What is the difference between primary key and foreign key in Oracle?
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What is the difference between primary key and foreign key in Oracle?
A primary key is used to assure the value in the particular column is unique. The foreign key provides the link between the two tables.
What is difference between foreign key and primary key?
A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table.
Can primary key and foreign key be same?
Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.
What is the relationship between primary key and foreign key?
A foreign key is a column or a set of columns in one table that references the primary key columns in another table. The primary key is defined as a column (or set of columns) where each value is unique and identifies a single row of the table.
Is foreign key can be null?
Foreign keys allow key values that are all NULL , even if there are no matching PRIMARY or UNIQUE keys.
Can foreign keys be null?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.
Can primary keys be null?
A primary key defines the set of columns that uniquely identifies rows in a table. When you create a primary key constraint, none of the columns included in the primary key can have NULL constraints; that is, they must not permit NULL values.
IS null values allowed in foreign key?
What is delete rule in SQL?
The delete rule works as follows: With RESTRICT or NO ACTION, an error occurs and no rows are deleted. With CASCADE, the delete operation is propagated to the dependents of p in table D. With SET NULL, each nullable column of the foreign key of each dependent of p in table D is set to null.
Can foreign key reference multiple tables?
The FOREIGN KEY constraint is a key used to link two tables together.
