Dominoes Dominoes Dominoes is a family of tile-based games played with rectangular "domino" tiles. Each domino is a rectangular tile with a line dividing its face into two square "ends". Each end is marked with a number of spots (also called "pips", "nips", or "dobs") or is blank. The backs of the dominoes in a set are indistinguishable, either blank or having some common design. The domino gaming pieces make up a domino set, sometimes called a "deck" or "pack". The traditional Sino-European domino set consists of 28 dominoes, featuring all combinations of spot counts between zero and six. A domino
Unique key According to the SQL standard a unique constraint does not enforce uniqueness in the presence of nulls and can therefore contain several rows with identical combinations of nulls and non-null values — however not all RDBMS implement this feature according to the SQL standard. Unique key In database relational modeling and implementation, a unique key (also known as a candidate key) of a relation is a minimal superkey for that relation; that is, a set of attributes such that: When a column or set of columns is defined as unique to the database management system, the system verifies that each