Question: How set multiple foreign key in SQL Server?

How set multiple foreign key in SQL?

You can use the FOREIGN KEY REFERENCES constraint to implement a foreign key relationship in SQL Server. Specify the table name. Then specify in parenthesis the column name for the foreign key to reference it.

Can we have multiple foreign keys in a table in SQL Server?

A Foreign Key is a database key that is used to link two tables together. … 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.

How do you add multiple foreign key constraints?

3 Answers. The second code block from your question: ALTER TABLE ORDERS ADD FOREIGN KEY (customer_sid) REFERENCES CUSTOMER(SID), ADD FOREIGN KEY (customer_sid2) REFERENCES CUSTOMER(SID2); will take care of what you are trying to do.

Can we make 2 foreign keys in a table?

A table can have multiple foreign keys and no composite keys. A composite key simply means that there are two or more columns making up the key value.

THIS IS IMPORTANT:  How do I limit rows in Oracle SQL query?

Can foreign key be duplicate?

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 foreign keys 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. … A foreign key value is null if any part is null.

Can a table have multiple unique keys?

A table can have more than one unique key unlike primary key. Unique key constraints can accept only one NULL value for column. Unique constraints are also referenced by the foreign key of another table.

Does foreign key have to be primary key?

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.

Can a primary key be a foreign key?

Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).

THIS IS IMPORTANT:  Your question: How disconnect MySQL database in PHP?

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. … NULL values are not allowed.

Can a table have no 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.

Which key accepts multiple NULL values?

Which key accepts multiple NULL values? Explanation: A foreign key is a key used to link two tables together. This is sometimes called a referencing key. Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.

Does a foreign key have to be unique?

3 Answers. 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.

Categories PHP