Which of the following statements about creating constraints is incorrect?

Question

BlackTom AI · Accepted Answer

The question asks about creating constraints and which statement is incorrect.
Option 1: 'Creating a constraint at the column level means that the constraint applies only to the specified column.' This is generally true: a column-level constraint is attached to a single column and governs only that column's values.
Option 2: 'All constraints can be created at either the column level or the table level'. This is not accurate in all cases. While many constraints (like PRIMARY KEY, FOREIGN KEY, CHECK) can be defined either as column constraints or as table constraints, some constraints cannot be defined at the table level. A classic example is NOT NULL, which is defined on a column itself and cannot be declared as a table-level constraint. Therefore, this statement overgeneralizes and is incorrect.
Option 3: 'A table can have multiple FOREIGN KEY constraints.' This is correct in most relational database systems: a table can reference multiple other tables, each via its own FOREIGN KEY constraint, and they can be defined separately.
Option 4: 'When a constraint applies to multiple columns (e.g., a composite primary key), it must be created at the table level.' This is typically true: composite keys or multi-column constraints are defined as table constraints since they span multiple columns and cannot be represented by a single column-level constraint.
In summary, the key issue lies with the blanket claim in option 2: not all constraints can be created at either the column level or the table level, because some constraints (such as NOT NULL) are restricted to the column level and cannot be expressed as table constraints.

Which of the following statements about creating constraints is incorrect?单项选择题

类似问题

Which of the following statements about the FOREIGN KEY constraint is incorrect?