SQL
Not Null Constraint
A not-null constraint specifies that the column cannot contain any `NULL` values. By default, all columns in tables are nullable. Adding `NOT NULL` to a column definition enforces that a column cannot contain `NULL` values.  
```sql
CREATE TABLE students (name VARCHAR NOT NULL);
INSERT INTO students VALUES (NULL);
```  
```console
Constraint Error: NOT NULL constraint failed: students.name
```