What is the SQL keyword used to specify that a column can contain NULL values?OptionsNOT NULLALLOW NULLNULL ABLEWITH NULLS

Breaking Down the Problem

1. We need to identify the correct SQL keyword that indicates a column can contain NULL values.
2. The options given are:
   • NOT NULL
   • ALLOW NULL
   • NULL ABLE
   • WITH NULLS.

Relevant Concepts

1. In SQL, the term used to indicate that a column is allowed to have NULL values must be distinguished from the term used to enforce that a column cannot have NULLs.

Analysis and Detail

1. NOT NULL - This keyword is used to specify that a column must contain a value and cannot be left blank (i.e., it cannot be NULL).
2. ALLOW NULL - This is not a standard SQL keyword.
3. NULL ABLE - This is a non-standard way to refer to a column that can contain NULL values but is not the correct SQL syntax.
4. WITH NULLS - This phrase is not used to define NULL constraints in SQL.

Verify and Summarize

From the analysis, "NOT NULL" is incorrect as it prevents NULL values. "ALLOW NULL" and "NULL ABLE" are not recognized SQL keywords. "WITH NULLS" does not pertain to column definitions.

Final Answer

The correct SQL keyword to specify that a column can contain NULL values is ALLOW NULL. However, note that the most standard practice simply does not include any declaration, as columns default to allowing NULL values in SQL unless specified otherwise (with NO keywords needed).