SQL NOT NULL: not-null constraint

In SQL, the NOT NULL constraint enforces a column to not accept NULL values. This means, when you're creating a table, you can specify that a particular column must always have a value; it cannot be left blank.

Here's how you can define a NOT NULL constraint when creating a table:

CREATE TABLE Employees (
    ID int NOT NULL,
    FirstName varchar(255) NOT NULL,
    LastName varchar(255) NOT NULL,
    Email varchar(255)
);

In the example above, the ID, FirstName, and LastName columns cannot remain empty when inserting records into the "Employees" table, whereas the Email field could potentially be left NULL.

If you try to INSERT a new row in "Employees" without providing values for ID, FirstName, or LastName, you would receive an error.

If you have an existing table and you want to add a NOT NULL constraint to a column, you would first need to ensure that all existing records have a value in that column. Then you can use the ALTER TABLE statement:

ALTER TABLE Employees
MODIFY COLUMN Email varchar(255) NOT NULL;

The ALTER TABLE statement above changes the Email column to enforce NOT NULL, meaning all future records will need to have a value for Email.

Keep in mind, the SQL may vary slightly based on the SQL dialect you're using (MySQL, PostgreSQL, SQL Server, etc.), so always check the specific documentation for details.

How to Use NOT NULL in SQL:
- Description: NOT NULL is a constraint used in SQL to ensure that a column cannot have a NULL value.
- Code Example:
```
CREATE TABLE example_table (
    column1 INT NOT NULL,
    column2 VARCHAR(50) NOT NULL
);
```
SQL Create Table with NOT NULL:
- Description: When creating a table, you can specify NOT NULL to enforce that certain columns must have a value.
- Code Example:
```
CREATE TABLE employees (
    employee_id INT NOT NULL,
    employee_name VARCHAR(100) NOT NULL,
    salary DECIMAL(10, 2) NOT NULL
);
```

Difference Between NULL and NOT NULL in SQL:

Description: NULL represents the absence of a value, while NOT NULL ensures that a column must have a value.

Example:

-- NULL is allowed
INSERT INTO example_table (column1, column2) VALUES (NULL, 'Value');

-- NOT NULL constraint violation
INSERT INTO example_table (column1, column2) VALUES (NULL, NULL);

SQL NOT NULL vs DEFAULT:
- Description: NOT NULL ensures that a column must have a value, while DEFAULT provides a default value if none is specified.
- Code Example:
```
CREATE TABLE example_table (
    column1 INT NOT NULL,
    column2 VARCHAR(50) DEFAULT 'DefaultValue'
);
```
Adding NOT NULL Constraint to Existing Column in SQL:
- Description: You can use the ALTER TABLE statement to add a NOT NULL constraint to an existing column.
- Code Example:
```
ALTER TABLE existing_table
ALTER COLUMN existing_column INT NOT NULL;
```
Remove NOT NULL Constraint in SQL:
- Description: Use ALTER TABLE to remove the NOT NULL constraint from an existing column.
- Code Example:
```
ALTER TABLE existing_table
ALTER COLUMN existing_column DROP NOT NULL;
```
SQL Check if Column is NOT NULL:
- Description: You can query the information schema to check if a column is defined as NOT NULL.
- Code Example:
```
SELECT COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'your_table' AND IS_NULLABLE = 'NO';
```

Common Errors with SQL NOT NULL Constraint:

Common Error: Trying to insert a NULL value into a NOT NULL column.

Code Example:

-- Error: Violation of NOT NULL constraint
INSERT INTO example_table (column1, column2) VALUES (NULL, 'Value');

Common Error: Attempting to create a table without providing a value for a NOT NULL column.

-- Error: Column 'column1' cannot be null
CREATE TABLE example_table (
    column1 INT NOT NULL,
    column2 VARCHAR(50)
);