How to create not null constraint in oracle

What is Not Null constraint in oracle

Not Null constraints in oracle are used to enforce not null values in the column in the table. So if you try to insert null into that column, it will throw an error.

How to create not null constraint

Not null constraints are defined at the table creation or you can modify the table, later on, to have a column changed to Not null. Let’s take a look at the examples.

Table creation
Not null constraint is specified inline with the column.

SQL> CREATE TABLE "EMP"
    ("EMPNO" NUMBER(4,0) Not Null,
     "ENAME" VARCHAR2(10) Not null,
     "JOB" VARCHAR2(9),  
     "MGR" NUMBER(4,0),       
     "HIREDATE" DATE,
     "SAL" NUMBER(7,2) ,  
     "COMM" NUMBER(7,2),
     "DEPTNO" NUMBER(2,0)
      );

 Table created. 

Here we have defined two not null constraints on the table emp. We can check the Not null constraints on the table using the below query

SQL> column CONSTRAINT_NAME format a20
SQL> column SEARCH_CONDITION format a50
SQL> SELECT Constraint_name, Search_condition
     FROM User_constraints
     WHERE Table_name = 'EMP'  AND    
     Constraint_type = 'C';

Not Null constraints are also defined as check constraints and have search conditions as column name not null

Adding Not constraint after table creation

We can modify the table to add the not null constraint

SQL> alter table emp modify ("HIREDATE" not null);
 Table altered.

If we have null data in the column existing then this statement will fail

SQL> SELECT Constraint_name, Search_condition
     FROM User_constraints
     WHERE Table_name = 'EMP'  AND
     Constraint_type = 'C'

We can give the custom name to the Not Null constraint also. Here is the example to demonstrate it

SQL> CREATE TABLE "EMP"
 ("EMPNO" NUMBER(4,0) Constraint EMP_NOTNULL Not Null,  
 "ENAME" VARCHAR2(10) Not null,
 "JOB" VARCHAR2(9),
 "MGR" NUMBER(4,0),
 "HIREDATE" DATE,
 "SAL" NUMBER(7,2) ,
 "COMM" NUMBER(7,2),
 "DEPTNO" NUMBER(2,0)
  );

 Table created.
 SQL> column CONSTRAINT_NAME format a20
 SQL> column SEARCH_CONDITION format a50
 SQL> SELECT Constraint_name, Search_condition
     FROM User_constraints
     WHERE Table_name = 'EMP'  AND
     Constraint_type = 'C';

 SQL> alter table emp modify ("HIREDATE" Constraint H_NOTNULL  Not    Null);
 
Table altered.
 SQL> SELECT Constraint_name, Search_condition
      FROM User_constraints
      WHERE Table_name = 'EMP'  AND
      Constraint_type = 'C';

How to drop the Not Null constraint

We need to modify the column to accept null values

SQL>alter table emp modify "HIREDATE" Null;

Table altered.

SQL> SELECT Constraint_name, Search_condition
     FROM User_constraints
     WHERE Table_name = 'EMP'  AND
     Constraint_type = 'C';

We can drop the not null constraint using the alter table drop constraint command also

SQL> alter table emp drop constraint  EMP_NOTNULL ;
Table altered;

SQL> SELECT Constraint_name, Search_condition
     FROM User_constraints
     WHERE Table_name = 'EMP'  AND
     Constraint_type = 'C';

How to add not null constraint on the column containing null values

If you try to add a not-null constraint on the column containing null values, then that command will fail as it will validate all the rows all the existing rows for not null constraint. Sometimes we want to keep the existing null values and check for future rows that they contain a not null value for this column, then, in that case, we may enable the constraint with novalidate clause. With this clause, it will not check the existing rows, but future rows will be checked

alter table emp modify "HIREDATE" not Null novalidate;

Also Read
drop primary key oracle
Foreign key in oracle
Unique Key constraint in Oracle
how to check all constraints on a table in oracle
oracle alter table modify column type
https://docs.oracle.com/cd/B28359_01/server.111/b28310/general005.htm#ADMIN11554