In Oracle Database 12c, it is important to understand the behavior and properties of constraints.
A. This statement is false. A FOREIGN KEY column in a child table does not need to have the same name as the referenced PRIMARY KEY column in the parent table. What is required is that the data type is the same and that the values in the FOREIGN KEY column correspond to values in the PRIMARY KEY column of the parent table.
B. This statement is false. A table cannot have multiple PRIMARY KEY constraints. By definition, a PRIMARY KEY is a unique identifier for a row in a table, and there can only be one such identifier.
C. This statement is false for the same reasons as B; a table can have only one PRIMARY KEY. However, it can have multiple FOREIGN KEY constraints that reference PRIMARY KEYS in other tables.
D. This is true. PRIMARY KEY and FOREIGN KEY constraints can indeed be specified at the column level with the column definition or at the table level with the ALTER TABLE statement.
E. This is true. A table can have only one PRIMARY KEY constraint because it defines a unique row identifier. However, it can have multiple FOREIGN KEY constraints referencing keys in other tables, allowing for complex relational structures.
F. This statement is false. NOT NULL constraints are always defined at the column level, as they apply to individual columns. They cannot be specified at the table level.
References:
Oracle Documentation on Constraints: https://docs.oracle.com/database/121/SQLRF/clauses002.htm#SQLRF52271
Oracle Documentation on NOT NULL Constraints: https://docs.oracle.com/database/121/SQLRF/clauses002.htm#SQLRF52162
