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Las claves principales y las claves externas son dos tipos de restricciones que se pueden usar para aplicar la integridad de datos en las tablas de SQL Server SQL Server. Se trata de objetos de base de datos importantes. These are important database objects.

Este tema contiene las siguientes secciones. This topic contains the following sections. Restricciones de clave principal Primary Key Constraints. Tareas relacionadas Related Tasks. A table typically has a column or combination of columns that contain values that uniquely identify each row in the table.

Estas columnas se denominan claves principales de la tabla y exigen la integridad de entidad de la tabla. This column, or columns, is called the primary key PK of the table and enforces the entity integrity of the table.

Because primary key constraints guarantee unique data, they are frequently defined on an identity column. When you specify a primary key constraint for a table, the Motor de base de datos Database Engine enforces data uniqueness by automatically creating a unique index for the primary key columns. This index also permits fast access to data when the primary key is used in queries. If a primary key constraint is defined on more than one column, values may be duplicated within one column, but each como se denominan las opciones sobre acciones of values from all the columns in the primary key constraint definition must be unique.

ProductVendor table form a composite primary key constraint for this table. This prevents the insertion of duplicate como se denominan las opciones sobre acciones. A table can contain only one primary key constraint. Una clave principal no puede superar las 16 columnas y una longitud de clave total de bytes. A primary key cannot exceed 16 columns and a total key length of bytes.

The index generated by a primary key constraint cannot cause the number of indexes on the como se denominan las opciones sobre acciones to exceed nonclustered indexes and 1 clustered index. If clustered or nonclustered is not specified for a primary key constraint, clustered is used if there no clustered index on the table.

All columns defined within a primary key constraint must be defined as not null. If nullability is not specified, all columns participating in a primary key constraint have their nullability set to not null. If a primary como se denominan las opciones sobre acciones is defined on a CLR user-defined type column, the implementation of the type must support binary ordering.

A foreign key FK is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in como se denominan las opciones sobre acciones foreign key table.

In a foreign key reference, como se denominan las opciones sobre acciones link is created between two tables when the column or columns that hold the primary key value for one table are referenced by the column or columns in another table. Esta columna se convierte en una clave externa para la segunda tabla. This column becomes a foreign key in the second table.

Por ejemplo, la tabla Sales. For example, the Sales. SalesOrderHeader table has a foreign key link to the Sales. SalesPerson table because there is a logical relationship between sales orders and salespeople. By creating this foreign key relationship, a value for SalesPersonID cannot be inserted into the SalesOrderHeader table if it does not already exist in the SalesPerson table.

A table can reference a maximum of other tables and columns as foreign keys outgoing references. SQL Server Requiere al menos el nivel de compatibilidad El aumento conlleva las siguientes restricciones: Requires at least compatibility level. The increase has the following restrictions: A table with a foreign key reference to itself is still limited to foreign key references.

Greater than foreign key references are not currently available for columnstore indexes, memory-optimized tables, Stretch Database, or partitioned foreign como se denominan las opciones sobre acciones tables. Unlike primary key constraints, creating a foreign key constraint does not automatically create a corresponding index.

However, manually creating an index on a foreign key is often useful for the following reasons: Foreign key columns are frequently used in join criteria when the data from related tables is combined in queries by matching the column or columns in the foreign key constraint of one table with the primary or unique key column or columns in the other table.

An index enables the Motor de base de datos Database Engine to quickly como se denominan las opciones sobre acciones related data in the foreign key table. However, creating this index is not required. Data from two related tables can be combined even if no primary key or foreign key constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.

Los cambios en las restricciones de clave principal se comprueban con restricciones de clave externa en las tablas relacionadas. Changes to primary key constraints are checked with foreign key constraints in related tables. Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table. Por ejemplo, si se elimina la fila de un vendedor de la tabla Sales.

SalesPerson y el identificador del vendedor se usa para pedidos de ventas en la tabla Sales. SalesOrderHeaderse rompe la integridad relacional entre ambas tablas: For example, if the row for a salesperson is deleted from the Sales. SalesPerson table, and the salesperson's ID is used for sales orders in the Sales. SalesOrderHeader table, the relational integrity between the two tables is broken; the deleted salesperson's sales orders are orphaned in the SalesOrderHeader table without a link to the data in the SalesPerson table.

A foreign key constraint prevents this situation. The constraint enforces referential integrity by guaranteeing that changes cannot be made to data in the primary key table if those changes invalidate the link to data in the foreign key table.

If an attempt is made to delete the row in a primary key table or to change a primary key value, the action will fail when the deleted or changed primary key value corresponds to a value in the foreign key constraint como se denominan las opciones sobre acciones another table. To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data.

Las restricciones de integridad referencial en cascada permiten definir las acciones que Motor de base de datos Database Engine lleva a cabo cuando un usuario intenta eliminar o actualizar una clave a la que apuntan las claves externas existentes. By using cascading referential integrity constraints, you can define the actions that the Motor de base de datos Database Engine takes when a user tries to delete or update a key to which existing foreign keys point.

Se pueden definir las acciones en cascada. The following cascading actions can be defined. The Motor de base de datos Database Engine raises an error and the delete or update action on the row in the parent table is rolled back. Corresponding rows are updated or deleted in the referencing table when that row is updated or deleted in the parent table. All the values that make up the foreign key are set to NULL when the corresponding row in the parent table is updated or deleted.

For this constraint to execute, the foreign key columns must be nullable. All the values that make up como se denominan las opciones sobre acciones foreign key are set to their default values if the corresponding row in the parent table is updated or deleted.

For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable, and there is no explicit default value set, NULL becomes the implicit default value of the column.

If there are any AFTER triggers defined on the affected tables, these triggers fire after all cascading actions are performed.

These triggers fire in opposite order of the cascading action. If there are multiple triggers on a single table, they fire in random order, unless there is a dedicated first or last trigger for the table. However, one chain always fires all its triggers before another chain starts firing. En este caso, ninguna otra tabla se ve afectada por la cascada. There como se denominan las opciones sobre acciones no other tables affected by cascading in this case. En la tabla siguiente se enumeran las tareas comunes asociadas a restricciones de clave principal y clave externa.

The following table lists the common tasks associated with primary key and foreign key constraints. Integridad referencial en cascada Cascading Referential Integrity Las restricciones de integridad referencial en cascada permiten definir las acciones que Motor de base de datos Database Engine lleva a cabo cuando un usuario intenta eliminar o actualizar una clave a la que apuntan las claves externas existentes.

Tareas relacionadas Related Tasks En la tabla como se denominan las opciones sobre acciones se enumeran las tareas comunes asociadas a restricciones de clave principal y clave externa. Describes how to create a primary key. Describes how to delete a primary key. Describes how to modify a primary key. Describes how to modify foreign key relationships.

Modificar relaciones de claves externas. Describes how to delete foreign key relationships. Eliminar relaciones entre claves externas. Describes how to view foreign key properties. Describes how to disable foreign key constraints for replication. Crear claves principales Create Primary Keys. Eliminar claves principales Delete Primary Keys. Modificar claves principales Modify Primary Keys. Crear relaciones de clave externa Create Foreign Key Relationships.

Modify Foreign Key Relationships. Delete Foreign Key Relationships. Ver las propiedades de clave externa Como se denominan las opciones sobre acciones Foreign Key Properties.

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