SQL NULL: null value

In SQL, NULL is a special marker used to indicate that a data value does not exist in the database. A NULL value is different from a zero value or a field that contains spaces.

Here are a few key things to understand about NULL:

NULL is not the same as an empty string or the number zero.
NULL is not the same as a value of zero or a value of blank space. NULL means no entry was made or an explicit NULL was supplied, and it generally implies that the value is either unknown or not applicable.

Here's how you could use NULL:

1. Inserting a NULL value into a database:

You can insert a NULL value in a database when you don't have the data for a specific column.

INSERT INTO table_name (column1, column2, column3) 
VALUES ('Value1', NULL, 'Value3');

2. Updating a column with NULL:

You can update a column's value to NULL.

UPDATE table_name 
SET column1 = NULL 
WHERE condition;

3. Selecting rows with NULL in the column:

When selecting data from a database, you might want to filter out rows that contain NULL in the column. You can use the IS NULL and IS NOT NULL operators to do this.

SELECT column1, column2, ...
FROM table_name
WHERE column_name IS NULL;

SELECT column1, column2, ...
FROM table_name
WHERE column_name IS NOT NULL;

Remember, comparisons with NULL can be tricky. For example, the condition NULL = NULL does not return true but instead returns unknown. This is because NULL represents a lack of data, so it's unknown whether it's equal to another NULL value.

As always, the exact syntax may vary between different SQL dialects, so be sure to consult the documentation for the SQL dialect you're using.

How to Insert NULL in SQL:

Description: To insert a NULL value into a column, you can explicitly use the keyword NULL or omit the column in the INSERT statement.

Code Example:

-- Explicitly insert NULL
INSERT INTO example_table (column1, column2) VALUES (NULL, 'Value');

-- Omit column to insert NULL
INSERT INTO example_table (column2) VALUES ('Value');

Check if Column is NULL in SQL:
- Description: You can use the IS NULL condition to check if a column contains a NULL value.
- Code Example:
```
SELECT *
FROM example_table
WHERE column1 IS NULL;
```
SQL IS NULL vs = NULL:
- Description: Use IS NULL to check for NULL values. Avoid using = NULL as it might not work as expected.
- Code Example:
```
SELECT *
FROM example_table
WHERE column1 IS NULL;
```
Update Column to NULL in SQL:
- Description: Use the UPDATE statement to set a column to NULL.
- Code Example:
```
UPDATE example_table
SET column1 = NULL
WHERE condition;
```
Handling NULL Values in SQL Queries:
- Description: When querying, consider using functions like COALESCE or IS NULL to handle NULL values appropriately.
- Code Example:
```
SELECT column1, COALESCE(column2, 'DefaultValue') AS column2_alias
FROM example_table
WHERE column1 IS NOT NULL;
```
Coalesce Function in SQL for Handling NULL:
- Description: The COALESCE function returns the first non-NULL expression in a list.
- Code Example:
```
SELECT column1, COALESCE(column2, 'DefaultValue') AS column2_alias
FROM example_table;
```
SQL NULLIF Function Usage:
- Description: The NULLIF function returns NULL if two expressions are equal; otherwise, it returns the first expression.
- Code Example:
```
SELECT NULLIF(column1, 0) AS column1_no_zeros
FROM example_table;
```

Common Pitfalls with SQL NULL Values:

Common Pitfall: Using = NULL for comparison.

-- Pitfall: This might not work as expected
SELECT *
FROM example_table
WHERE column1 = NULL;

Common Pitfall: Forgetting to handle NULLs in calculations.

-- Pitfall: This could result in unexpected behavior if column1 is NULL
SELECT column1 + 5 AS calculated_value
FROM example_table;