SQL | NULL

In SQL, NULL represents missing or undefined data in a database. It's essential to understand that NULL is not the same as an empty string or zero; it signifies the absence of a value.

Here are some key points and operations involving NULL:

1. Inserting NULL:

You can explicitly insert a NULL value into a table column, assuming the column allows for null values:

INSERT INTO table_name (column1, column2) VALUES ('value', NULL);

2. Checking for NULL:

You cannot use standard comparison operators (e.g., =, !=) to check for NULL. Instead, you use IS NULL or IS NOT NULL:

-- Select rows where column_name is NULL
SELECT * FROM table_name WHERE column_name IS NULL;

-- Select rows where column_name is not NULL
SELECT * FROM table_name WHERE column_name IS NOT NULL;

3. NULL in Arithmetic Operations:

In most databases, any arithmetic operation involving NULL will result in NULL:

SELECT 5 + NULL;  -- Result is NULL

4. COALESCE() and NULLIF():

COALESCE(): Returns the first non-null value in the list. This function is handy for replacing null values:
```
SELECT COALESCE(column_name, 'Default Value') FROM table_name;
```
If column_name is NULL, the above query will return 'Default Value'.
NULLIF(): Compares two expressions and returns null if they are equal, or the first expression if they are not:
```
SELECT NULLIF(column_name, 'value_to_compare') FROM table_name;
```
If column_name is equal to 'value_to_compare', the query will return NULL. Otherwise, it will return the value in column_name.

5. Aggregate Functions:

Most aggregate functions, like SUM(), AVG(), MAX(), etc., typically ignore NULL values. For example, if you're calculating the average of a column, rows with NULL will not be considered in the computation.

6. JOIN Operations:

When joining tables, rows with NULL values might not match up as you might expect because NULL does not equal any value, including another NULL. To link rows based on NULL values, you'd have to use special conditions in your JOIN clause.

7. Default NULL Behavior:

When defining table structures, if a column doesn't explicitly specify a default value and isn't set as NOT NULL, it will default to NULL if no value is provided during an insert operation.

Note:

Handling NULL values properly is essential, especially during data analysis or reporting, as overlooked or mismanaged NULL values can lead to inaccurate results or interpretations. Always be mindful of potential NULL values in your data and handle them appropriately.

NULL vs. NOT NULL in SQL:
- NULL represents the absence of a value in a column.
- NOT NULL constraint ensures that a column cannot have a NULL value.
```
CREATE TABLE example_table (
    column1 INT NULL,
    column2 VARCHAR(255) NOT NULL
);
```

Using IS NULL and IS NOT NULL in SQL:

IS NULL checks for NULL values.
IS NOT NULL checks for non-NULL values.

SELECT * FROM example_table
WHERE column1 IS NULL;

SELECT * FROM example_table
WHERE column2 IS NOT NULL;

NULL values in arithmetic operations:
- Arithmetic operations involving NULL result in NULL.
```
SELECT column1 + 5 FROM example_table;
```
NULL values in aggregate functions in SQL:
- Aggregate functions ignore NULL values unless specified otherwise.
```
SELECT AVG(column1) FROM example_table;
```
Coalesce and NULLIF functions in SQL:
- COALESCE returns the first non-NULL expression in the list.
- NULLIF returns NULL if the two expressions are equal.
```
SELECT COALESCE(column1, 0) FROM example_table;

SELECT NULLIF(column1, 0) FROM example_table;
```
Dealing with NULL values in JOIN operations:
- INNER JOIN excludes rows with NULL values in the joining columns.
```
SELECT * FROM table1
INNER JOIN table2 ON table1.column1 = table2.column1;
```
NULL values and GROUP BY in SQL:
- GROUP BY treats NULL values as a single group.
```
SELECT column1, COUNT(*) FROM example_table
GROUP BY column1;
```
NULL values and ORDER BY in SQL:
- ORDER BY places NULL values at the end by default.
```
SELECT * FROM example_table
ORDER BY column1;
```

NULL values and subqueries in SQL:

Subqueries involving NULL may need special handling.

SELECT * FROM example_table
WHERE column1 IN (SELECT column1 FROM another_table WHERE column2 IS NULL);

Setting default values for NULL columns:
- Set default values for columns to replace NULL.
```
ALTER TABLE example_table
ALTER COLUMN column1 SET DEFAULT 0;
```

NULL values and foreign key constraints in SQL:

Foreign key columns can allow NULL values, but constraints must be handled appropriately.

CREATE TABLE table1 (
    id INT PRIMARY KEY,
    foreign_key_column INT NULL,
    FOREIGN KEY (foreign_key_column) REFERENCES table2(id)
);