some questions about DS,please help me ,thanks.

HI

I'M a freshman in DS,so i got some problems to be solved, please help me .

1.when i view data in DBtables , it raises error 'the total length of fields is exceeded than 512 byte', why?

2.please give me some usages of frequently-used variables.

3.when i view data in the template table it raises error 'SQL failed and view or table is not existing' why?