c# print barcode zebra The Relational Model in Visual C#

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The Relational Model
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Referential Integrity
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A foreign key is a set of columns whose values match some key of another table in other words, a copy of a key from another relation. Foreign keys denote associations between relations; they are the glue that keeps relations in a database together. The rule foreign keys enforce can be expressed brie y: There must be no unmatched foreign keys in a database at any time. Foreign keys maintain references between relations in other words, they enforce referential integrity. The foreign key rule can be maintained during update and delete operations in different ways. In SQL Server, four possibilities exist for enforcing the foreign key rule, and each possibility consists of two pairs of rules. One pair of rules deals with the primary (parent) table, and one pair deals with the secondary (child) table. The pair of rules for the child table is immutable; the rules are always the same for all four possibilities of implementing a foreign key:
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You cannot insert a row in the child table if it has no related row in the parent table. You cannot update the foreign key columns in the child table in a way that would leave them without a related row in the parent table.
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The two rules for the parent table differ with each of the four possible implementations. The four standard possibilities and the implementation of the two rules for the parent table are the following:
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No Action implementation
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You cannot delete a row in the parent table if it has related rows in the child table. You cannot update the key columns in the parent table if they have related rows in the child table that would become orphaned.
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If you delete a row in the parent table, you have to delete all related rows in the child table. If you update a primary key in the parent table, you have to update foreign keys in all related child tables to the same new value.
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Set Null implementation
If you delete a row in the parent table, you have to set to unknown (NULL) all foreign keys of related rows in the child table. If you update a primary key in the parent table, you have to set to unknown (NULL) all foreign keys of related rows in the child table.
Set Default implementation
If you delete a row in the parent table, you have to set to a prede ned default value all foreign keys of related rows in the child table. If you update a primary key in the parent table, you have to set to a prede ned default value all foreign keys of related rows in the child table.
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In short, whatever you do, never leave rows in the child table orphaned. You would normally use the No Action implementation. You should use the Cascade implementation for deletes only in case you want to implement a strong relationship between the parent and the child table. In such a relationship the child table rows make no sense without parent rows. A classical example is orders and order line items: order line items cannot exist without an order. If you delete an order, you should delete all of its line items as well. I do not like to use Cascade updates. Cascade updates indicate that your key in the parent table is not stable, and stability is one of the desired properties of a key. The Set Null and Set Default rules are useful for maintaining history of the child table; for example, an order with unknown customer gives you information that something was ordered and when it was ordered but not who ordered it. Nevertheless, today history is commonly maintained in a data warehouse, and you usually do not need these rules. A foreign key constraint must reference a key in the parent table. The parent table can be the same as the child table; a foreign key can refer to the table itself. This is how you can represent graphs, trees, and hierarchies using the adjacency list model.
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