barcode generator in vb.net code project Understand Integrity Constraints in Software

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Understand Integrity Constraints
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SQL integrity constraints, which are usually referred to simply as constraints, can be divided into three categories:
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Table-related constraints A type of constraint that is defined within a table definition. The constraint can be defined as part of the column definition or as an element in the table definition. Constraints defined at the table level can apply to one or more columns. Assertions A type of constraint that is defined within an assertion definition (separate from the table definition). An assertion can be related to one or more tables. Domain constraints A type of constraint that is defined within a domain definition (separate from the table definition). A domain constraint is associated with any column that is defined within the specific domain.
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Of these three categories of constraints, table-related constraints are the most common and include the greatest number of constraint options. Table-related constraints can be divided into two subcategories: table constraints and column constraints. The constraints in both these subcategories are defined in the table definition. A column constraint is included with the column definition, and a table constraint is included as a table element, similar to the way columns are defined as table elements. ( 3 discusses table elements and column definitions.)
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Enforcing Data Integrity
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Both column constraints and table constraints support a number of different types of constraints. This is not the case for assertions and domain constraints, which are limited to only one type of constraint. Figure 4-1 provides an overview of the types of constraints that can be created. At the top of the illustration, you can see the three categories of constraints. Beneath the Table-Related Constraints category are the Column Constraints subcategory and the Table Constraints subcategory, each of which contains specific types of constraints. For example, table constraints can include unique (UNIQUE constraints and PRIMARY KEY constraints), referential (FOREIGN KEY constraints), and CHECK constraints, while column constraints can include the NOT NULL constraint as well as unique, referential, and CHECK constraints. However, domains and assertions support only CHECK constraints.
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In some places, the SQL:2006 standard uses the term table constraint to refer to both types of table-related constraints. I use the term table-related to avoid confusion.
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As Figure 4-1 shows, there are five different types of constraints: NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK. In SQL, UNIQUE constraints and PRIMARY KEY constraints are both considered unique constraints, and FOREIGN KEY constraints are considered referential constraints. The rest of the chapter is devoted to explaining what each of these constraints means and how to apply them.
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Integrity Constraints
Domain Constraints (within domain definitions) CHECK
Table-Related Constraints (within table definitions)
Assertions (within assertion definitions) CHECK
Column Constraints
Table Constraints
NOT NULL
Unique UNIQUE PRIMARY KEY
Referential FOREIGN KEY
CHECK
Unique UNIQUE PRIMARY KEY
Referential FOREIGN KEY
CHECK
Figure 4-1
Types of SQL integrity constraints
SQL: A Beginner s Guide
Use NOT NULL Constraints
In 3, I told you that null signifies that a value is undefined or not known. This is not the same as a zero, a blank, an empty string, or a default value. Instead, it indicates that a data value is absent. You can think of a null value as being a flag. (A flag is a character, number, or bit that indicates a certain fact about a column. The flag serves as a marker that designates a particular condition or existence of something.) In the case of null, if no value is provided for a column, the flag is set, indicating that the value is unknown, or null. Every column has a nullability characteristic that indicates whether the column will accept null values. By default, all columns accept null values. However, you can override the default nullability characteristic by using a NOT NULL constraint, which indicates that the column will not accept null values.
NOTE
Some RDBMSs allow you to change the default nullability of any new column you create. In addition, some systems support a NULL constraint, which you can use to designate that a column will accept null values.
The NOT NULL constraint can only be used as a column constraint. It is not supported for table constraints, assertions, or domain constraints. Implementing a NOT NULL constraint is a very straightforward process. Simply use the following syntax when creating a column definition: <column name> { <data type> | <domain> } NOT NULL For example, suppose you want to create a table named COMPACT_DISC_ARTISTS that requires three columns: ARTIST_ID, ARTIST_NAME, and PLACE_OF_BIRTH. You want to make sure that any new rows that are added to the table include a value for the ARTIST_ID column and a value for the ARTIST_NAME column. To do this, you add a NOT NULL constraint to both column definitions, as shown in the following SQL statement:
CREATE TABLE COMPACT_DISC_ARTISTS ( ARTIST_ID INT NOT NULL, ARTIST_NAME VARCHAR(60) NOT NULL, PLACE_OF_BIRTH VARCHAR(60) );
Notice that the PLACE_OF_BIRTH column does not include a NOT NULL constraint. As a result, if a value isn t supplied for this column (when a row is inserted), a null value will be inserted. (The null flag will be set.) Figure 4-2 shows how the table might look if rows were inserted that contained no value for the PLACE_OF_BIRTH column. As you can see, the ARTIST_ID and ARTIST_NAME columns do not , and cannot, contain null values. The PLACE_OF_BIRTH column, on the other hand, contains two null values.
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