What are the main problems of using relational database for XML database where the XML has an irregular structure? Select all that apply.

The database cannot be indexed.

XSL transformation cannot be performed.

It may use many tables for representing the data.

It may contain many null columns in a table for representing the data.

Which of the following line of the XQuery code is incorrect?

let $a :$=3$

let $b :$=7$

$$a=8**$$$a+4**$$$b$

return $a

let $a :$=3$

$$a=8**$$$a+4**$$$b$

return $a

None of them