By Neeraj Sharma, Liviu Perniu, Raul F. Chong, Abhishek Iyer
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Along with NOT NULL, the additional keyword “WITH DEFAULT” can optionally be specified so that the system generates a default value for the attribute in case it is null at the source while inserting. ) and date (date, time, timestamp) data types. For other types, default value should be provided explicitly in the DDL. 3 Unique constraint A unique constraint specifies that attribute values must be different. It is not possible to have two tuples in a relation with the same values for that attribute.
Sometimes, in database literature, you will also find it referenced as artificial key. Surrogate keys usually have unique numerical values. Those values grow or decrease automatically with an increment (usually by 1). 3 Foreign keys A foreign key is an attribute (or attribute combination) in one relation R2 whose values are required to match those of the primary key of some relation R1 (R1 and R2 not necessarily distinct). Note that a foreign key and the corresponding primary key should be defined on the same underlying domain.
The formal notation for a difference operation is DIFFERENCE operation is not associative and commutative. 7 provides an example of a DIFFERECE operation. The operands are relation R1 and relation R2 and the result is another relation R3 with two tuples. As you can see, the result of R1-R2 is different from R2-R1. 4 Cartesian product The Cartesian product between two relations R1 and R2, R1 TIMES R2, is the set of all tuples t such that t is the concatenation of a tuple r belonging to R1 and a tuple s belonging to R2.