A Foreign Key Constraints is a column or a set of columns in a table that refers to the primary key or a unique key of another table. It helps to create connection between two different table.
Here we have two different table author and book where in author table author_id is primary_key where as in book table id is primary key, In book table author_id is foreign key.
Now here is syntax and example of adding foreign key
Syntax:
ALTER TABLE TableName ADD CONSTRAINT FK_Key_Name FOREIGN KEY (column_name) REFERENCES foriegnTableName(column_name);
Example: ALTER TABLE book ADD CONSTRAINT FK_AUTHOR_ID FOREIGN KEY (author_id) REFERENCES author(author_id);
Now you will see there is Multiple Key created in in Key description.
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