Can We Delete Foreign Key Data?

Can we delete foreign key?

We can remove FOREIGN KEY constraint from a column of an existing table by using DROP keyword along with ALTER TABLE statement..

How do I delete a foreign key constraint in SQL Server?

So, you need to DELETE related rows from conflicted tables or more logical to UPDATE their FOREIGN KEY column to reference other PRIMARY KEY ‘s from the parent table. Set the FOREIGN_KEY_CHECKS before and after your delete SQL statements. SET FOREIGN_KEY_CHECKS = 0; DELETE FROM table WHERE …

How do I delete a foreign key in mysql?

To drop a foreign key constraint, you use the ALTER TABLE statement: ALTER TABLE table_name DROP FOREIGN KEY constraint_name; In this syntax: First, specify the name of the table from which you want to drop the foreign key after the ALTER TABLE keywords.

What is 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.

What happens when you delete a foreign key?

A foreign key with cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table will automatically be deleted.

Can we drop a table with primary key?

The syntax to drop a primary key in SQL is: ALTER TABLE table_name DROP PRIMARY KEY; table_name. The name of the table to modify.

Which keyword is used to specify the foreign key after the table is created?

Which keyword is used to specify the foreign key after the table is created? Explanation: When the table has already been created but the foreign key has not been set of foreign key constraints are not specified, the ‘ALTER TABLE – ADD FOREIGN KEY’ clause is used.

What is the difference between primary key and unique key?

Primary Key is a column that is used to uniquely identify each tuple of the table. It is used to add integrity constraints to the table. Only one primary key is allowed to be used in a table. … Unique key is a constraint that is used to uniquely identify a tuple in a table.

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).

Can we delete constraints in primary or foreign key?

The DROP CONSTRAINT command is used to delete a UNIQUE, PRIMARY KEY, FOREIGN KEY, or CHECK constraint.

What is cascade delete?

DELETE CASCADE: When we create a foreign key using this option, it deletes the referencing rows in the child table when the referenced row is deleted in the parent table which has a primary key.

What is an example of a foreign key?

A foreign key is a column (or columns) that references a column (most often the primary key) of another table. … For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders.

How do you assign a foreign key?

Using SQL Server Management StudioIn Object Explorer, right-click the table that will be on the foreign-key side of the relationship and click Design. … From the Table Designer menu, click Relationships.In the Foreign-key Relationships dialog box, click Add. … Click the relationship in the Selected Relationship list.More items…•

What is the value of primary key?

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).