[Continue to supplement]

1. In statement
I tried it many times. It seems that the in statement does not generate in statements, but can only be changed.

For example, to find out the "Mobile Phone category" and "all sub-classes of mobile phone" Products (mobile phone category ID for D0C37275-2279-4590-A679-C65A4F28918F)

Select F_id, f_name, f_clsid From T_product Where F_clsid In (
  Select F_id From T_class Where F_id = ' D0C37275-2279-4590-A679-C65A4F28918F '   Or F_parentidstr Like   ' % D0C37275-2279-4590-A679-C65A4F28918F % ' )

The effect is equivalent

Select P.f _ id, p.f _ name, p.f _ CLSID From T_product As P Inner   Join T_class As C On C. f _ id = P.f _ CLSID Where  
C. f _ id = ' D0C37275-2279-4590-A679-C65A4F28918F '   Or C. f _ parentidstr Like   ' % D0C37275-2279-4590-A679-C65A4F28918F % '

Or

Select P.f _ id, p.f _ name, p.f _ CLSID From T_product As P, t_class As C Where  
P.f _ CLSID = C. f _ id And (
C. f _ id = ' D0C37275-2279-4590-A679-C65A4F28918F '   Or  
C. f _ parentidstr Like   ' % D0C37275-2279-4590-A679-C65A4F28918F % ' )

Equivalent Writing Method of LINQ to SQL:

VaR Query = From P In DB. t_products join
C In DB. t_classes on p. f_clsid equals C. f_id
Where C. f_id =   New GUID ( " D0C37275-2279-4590-A679-C65A4F28918F " ) | C. f_parentidstr.contains ( " D0C37275-2279-4590-A679-C65A4F28918F " )
Select New   {P. f_id, P. f_name} ;

Or

VaR Query = From P In DB. t_products
From C In DB. t_classes
Where P. f_clsid = C. f_id && (C. f_id =   New GUID ( " D0C37275-2279-4590-A679-C65A4F28918F " ) | C. f_parentidstr.contains ( " D0C37275-2279-4590-A679-C65A4F28918F " ))
Select New   {P. f_id, P. f_name} ;