create barcode using vb.net Out-of-Line Constraints in Java

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Out-of-Line Constraints
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Figure 7-6 shows the syntax details for out-of-line constraints. This syntax is slightly different from the inline constraint syntax. In the syntax diagram, col name list refers to a comma-separated list of one or more column names. The type of constraint can be UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK. By default, constraints become active immediately, unless you specify the DISABLE option; in other words, the default option is ENABLE. The four types of constraints work as follows: UNIQUE allows you to prevent duplicate values in a column or a column combination. PRIMARY KEY and FOREIGN KEY allow you to implement entity integrity and referential integrity. See 1 for a detailed discussion of these concepts. CHECK allows you to specify any arbitrary condition as a constraint.
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Figure 7-7 shows the syntax details of a foreign key constraint reference (FK-REF in Figure 7-6).
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DATA DEFINITION, PART II
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Figure 7-6. Out-of-line constraint syntax diagram
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Figure 7-7. Foreign key reference syntax diagram You can omit the comma-separated list of column names (col name list in Figure 7-7) in the foreign key reference. In that case, the foreign key constraint automatically refers to the primary key of the referenced table.
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Tip In general, it is considered good practice to have foreign keys always refer to primary keys, although foreign keys may also reference unique keys.
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DATA DEFINITION, PART II
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To understand the ON DELETE option of the foreign key reference, consider the example of a foreign key constraint violation shown in Listing 7-3. Normally, it is impossible to remove parent (master) rows if the database still contains child (detail) rows. In Listing 7-3, we try to remove the XML course while the database still apparently contains XML course offerings. Listing 7-3. Example of a Foreign Key Constraint Violation SQL> delete from courses 2 where code = 'XML'; delete from courses * ERROR at line 1: ORA-02292: integrity constraint (BOOK.O_COURSE_FK) violated child record found SQL>
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Note Listing 7-10 shows the definition of the O_COURSE_FK constraint.
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The ON DELETE CASCADE option (see Figure 7-7) changes the behavior in such situations. The master/detail problems are solved by a cascading effect, in which, apart from the parent row, all child rows are implicitly deleted, too. The ON DELETE SET NULL option solves the same problem in a different way: the child rows are updated, rather than deleted. This approach is applicable only if the foreign key columns involved may contain null values, of course.
Inline Constraints
The inline constraint syntax is shown in Figure 7-8. There are some subtle differences from the syntax for out-of-line constraints: You don t specify column names in inline constraints, because inline constraints always belong to the column definition in which they are embedded. The foreign key constraint reference (FK-REF) is the same for both constraint types (see Figure 7-7), but you don t specify the keywords FOREIGN KEY for an inline constraint REFERENCES is enough. In the context of inline constraints, a NOT NULL constraint is allowed. In out-of-line constraints, this is impossible, unless you rewrite it as a CHECK constraint.
DATA DEFINITION, PART II
Figure 7-8. Inline constraint syntax diagram
Constraint Definitions in the Data Dictionary
Constraint definitions are stored in the data dictionary. The two most important views are USER_CONSTRAINTS and USER_CONS_COLUMNS. Listing 7-4 shows how you can produce an overview of all referential integrity constraints for the current user. Listing 7-4. Foreign Key Constraints in the Data Dictionary SQL> 2 3 4 5 6 select , , , from where table_name constraint_name status r_constraint_name as references user_constraints constraint_type = 'R'; CONSTRAINT_NAME -------------------E_MGR_FK D_MGR_FK E_DEPT_FK O_TRAIN_FK O_COURSE_FK R_OFF_FK R_ATT_FK H_DEPT_FK H_EMPNO_FK STATUS -------ENABLED ENABLED ENABLED ENABLED ENABLED ENABLED ENABLED ENABLED ENABLED REFERENCES ---------E_PK E_PK D_PK E_PK C_PK O_PK E_PK D_PK E_PK
TABLE_NAME -------------------EMPLOYEES DEPARTMENTS EMPLOYEES OFFERINGS OFFERINGS REGISTRATIONS REGISTRATIONS HISTORY HISTORY SQL>
DATA DEFINITION, PART II
Tools like Oracle Forms can use constraint definitions from the data dictionary; for example, to generate code for constraint checking in database applications. Last but not least, the Oracle optimizer uses knowledge about constraint information from the data dictionary to decide about efficient execution plans for SQL statements. To reiterate what we discussed in 1, constraints are very important, and they must be defined in the database.
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