4/12/2023 0 Comments Sqlitemanager foreign key![]() ![]() The first table will act as the parent table with the ID column defined as a PRIMARY KEY column. We will create two new tables to understand the FOREIGN KEY constraint functionality. ![]() You can add the FOREIGN KEY constraint while defining the column using the CREATE TABLE T-SQL statement, or add it after the table creation using the ALTER TABLE T-SQL statement. SET DEFAULT: Using the SET DEFAULT option of the ON UPDATE and ON DELETE clauses specifies that, if the referenced values in the parent table are updated or deleted, the related values in the child table with FOREIGN KEY columns will be set to its default value.SET NULL: With this ON UPDATE and ON DELETE clauses option, if the referenced values in the parent table are deleted or modified, all related values in the child table are set to NULL value.For example, if the referenced value is deleted in the parent table, all related rows in the child table are also deleted. CASCADE: Setting the ON UPDATE or ON DELETE clauses to CASCADE, the same action performed on the referenced values of the parent table will be reflected to the related values in the child table.NO ACTION: When the ON UPDATE or ON DELETE clauses are set to NO ACTION, the performed update or delete operation in the parent table will fail with an error.The supported actions that can be taken when deleting or updating the parent table’s values include: The FOREIGN KEY constraint provides you also with the ability to control what action will be taken when the referenced value in the parent table is updated or deleted, using the ON UPDATE and ON DELETE clauses. Another difference is that the FOREIGN KEY allows inserting NULL values if there is no NOT NULL constraint defined on this key, but the PRIMARY KEY does not accept NULLs. The FOREIGN KEY constraint differs from the PRIMARY KEY constraint in that, you can create only one PRIMARY KEY per each table, with the ability to create multiple FOREIGN KEY constraints in each table by referencing multiple parent table. The entity integrity enforced by the PRIMARY KEY and the referential integrity enforced by the FOREIGN KEY together form the key integrity. Recall that the PRIMARY KEY constraint guarantees that no NULL or duplicate values for the selected column or columns will be inserted into that table, enforcing the entity integrity for that table. Referential integrity ensures that the relationship between the database tables is preserved during the data insertion process. In this way, the FOREIGN KEY constraint, in the child table that references the PRIMARY KEY in the parent table, will enforce database referential integrity. ![]() The relationship between the child and the parent tables is maintained by checking the existence of the child table FOREIGN KEY values in the referenced parent table’s PRIMARY KEY before inserting these values into the child table. The FOREIGN KEY constraint identifies the relationships between the database tables by referencing a column, or set of columns, in the Child table that contains the foreign key, to the PRIMARY KEY column or set of columns, in the Parent table. In this article, we will discuss the other three constraints FOREIGN KEY, CHECK and DEFAULT by describing each one briefly and providing practical examples.Ī Foreign Key is a database key that is used to link two tables together. In the previous article Commonly used SQL Server Constraints: NOT NULL, UNIQUE and PRIMARY KEY, we described, in detail, the first three types of the SQL Server constraints NOT NULL, UNIQUE and PRIMARY KEY. ![]()
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