Can a key be both primary and foreign?

Yes, foreign key has to be primary key of parent table. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table (as it is a primary key).

Can a key be both primary and foreign key?

Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. 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.

Can primary and foreign key be in the same table?

If you mean "can foreign key 'refer' to a primary key in the same table?", the answer is a firm yes as some replied.

Can a key be both primary and foreign SQL?

"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." Not used often, but useful in some circumstances.

Can an entity be a primary and foreign key?

Each entity must have a attribute or attributes, the primary key, whose values uniquely identify each instance of the entity. Every child entity must have an attribute, the foreign key, that completes the association with the parent entity.

Primary

Can attribute be PK and FK?

Yes of course you can have PK and FK at the same time. PK & FK at the same time means that you want to index on the primary key, but at the same time you want to restrict the values of the PK based on values that exist in another attribute.

Does a foreign key have to be a primary key?

Yes, foreign key has to be primary key of parent table. Yes, it may not be unique and may have duplicate entries in child table, but it must be unique and does not have any duplicate entries at the parent table (as it is a primary key).

Can a primary key be a composite key?

Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.

Can a table have 2 foreign keys?

A table can have multiple foreign keys based on the requirement.

Can a table have only a foreign key and no primary key?

Yes, you can make one without a Primary Key (or, another option is a Compound Primary Key - making the two references a unique pair, and using that as the unique identifying key - but even this isn't necessary (note: just because it "isn't necessary" doesn't mean it isn't "good practice"; it wouldn't generally be a ...

Can a table have 3 primary keys?

A table can only ever have a one primary key. It is not possible to create a table with two different primary keys. You can create a table with two different unique indexes (which are much like a primary key) but only one primary key can exist.

Can foreign key be NULL?

A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts.

Can a primary key 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.

Can foreign key be composite?

A composite foreign key is a foreign key that consists of two or more columns. It is important to note that all the columns in a single foreign key must point to the same table. In other words, it is not possible to have a foreign key that references to a column in Table 1 and a column in Table 2.

Can composite key be null?

In composite primary key columns you cannot pass null values. Each column defined as a primary key would be validated so that null values are not passed on to them. If you have given a Unique constraint then we have a chance of NULL values being accepted. But in case of primary keys they cannot hold null values.

Is composite key and foreign key same?

Composite key is a Candidate key that consists of more than one attribute. Foreign key is an attribute which is a Primary key in its parent table but is included as an attribute in the host table.

Can primary key and foreign key have different names?

A foreign key can also have different column names than the primary key. The foreign key and primary key can also have different default values. However, since values in the referenced table must be unique, default values are not much used and are rarely used for columns that are part of a primary key.

Can primary key be unique?

A Primary key is a unique key. Each table must have at most ONE primary key but it can have multiple unique key. A primary key is used to uniquely identify a table row. A primary key cannot be NULL since NULL is not a value.

Is a Superkey a candidate key?

Super Key is an attribute (or set of attributes) that is used to uniquely identifies all attributes in a relation. Candidate Key is a subset of a super key. 2. All super keys can't be candidate keys.

How many primary key A table can max have?

A table can contain only one primary key constraint. A primary key cannot exceed 16 columns and a total key length of 900 bytes.

Can a foreign key be non unique?

No, foreign keys do not have to be unique. Indeed, a lack of uniqueness is requisite for one-to-many or many-to-many relations. Show activity on this post. Foreign key(s) must reference a unique set of attributes in the referenced table.

Can we insert value in foreign key column?

Each non-null value you insert into a foreign key column must be equal to some value in the corresponding parent key of the parent table. If any column in the foreign key is null, the entire foreign key is considered null.

Can a database table exist without a 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 primary key be multiple columns?

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

What is a surrogate key in database?

A surrogate key is a unique key for an entity in the client's business or for an object in the database. Sometimes natural keys cannot be used to create a unique primary key of the table. This is when the data modeler or architect decides to use surrogate or helping keys for a table in the LDM.