Can foreign key references unique key?
By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.
Does foreign key have to reference primary key?
Partial foreign key reference. A foreign key must refer to an entire primary key, and not just part of it. Consider a Department table with a primary key of company_name + department_name. An Employee table should only refer to both attributes and not to department_name alone.
What is primary key and unique key?
Both Primary key and Unique Key are used to uniquely define of a row in a table. Primary Key creates a clustered index of the column whereas a Unique creates an unclustered index of the column . A Primary Key doesn’t allow NULL value , however a Unique Key does allow one NULL value .
Can a table have a foreign key without a primary key?
A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table. So in your case if you make AnotherID unique, it will be allowed.
Can a SQL table only have foreign keys?
There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a “good” table.
Can a table have only two foreign keys?
A table can have multiple foreign keys based on the requirement.
Can a table have no primary key but foreign key?
Yes, you can. But you need to delete the foreign key constraint first in the database. Without the relationship constraints, you can pretty much drop the entire table containing the primary key as there are no more dependencies.
Can a table have a foreign key and not a primary key?
A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you must specify the column names of the key explicitly.
Is it okay to have a table without primary key?
Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. The primary key of one table may also help to identify records in other tables, and be part of the second table’s primary key.
Can two tables have same primary key?
Does every table need a foreign key?
Note that foreign keys are not mandatory, and a table may have no foreign keys. Conversely, every column in a table may have a foreign key constraint.
Can a table have 2 foreign keys?
What is the difference between primary and foreign key?
Foreign key is denoted by fk.
What are primary and foreign keys?
– The column named with the table name plus ID is the primary key. – Any other column name that is suffixed with ID (has ID at the end of the name) is a foreign key. – Any column not suffixed with ID is a regular data field containing information.
What is primary foreign key?
Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. Primary Key can’t accept null values. Foreign key can accept multiple null value. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
What is a primary key?
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