MySQL NULL and NOT NULL

NULL is a special value that represents missing or unknown data. It is different from an empty string or a value of zero.

When a column is defined as allowing NULL values, it means that the column can contain NULL values in addition to regular values of the specified data type.

When a column is defined as NOT NULL, it means that the column cannot contain NULL values and must have a regular value of the specified data type. 

mysql> DESCRIBE friends;
+-----------+----------+------+-----+---------+----------------+
| Field     | Type     | Null | Key | Default | Extra          |
+-----------+----------+------+-----+---------+----------------+
| id        | int      | NO   | PRI | NULL    | auto_increment |
| Name      | char(30) | YES  |     | NULL    |                |
| Lastname  | char(30) | YES  |     | NULL    |                |
| Telephone | int      | YES  |     | NULL    |                |
+-----------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

mysql>

NOT NULL is a constraint that can be applied to a column when creating a table.

It ensures that the column cannot contain NULL values, meaning that every row in the table must have a value for that column.

If a value is not specified for a NOT NULL column during an INSERT operation, the operation will fail with an error. This constraint is commonly used to enforce data integrity and to prevent unexpected behavior when working with the database.

 

What is Primary Key in MySQL

A primary key is a column or set of columns that uniquely identify each row in a table. A primary key constraint ensures that the values in the primary key columns are unique and not null, meaning they cannot be empty.

By defining a primary key for a table, you can ensure data integrity and enable efficient data retrieval operations, as primary keys are typically used for indexing and searching.

In addition, primary keys can be referenced by foreign keys in other tables, enabling the creation of relationships between tables.

 

What are Default Values in MySQL

Default values are used to specify a value that is automatically assigned to a column if no value is provided during insertion. The default value is defined when a table is created, and can be specified for each column individually.

For example, if you have a "users" table with a "created_at" column that has a default value of the current timestamp, then if you insert a row without providing a value for "created_at", MySQL will automatically assign the current timestamp to that column.

Here's an example of creating a table with a default value for a column:

CREATE TABLE example (
   id INT NOT NULL PRIMARY KEY,
   name VARCHAR(50) DEFAULT 'John Doe',
   age INT DEFAULT 18
);

In this example, the "name" column has a default value of "John Doe" and the "age" column has a default value of 18. If you insert a row without providing a value for either of these columns, the default values will be used.