IMS 4212: Data Modeling—Attributes and Domains 1 Dr. Lawrence West, Management Dept., University of Central Florida [email protected] Attributes and Domains

IMS 4212: Data Modeling—Attributes and Domains

1Dr. Lawrence West, Management Dept., University of Central [email protected]

Attributes and Domains

• Attribute Data Types

– Numeric Data Types

– Text Data Types

– Date Data Types

– Other Data Types

• Domain Constraints



Non-key Attributes

• Previous discussion has focused on different kinds of attributes

– Primary keys

– Foreign keys

– Alternate keys (candidate keys)

• We have implied and sometimes illustrated other attributes

– Attributes that just contain information we want to record about an entity

– Do not serve as any sort of key



Non-key Attributes (cont.)

• Four major issues with non-key attributes

– Identifying descriptive attributes

– Placing attributes in the correct entity

• Rules of normalization

• Appropriate functional dependency

– Identifying the appropriate data type for the attribute (field)

– Specifying appropriate domain business rules for the attribute (field)



Attribute Data Types

• When attributes are instantiated in a database they become fields

• Fields must have a data type

– Similar to variable type in programming

– (Variants are not allowed)

• Data type specifies:

– Class of data (numeric, text, date, binary, other)

– Size of data within the class



Attribute Data Types (cont.)

• A field’s type supports processing and storage efficiency

– Storage

• The number 123 can be represented with one byte

• The text “123” requires three bytes

– Processing

• “123” must be converted to a number before it can be used in an arithmetic calculation

• The number 123 can be used directly in an arithmetic calculation




• Many DB operations require comparisons of values

• Computers perform numeric operations (comparisons and arithmetic) much more efficiently than textual comparisons or text operations

• But a tremendous amount of useful business data is textual

• Date values are stored and compared as numbers but displayed as conventional dates

• Choose the appropriate data type for your attribute




• Some special cases

– Leading zeros in numbers

• ‘Numeric’ fields such as zip codes and social security numbers can start with zeros

• Storing them as numbers will strip off the leading zeros

• Store as text to retain leading zeros

• (And you will rarely perform arithmetic on them)

– International implications

• Many countries use letters in their postal (zip) codes



Numeric Data Types in SQL Server 2000

• Integers

– bigint Integers from -2^63 to 2^63 – 1 (8 bytes)

– int Integers from -2^31 to 2^31 – 1 (4 bytes)

– smallint Integers from -2^15 to 2^15 – 1 (2 bytes)

– tinyint Integers from 0 to 255 (1 byte)

• Note: only positive for tinyint

– bit 0 or 1 only

• Used for boolean values

• No True/False data type in SQL Server



Numeric Data Types in SQL Server 2000 (cont.)

• Decimal numeric values

– Money and SmallMoney are the workhorses of business programming

– money

• ± 2^63 (~ ± 922 trillion)

• decimal point fixed to four digits (1/10,000)

• no monetary formatting ($ or commas)

– smallmoney +/- 214,748.3648 fixed to four places




• Decimal numeric values– Decimal or Numeric types are equivalent– Allow specifying large values of decimal precision– approximate range +/- 10^38 – decimal point can be anywhere in the value– Decimal(p, s)

• p=1-38: Number of digits represented• s<p: Digits right of decimal place• p affects storage size—examples:

– p<=9 5 Bytes– 29 <= p <=38 17 Bytes




• Decimal numeric values (cont.)

– Float and Real

• Values stored in scientific notation

• Float(n) where n=number of digits in coefficient

• These are very imprecise

• Not commonly used in business



Numeric Precision

• The type of numeric field also limits the size of calculated results on that field

• You might choose tinyint for a quantity field in a retail application

• If all quantities are summed and the result exceeds 255 the result will be limited to 255

• There is a hierarchy of precedence for data type conversions

– E.g., Money_value * SmallMoney_value gives a Money_value

– TinyInt + SmallInt gives a SmallInt



Numeric Precision (cont.)

• Date Precedence

– DateTime

– SmallDateTime

• Character Precedence

– nText

– Text

– nVarChar

– VarChar

– Char

• Numeric Precedence

– Float

– Real

– Decimal

– Money

– SmallMoney

– BigInt

– Int

– SmallInt

– TinyInt

– Bit



Numeric Precision (cont.)

Uncomment each pair of SET/SELECT statements and see results

DECLARE @SmallIntOne SmallInt,@SmallIntTwo SmallInt,@SmallIntResultOne SmallInt,@IntResultOne Int

SET @SmallIntOne = 1000SET @SmallIntTwo = 1000

--SET @SmallIntResultOne = @SmallIntOne * @SmallIntTwo--SELECT @SmallIntResultOne

--SET @IntResultOne = @SmallIntOne * @SmallIntTwo--SELECT @IntResultOne

SET @IntResultOne = CAST(@SmallIntOne AS int) * @SmallIntTwoSELECT @IntResultOne




• Identity attributes

– int and bigint attributes can be identity attributes

– The field’s value for the first record will be automatically set to the identity seed value

• I usually set to 1,000 (int) or 10,000 (bigint)

– Each new record will get a new value incremented by the identity increment

• Almost always one

– Widely used for PKs

– Foreign Keys will be the same data type but will not be an identity attribute



Numeric Data Types (cont.)

