SQL-ForeignKeyConstraint-Page6
👉Foreign Key Constraint
Foreign Key represent relationships between tables.
A foreign key is a column or group of columns whose values are derived from the primary key or unique key of some other table.
✅The table in which the foreign key is defined is called Foreign table or Detail table.
✅The table that defines the primary or unique key and is referenced by the foreign key is called the Primary table or Master table.
👉Features
1)Master table cannot be updated if child records exists.
2)Records cannot be inserted or deleted into a detail table if corresponding records in the master table do not exist.
3)Records of the master table cannot be deleted if corresponding records in the detail table already exist.
👉On Delete Cascade
If this option is set, a delete operation in the master table will trigger a Delete operation for corresponding records in all detail tables.
If the On Delete Set Null option is set, a Delete operation in the Master table will set the value held by the foreign key of Detail tables to null. Refer Fig 1 as shown below.
👉Foreign Key at column level
👉Foreign Key at table level
👉Naming Constraints
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