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Chapter 14
Getting to First Base:Introduction to Database Concepts
Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 14-2
Tables: Special Kinds Of Lists
• A database (DB) stores data
• A relational database stores data in tables and describes the relationships among the different kinds of data
• Databaspecifys are like the lists in spreadsheets, but with added constraints– Constraints limit the kinds of data they can
contain, but expand the operations they can perform
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Entities
• Anything that can de identified by a fixed number of its characteristics (attributes)– Attributes have names and values
– The values are the data that's stored in the table
• To create a table, we specify the entity's name (table name), the names of the entity's attributes, and the kinds of values each attribute can have
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Properties of Entities
• Table Instances:
– Table is initially empty. It has a name and column headings (the attribute names) but no rows
– Each row will represent an entity
– An instance of the database table is the table with a specific set of rows
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Properties Of Entities (Cont'd)
• Structure: Content and Metadata
– We separate the structure of the information from the information itself
– A table's metadata includes its name, its attributes' names, the types of values each attribute can have, and the primary key, at a minimum
– Some properties are not metadata: order of rows and columns is unimportant
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Properties Of Entities (cont'd)
• Entities and Uniqueness
– A database table doesn't have any duplicate rows
– There must be at least one distinguishing characteristic to each entity
– Some characteristics may be the same, but not all
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Properties Of Entities (cont'd)
• Keys
– Any set of attributes for which all attributes are different is a candidate key
– Pick one and call it the primary key
– Key must distinguish all potential and actual entities, not just those that happen to be in the table at a given times
– If no combination of attributes qualify as a candidate key, assign a unique ID to each entity
• Like a student ID number issued by school
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Operations on Tables
• Main use of database is to look up information– Users specify what they want to know, and
the database finds it
• Five fundamental operations that can be performed on tables: Select, Project, Union, Difference, Product
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Operations (cont'd)
• Select Operation
– Takes rows from one table to create a new table
• Specify the table from which rows are to be taken, and the test for selection
• Syntax: Select Test From Table
• Test is applied to each rows of the table to determine if it should be included in result table
• Test uses attribute names, constants and relational operators
• If the test is true for a given row, the row is included in the result table; otherwise it is ignored
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Operations (cont'd)
• Project Operation
– Builds a new table from the columns of an existing table
– Specify name of exiting table (or tables) and the columns (field names) to be included in the new table
– Syntax:• Project Field_List From Table
– The new table will have the number of columns specified, and the same number of rows as the original table, unless
• The new table eliminates a key field. If rows duplicate in the new table, duplicates will be eliminated
Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 14-13
Operations (cont'd)
• Union Operation– Combines two tables (that have the same attributes)– Syntax:
• Table1 + Table2
• Difference Operation– Remove from one table the rows also listed in a
second table– Syntax:
• Table1 - Table2
• Product Operation– Creates a super table with all fields from both tables– Puts the rows together– Syntax:
• Table1 Table2
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Join Operation
• Combines two tables, like the Product Operation, but doesn't necessarily produce all pairings– If the two tables each have fields with a
common data type, new table combines only the rows from the given tables that match on the fields
– Syntax:• Table1 Table2 On Match
Copyright © 2006 Pearson Addison-Wesley. All rights reserved. 14-15
Join Operation (cont'd)
• Match is a comparison test involving a field from each table
• If the same field in table 1 contains the same data in the row for that field
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