青云翻译

The only constraint where the column allows null values, but the primary key constraint where a column does not allow null values.

Uniqueness constraint the column allows null values, but the same column as the primary key constraint does not allow null values.

The unique restraint is at the row permits the spatial value, but the principal linkage restraint is at the row does not permit the spatial value.

The only restriction the column in which the NULL value, but allowed the column in which the primary key constraints do not allow empty value.

Uniqueness constraint the column allows null values, but the same column as the primary key constraint does not allow null values.