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A foreign key is a field or collection of fields in a table that refers to the Primary key of the other table. It is responsible for managing the relationship between the tables. The table which contains the foreign key is often called the child table, and the table whose primary key is being referred by the foreign key is called the Parent Table.
What is the table containing the foreign key called?
The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
How is the FOREIGN KEY constraint used in MySQL?
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table. The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.
Do you need a foreign key for a parent table?
For defining a Foreign key, we need a parent table with a Primary Key. Primary key creates clustered indexes on the table. Foreign key does not create indexes on the table neither clustered nor unclustered. In this tutorial, we have seen what a Primary and a Foreign Key is.
Can a table have more than one foreign key?
But the Foreign Key in the table can contain Null values and also can have duplicate values. A table can have only one Primary Key, whereas there can be more than one Foreign Key for a table. The index gets created for the Primary Key automatically, whereas, for the Foreign Key, indexes do not get created automatically.
Which is the child table with the foreign key?
The table which contains the foreign key is often called the child table, and the table whose primary key is being referred by the foreign key is called the Parent Table. For example: When we talk about students and the courses they have enrolled in, now if we try to store all the data in a single table, the problem of redundancy arises.
Can a foreign key have a null value?
The Primary Key column in a table cannot have Null values and should always have unique values. But the Foreign Key in the table can contain Null values and also can have duplicate values.
Modifying a table’s foreign key changes which columns are related to columns in the primary key table. The new foreign key column must match the data type and size of the primary key column to which it relates, with these exceptions: A char column or sysname column can relate to a varchar column.
How to avoid create table with foreign key reference?
If you’d like to make sure that the SQL engine won’t insert a row with a foreign key that references a non-existent primary key, then you can add a FOREIGN KEY constraint in your CREATE TABLE statement. You can see the error that it results in by removing the comments from the fourth INSERT in the code below.
How can I modify a foreign key in Object Explorer?
To modify a foreign key. In Object Explorer, expand the table with the foreign key and then expand Keys. Right-click the foreign key to be modified and select Modify. In the Foreign Key Relationships dialog box, you can make the following modifications. Selected Relationship Lists existing relationships.
How can I modify a FOREIGN KEY constraint?
To modify a foreign key. To modify a FOREIGN KEY constraint by using Transact-SQL, you must first delete the existing FOREIGN KEY constraint and then re-create it with the new definition. For more information, see Delete Foreign Key Relationships and Create Foreign Key Relationships.