Which of the following is a legal expression in SQL?

• SELECT NULL FROM EMPLOYEE;

• SELECT NAME FROM EMPLOYEE;

• SELECT NAME FROM EMPLOYEE WHERE SALARY=NULL;

• None of the above

...

"SELECT NAME FROM EMPLOYEE"

is a valid expression in SQL. It is used to retrieve the values of the "NAME" column from the "EMPLOYEE" table. The SELECT statement is one of the most basic and commonly used statements in SQL, and it is used to retrieve data from one or more tables in a database.

The expression "SELECT NULL FROM EMPLOYEE" is not a valid expression, as the keyword "NULL" cannot be selected on its own.

The expression "SELECT NAME FROM EMPLOYEE WHERE SALARY=NULL" is also not a valid expression, as the comparison operator "=" cannot be used to compare a value to "NULL". In SQL, "NULL" represents the absence of a value, and comparisons to "NULL" must be done using the IS NULL or IS NOT NULL operators.