Question

What will happen if you try to use the INSERT command to insert a NULL value into a column that has been assigned a PRIMARY KEY constraint?

a) Data will be inserted successfully
b) Error will occur
c) NULL will be replaced with default value
d) Depends on database settings

Answer 1

When you use the INSERT command to insert a NULL value into a PRIMARY KEY constrained column, the operation will result in an error because PRIMARY KEYs cannot accept NULL values(OPTION B).

Step-by-step explanation:

If you try to use the INSERT command to insert a NULL value into a column that has been assigned a PRIMARY KEY constraint, an error will occur(OPTION B).

A PRIMARY KEY constraint uniquely identifies each record in a database table, and it must contain a unique value for each row. Moreover, it cannot accept NULL values, as NULL represents an unknown or undefined value, which can't be unique or serve as a reliable identifier for a record.

Some databases may have settings that allow for the handling of NULLs differently but in standard SQL and in most databases, trying to insert a NULL value into a PRIMARY KEY field violates the fundamental rule of the PRIMARY KEY constraint. Normally, PRIMARY KEY fields are set to AUTO_INCREMENT if they are numerical, to avoid manual insertion of primary key values, including NULL.