Which of the following are true about SELECT command? $($Select all that apply.$)$

a$.$

The WHERE clause of a SELECT command can be used to compare different values so you can limit the result set.

b$.$

The SELECT command has several optional clauses, including the WHERE clause.

c$.$

The WHERE clause can be used to see if a field is empty.

d$.$

The GROUP BY clause in a SELECT command groups tables with matching columns.