In database relational modeling and implementation, a unique key is a set of zero or more attributes, the value(s) of which are guaranteed to be unique for each tuple (row) in a relation.The value or combination of values of unique key attributes for any tuple cannot be duplicated for any other tuple in that relation.As shown in the following illustration, the Product ID and Vendor ID columns in the Purchasing.Product Vendor table form a composite primary key constraint for this table.This makes sure that that every row in the Product Vendor table has a unique combination of Product ID and Vendor ID. A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in the foreign key table.In a foreign key reference, a link is created between two tables when the column or columns that hold the primary key value for one table are referenced by the column or columns in another table.He also covers creating a new database from scratch, as well as data types, transactions, subselects, views, and stored routines.Plus, learn about the multi-platform PHP PDO interface that will help you connect your database to web applications.
= new_diff 7 OR (old_diff IS NULL AND new_diff IS NOT NULL) 8 OR (old_diff IS NOT NULL AND new_diff IS NULL); Can you really update the expression like that?
Another table called Employee Details has a foreign key which references employee_id in order to uniquely identify the relationship between both the tables.
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.
This index also permits fast access to data when the primary key is used in queries.
If a primary key constraint is defined on more than one column, values may be duplicated within one column, but each combination of values from all the columns in the primary key constraint definition must be unique.