What is MySQL NOT NULL constraint and how can we declare a field NOT NULL while creating a table?

In fact, the MySQL NOT NULL constraint restricts a column of the table to have a NULL value. Once we apply NOT NULL constraint on a column, then we cannot pass null values ​​to that column. It cannot be declared on the entire table, in other words we can say NOT NULL is a column level constraint.

In order to declare a field NOT NULL, we must use the NOT NULL keyword when defining the column in the CREATE TABLE statement.

Example

mysql> Create table Employee(ID Int NOT NULL, First_Name Varchar(20), Last_name Varchar(20), Designation Varchar(15));
Query OK, 0 rows affected (0.59 sec)

In the above query, we have applied NOT NULL constraint on the "ID" field of the "Employee" table. The 'ID' column cannot now take NULL values. You can also check from the DESCRIBE statement that the ID field cannot accept NULL values.

mysql> DESCRIBE Employee123\G
*************************** 1. row ***************************
  Field: ID
   Type: int(11)
   Null: NO
    Key:
Default: NULL
  Extra:
*************************** 2. row ***************************
  Field: First_Name
   Type: varchar(20)
   Null: YES
    Key:
Default: NULL
  Extra:
*************************** 3. row ***************************
  Field: Last_name
   Type: varchar(20)
   Null: YES
    Key:
Default: NULL
  Extra:
*************************** 4. row ***************************
  Field: Designation
   Type: varchar(15)
   Null: YES
    Key:
Default: NULL
  Extra:
4 rows in set (0.03 sec)

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