• BigInt ± 9,223,372,036,854,775,808

• Int ± 2,147,483,648

• SmallInt ± 32,768

• TinyInt + 0-255 (positive only)

• Decimal ± 10^38

• Money ± 922,337,203,685,477.5807

• SmallMoney +/- 214,748.3648



Date Data Types in SQL Server 2000

• datetime

– January 1, 1753 to December 31, 9999

– Accurate to 3 hundredths of a second

• smalldatetime

– January 1, 1900 to June 6, 2079

– Accurate to one minute

– Remember the year 2000 problem

• Required time precision (rather than maximum date allowed) may determine datatype chosen

• Use “___Date” or “___Time” in naming to indicate stored precision



Text Data Types in SQL Server 2000

• Unicode vs. nonUnicode

– Unicode allows the extended character set needed to represent many foreign, especially Oriental, languages

– Unicode uses two bytes per character

– nonUnicode limited to 255 characters and supports most European languages

• Text/character fields are specified with their maximum length



Text Data Types in SQL Server 2000

• char(n) up to 8,000 characters

– Fixed length text field n characters long

– Shorter values will be padded with spaces

– Wasted space but efficient processing

– May require trimming when retrieving

• varchar(n) up to 8,000 characters

– variable length text field n characters long

• text(n) up to 2,147,483,647 characters

– Variable length text field allows for large text fields

– May not be searched with SQL comparisons



Text Data Types in SQL Server 2000 (cont.)

• nchar(n), nvarchar, ntext are Unicode versions of char, varchar, and text data types

• Don’t be stingy declaring text fields but don’t be wasteful, either

– char data types can blow up database size quickly

– varchar and text fields can require additional processing

• Avoid searching within long text values unless necessary

• Full text indexing is available in SQL Server and other databases



Other Data Types in SQL Server 2000

• binary, varbinary, and image are three different data types that store binary values

– images

– formatted documents such as Excel or Word documents

– Avoid storing binary data in the database

• uniqueidentifier is a field that automatically creates a value guaranteed to be unique anywhere in the world

• See text for info on cursor, rowversion, and sql_variant

– Don’t even think about using sql_variant for normal business data design



Data Type Considerations

• The attribute data types are also the same datatypes available for use in stored procedures

– Parameters

– Variables

• Ensure thatvariable/parameterdata type is consistent with its use

– Will hold required value

– Is consistent with field datatypes with which it is compared

DECLARE @SmallIntOne SmallInt,@SmallIntTwo SmallInt,@SmallIntResultOne SmallInt,@IntResultOne Int



Domain Constraints

• Domain constraints are critical limitations on the values that attributes can take on

– Is a value required?

• Always for PK attributes

• What about other attributes?

– Is value numeric, textual, boolean, date?

– Are there ranges the value must be in or must not be in?

– Are there limited choices for the value (e.g., subtype identifier codes, state abbreviations)

– Other examples



Domain Constraints (cont.)

• Some domain constraints are simple

– Last names may not be null and must be alphabetic characters between two and 30 characters in length

• Some domain constraints can be more complex

– Social security numbers will be recorded as character strings with three numeric characters, a dash, two numeric characters, a dash, and four numeric characters and will be exactly eleven characters long




• Many domain constraints contain ranges

– Minimum or maximum values

• Numbers

• Dates

– Price must be greater than zero and less than 10,000

– Membership date must be after 6/6/55

• Domain constraints may specify a default value

– Quantity defaults to 1

– Purchase date defaults to current date




• Some domain constraints can be expressed in terms of calculated values when calculations refer to system values

– Birth date must be more than 21 years earlier than today

• More complex calculations to enforce business rules are implemented as operational constraints (triggers)

– Inventory quantity on hand is the sum of all items received less the sum of all items sold less the sum of all inventory adjustments

– Covered in the next lesson



Domain Constraints

• Identifier Attributes (Primary Keys) will have special domain constraints

– Cannot be null

– Single attribute identifiers must be unique

– Composite identifiers must be unique (though each component attribute will not be)

• Some other attributes or sets of attributes may also be set to be unique

– SSN and driver's license number in the same entity

– Use an Index required to be unique to implement




• Two ways to implement domain constraints

– Specify each aspect of the domain for each attribute

– Specify named domains (rules)

• A "Last Name" domain may have all the rules for last name attributes

• Different entities with last name attributes would use the named domain for those attributesE.g., Salesrep, Employee, Customer, Supplier

• DBMS must support named domains (called rules in SQL Server)

• Bind rule to a column

Documents

IMS 4212: Data Modeling—Attributes and Domains 1 Dr. Lawrence West, Management Dept., University of Central Florida [email protected] Attributes and Domains