result Assisting the RDBMS in efficiently storing the column data For example, numbers can often be stored in an internal numeric format that saves space, compared with merely storing the numeric digits as a string of characters
Figure 2-7 shows the table definition of the Northwind Orders table from Microsoft Access 2007 (the same table listed in Figure 2-6) The data type for each column appears in the second column The data type names are usually self-evident, but if you find any of them confusing, you can view definitions of each in the Microsoft Access help pages
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