The table structure is as follows: (1)guest(guestid char(4),guestname char(20),g_age number(3,0),g_addr char(20),g_work char(30)) (2)goods(goodsid char(4),goodsname char(8),g_price number(5)) (3)purchase(goodsid char(4),guestid char(4),purchase_date date,purchase_num number(7)) where : (1)In table guest, the guestid is primary key, the guestname is not null, the value of g_age is between 0 and 200 ; (2) In table goods, the goodsid is primary key, the value of g_price is more then 0 ; (3)In table purchase, the guestid references the column guestid in table guest , the goodsid references goodsid in table goods , the primary key is(guestid , goodsid , purchase_date) 。 Write out the following SQL command: (1) Create the above three tables and create an backup table for the guest table: guest_back. (2) add a column of g_tel char(13) to the guest table. (3) change the data type of g_age to Number(5). (4) delete the guestname column-level integrity constraint in the guest table. (5) delete the new column g_tel in the table guest. (6) delete guest_back table.