Given the following statements: S1: A foreign key declaration can always be replaced by an equivalent check assertion in SQL. S2: Given the table R(a,b,c) where a and b together form the primary key, the following is a valid table definition. CREATE TABLE S ( ConstINTEGER, d INTEGER, e INTEGER, PRIMARY KEY (d), FOREIGN KEY (a) references R) Which one of the following statements is CORRECT?
Question
Given the following statements: S1: A foreign key declaration can always be replaced by an equivalent check assertion in SQL. S2: Given the table R(a,b,c) where a and b together form the primary key, the following is a valid table definition. CREATE TABLE S ( ConstINTEGER, d INTEGER, e INTEGER, PRIMARY KEY (d), FOREIGN KEY (a) references R) Which one of the following statements is CORRECT?
Solution
The correct statement is S2.
In S1, a foreign key declaration cannot always be replaced by an equivalent check assertion in SQL. A foreign key is a field (or collection of fields) in one table, that is used to ensure referential integrity of the data. It is used to prevent actions that would destroy links between tables. A check constraint is a type of integrity constraint in SQL which specifies a requirement that must always be met by each row in a database table. The constraint must be a predicate. It can refer to a single column, or multiple columns of the table. However, it cannot refer to columns of other tables.
In S2, the table definition is valid. The table S is created with columns Const, d, e. The primary key is d. The foreign key is a which references column a in table R. This is a valid table definition as long as table R exists and has a column a.
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