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Can foreign key refer to two tables?
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.
Can a foreign key reference two tables MySQL?
MySQL supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, a “child table record” refers to a dependent record within the same table.
Can a foreign key be one to many?
A foreign key relationship could be one-to-one (a record in one table is linked to one and only one record in another table) or one-to-many (a record in one table is linked to multiple records in another table). Foreign keys are only supported on InnoDB tables.
Can one column have two foreign keys?
A single column can have multiple foreign key constraints. For an example, see Add multiple foreign key constraints to a single column.
Can a foreign key reference multiple columns?
MySQL allows us to add a FOREIGN KEY constraint on multiple columns in a table. The condition is that each Foreign Key in the child table must refer to the different parent table.
How do I add a foreign key to a MySQL table?
After creating a table, if we want to add a foreign key to an existing table, we need to execute the ALTER TABLE statement as below:
- ALTER TABLE Contact ADD INDEX par_ind ( Person_Id );
- ALTER TABLE Contact ADD CONSTRAINT fk_person.
- FOREIGN KEY ( Person_Id ) REFERENCES Person ( ID ) ON DELETE CASCADE ON UPDATE RESTRICT;
How many foreign keys can you have in a table?
253 foreign key references
A table with a foreign key reference to itself is still limited to 253 foreign key references. Greater than 253 foreign key references are not currently available for columnstore indexes, memory-optimized tables, Stretch Database, or partitioned foreign key tables.
Can one column referencing multiple tables?
In SQL can a single column in a table reference multiple tables – no this is not possible. A foreign key always references one target table (and one table only).
Can a column be primary key and foreign key at the same time?
These are totally different constructs. A Primary Key is used to enforce uniqueness within a table, and be a unique identifier for a certain record. A Foreign Key is used for referential integrity, to make sure that a value exists in another table. The Foreign key needs to reference the primary key in another table.
Can we update foreign key in a table?
The foreign key relation can be created either through SSMS GUI or T-SQL. Rules for update/delete operations may be specified explicitly. However if nothing is specified then the default rule is No Action. The rule may be changed to any other option at any time later by recreating the FK relation.
How do I create an existing column as a foreign key in MySQL?
Following are the syntax of the ALTER TABLE statement to add a foreign key in the existing table:
- ALTER TABLE table_name.
- ADD [CONSTRAINT [symbol]] FOREIGN KEY.
- [index_name] (column_name.)
- REFERENCES table_name (column_name,…)
- ON DELETE referenceOption.
- ON UPDATE referenceOption.