Dominoes spots from one to six are generally arranged as they are on six-sided dice, but because blank ends having no spots are used, seven faces are possible, allowing 28 unique pieces in a double-six set. However, this is a relatively small number especially when playing with more than four people, so many domino sets are "extended" by introducing ends with greater numbers of spots, which increases the number of unique combinations of ends and thus of pieces. Each progressively larger set increases the maximum number of pips on an end by three, so the common extended sets are double-nine, double-12,
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