Primary key and foreign key examples

Foreign key in a table refers to the primary key of other table. No two tuples in a relation carry duplicate values for a primary key attribute. Primary key value can never be NULL.

In their simplest form, a foreign key constraint stops you from entering values that aren’t found in the related table’s primary key. CurrencyRateID if it doesn’t already exist in the CurrencyRate table. Attributes (columns in the table) will help us distinguish one entity from another (one row from the other - A row in the students table will have all the details about a particular student, and a row in the courses table will describe course details). A Composite primary key is a set of columns whose values uniquely identify every row in a table. In this case , both the columns will be represented as primary key.

Final Words on Foreign Keys. On that page is an example in which a “product_order” table has foreign keys for two other tables. One foreign key references a two-column index in the “Product” table.

The other references a single-column index in the “Customer” table. SalesPerson table because there is a logical relationship between sales orders and salespeople. SalesOrderHeader table has a foreign key link to the Sales.

A single table can consist of multiple foreign keys. For more than one reference key single column can be used. There are two ways to declare foreign key 1) Inline declaration ) Out-of-line declaration. The column is defined as foreign key at column level in an inline declaration. A FOREIGN KEY is a key used to link two tables together.

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. For example in Employee and Department relationship, we have two tables Department(dept_i dept_name) and Employee (emp_i emp_name, dept_id). Duplicate: No two tuples in a relation carry duplicate values for a primary key attribute. Identity is used to make a column Auto Increment.

If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s). Consider the structure of the following two tables. Creating a foreign key is almost as easy as creating a primary key , except that SQL Server imposes several more rules on foreign keys. MySQL syntax examples - create table, primary key , foreign key.

Just a quick note here today that if you need some example MySQL database tables, you can use these. I created them for some experiments I ran last night. It shows many examples of creating tables and fields, drop table commands, constraints, and more.

And the table to which it references is called a parent table. The term for the foreign key in relational database terminology is referential integrity constraint. Note: The column or set of columns declared as a reference key in child table should be a primary key in a parent table. For instance, the PK to the ISO Country Region table above is ISO Country Code plus ISO Country Region code.

Region is province, state, départment, canton…. Some people call this a Super Key.