Which of the following statements about creating constraints is incorrect?

Question

​Which of the following statements about creating constraints is incorrect?​

BlackTom AI · Accepted Answer

Let’s break down each statement about creating constraints and assess its accuracy one by one.
Option 1: 'If a constraint applies to more than one column, the constraint must be created at the table level.' In many database designs, multi-column constraints (such as a composite foreign key or a composite check constraint) are defined at the table level because they reference multiple columns together. This makes this statement largely correct in common practice.
Option 2: 'When using the column-level approach, the definition of the constraint is included as part of the column definition.' This is true: when you declare a constraint at the column level, you place the constraint clause directly after the column’s data type in the column definition, tying the constraint to that single column.
Option 3: 'When you create constraints at the column level, the constraint being created applies to the column specified.' This is also true: a column-level constraint directly governs the data for the specified column and enforces rules on that column alone.
Option 4: 'The NOT NULL constraint can be created at both the column level and the table level.' Here lies the inaccuracy. In standard SQL and typical relational databases, NOT NULL is a column-level constraint; there isn’t a syntax for declaring NOT NULL as a table-level constraint that applies to multiple columns as a single table-level clause. If you want to enforce a non-null condition across multiple columns, you would generally use a table-level CHECK constraint or separate column-level NOT NULL constraints. Some systems may emulate similar behavior with CHECK constraints, but NOT NULL itself is not defined as a table-level constraint in the usual SQL syntax. Therefore, this statement is incorrect, while the other three align with standard constraint placement rules.

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

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