Oracle table level and column-level constraint
Table 1. Level defining constraints
refers After defining all the columns in a table, go to define all relevant constraints.
Note: not null constraints can only be defined at the column level.
2. The column-level constraint is defined
refers to a definition of each column in a table at the same time define each column of the constraint, the constraint which
is located after each column.
约束：FOREIGN KEY,PRIMARY KEY,NOT NULL,CHECK,UNIQUE
check constraints: a field is to allow the input values in the table can be a fixed value.
unique constraints: is the only constraint required for the column does not allow duplicate seed value. May be constrained to add a plurality of columns of unique individual columns add a unique constraint (constraint belonging to the table level)