Different Actions of Constraints

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\pard\keepn\s5\li720\b Update and Delete No Action\par
\pard\li720\b0 The No Action (default) option specifies that referenced key values cannot be updated or deleted if the resulting data would violate a referential integrity constraint. For example, if a primary key value is referenced by a value in the foreign key, then the referenced primary key value cannot be deleted because of the dependent data.\par
\b Delete Cascade\b0\par
A \b delete cascades\b0 when rows containing referenced key values are deleted, causing all rows in child tables with dependent foreign key values to also be deleted. For example, if a row in a parent table is deleted, and this row's primary key value is referenced by one or more foreign key values in a child table, then the rows in the child table that reference the primary key value are also deleted from the child table.\par
\b Delete Set Null\b0\par
A delete \b sets null\b0 when rows containing referenced key values are deleted, causing all rows in child tables with dependent foreign key values to set those values to null. For example, if employee_id references manager_id in the TMP table, then deleting a manager causes the rows for all employees working for that manager to have their manager_id value set to null.\par
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