Constraints - Nulls? Table/Column?

• A Foreign Key is called a Referential Integrity Constraint
• A Foreign Key can't be in a remote database.
• A Foreign Key can refer to a unique or primary key, even in the same table.
  
• A check can't use the pseudocolumns.
• Use the table constraint form to check more than one column.
  

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Drop Table T1 Cascade Constraints;
Drop Table T2 Cascade Constraints;
Drop Table T3 Cascade Constraints;
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Create Table T3
( C3 Char(6)
, C6 Number(7,2) Constraint T3_Uk Unique
, C7 Date
, Constraint T3_Pk Primary Key (C3)
);
Create Table T2
( C2 Varchar2(4)  Constraint T2_Pk Primary Key
, C3 Char(6)      Constraint T2_Fk References T3(C3)
, C4 Date         Constraint T2_C4_NN Not Null
, C5 Varchar2(10) Check (C5 <=9)
);  
Create Table T1  
( C1 Number(4)   Not Null          
, C2 Varchar2(4) 
, Constraint T1_Pk Primary Key (C1,C2) 
, Constraint T1_Fk Foreign Key (C2) References T2(C2)  
);  
Clear Screen 
Ttitle Left 'Constraint_Type: ' xConstraint_Type ' Owner: ' Xowner ' R_Owner: ' Xr_Owner ' Index_Owner: ' Xindex_Owner Skip 2 
Break On Constraint_Type Skip Page Dup  

Column Owner            Noprint New_Value Xowner 
Column R_Owner          Noprint New_Value Xr_Owner 
Column Index_Owner      Noprint New_Value Xindex_Owner 
Column Constraint_Type  Noprint New_Value Xconstraint_Type  
Column Index_Name Format A5 
Column Table_Name Format A5 
Column Validated  Format A9  

Select *
  From User_Constraints   
 Where Table_Name Like 'T_'  
 Order By Constraint_Type;