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.

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Which of the following statements about creating constraints is incorrect?

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