How to use MySQL Foreign Key Constraints

In this syntax: First, specify the name of foreign key constraint that you want to create after the CONSTRAINT keyword. If you omit the constraint name, MySQL automatically generates a name for the foreign key constraint. Second, specify a list of comma-separated foreign key columns after the FOREIGN KEY keywords.

1. What is foreign key constraint in MySQL?
2. How do you write a foreign key constraint?
3. How do I add a foreign key constraint to an existing table in MySQL?
4. How do I find foreign key constraints in SQL?
5. What is constraint in MySQL?
6. What is a foreign key constraint?
7. What is constraint key in SQL?
8. What is a primary key constraint?
9. Can foreign key be null?
10. How can I find the foreign key of a table in mysql?
11. How do I add a foreign key constraint in mysql workbench?
12. What is difference between primary key and foreign key?

What is foreign key constraint 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.

How do you write a foreign key constraint?

Creating a foreign key constraint requires at least one of the SELECT , INSERT , UPDATE , DELETE , or REFERENCES privileges on the parent table as of 5.6. 22. Corresponding columns in the foreign key and the referenced key must have similar data types.

How do I add a foreign key constraint to an existing table in MySQL?

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:

1. ALTER TABLE Contact ADD INDEX par_ind ( Person_Id );
2. ALTER TABLE Contact ADD CONSTRAINT fk_person.
3. FOREIGN KEY ( Person_Id ) REFERENCES Person ( ID ) ON DELETE CASCADE ON UPDATE RESTRICT;

How do I find foreign key constraints in SQL?

Using SQL Server Management Studio

1. Open the Table Designer for the table containing the foreign key you want to view, right-click in the Table Designer, and choose Relationships from the shortcut menu.
2. In the Foreign Key Relationships dialog box, select the relationship with properties you want to view.

What is constraint in MySQL?

The constraint in MySQL is used to specify the rule that allows or restricts what values/data will be stored in the table. They provide a suitable method to ensure data accuracy and integrity inside the table. It also helps to limit the type of data that will be inserted inside the table.

What is a foreign key constraint?

A foreign key constraint specifies that the key can only contain values that are in the referenced primary key, and thus ensures the referential integrity of data that is joined on the two keys. You can identify a table's foreign key when you create the table, or in an existing table with ALTER TABLE .

What is constraint key in SQL?

SQL constraints are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. ... Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.

What is a primary key constraint?

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).

Can foreign key be null?

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). ... Null by definition is not a value.

How can I find the foreign key of a table in mysql?

To see foreign key relationships of a table: SELECT TABLE_NAME, COLUMN_NAME, CONSTRAINT_NAME, REFERENCED_TABLE_NAME, REFERENCED_COLUMN_NAME FROM INFORMATION_SCHEMA. KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_SCHEMA = 'db_name' AND REFERENCED_TABLE_NAME = 'table_name';

How do I add a foreign key constraint in mysql workbench?

To add a foreign key, click the last row in the Foreign Key Name list. Enter a name for the foreign key and select the column or columns that you wish to index by checking the column name in the Column list.
...
Figure 8.16 The Foreign Keys Tab

1. RESTRICT.
2. CASCADE.
3. SET NULL.
4. NO ACTION.

What is difference between primary key and foreign key?

Primary key is used to identify data uniquely therefore two rows can't have the same primary key. It can't be null. On the other hand, foreign key is used to maintain relationship between two tables. Primary of a table act as forgein key in the other table.