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LESSON 8

SQL (Core) - A short note on NULLs

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A short note on NULLs

In SQL, NULL represents missing, unknown, or unassigned data. Because NULL is not a value but the absence of a value, it requires special comparison techniques.

1. Three-Valued Logic & Comparing NULLs

Standard comparison operators (like = or !=) do not work with NULL because evaluating building = NULL yields UNKNOWN (NULL), not TRUE or FALSE. 

-- Correct way to query missing values
SELECT name, role FROM employees WHERE building IS NULL;

-- Correct way to query assigned values
SELECT name, role FROM employees WHERE building IS NOT NULL;

2. Default Values vs. NULL

An alternative to NULL values is providing data-type appropriate default values (like 0 for numbers or empty strings for text). However, if your database must track missing information accurately without skewing statistical averages, NULL is the proper choice.

🗄️ Schema Explorer SELECTABLE

› employees
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🎯 Exercises Checklist:

☐ 1. Find the name and role of all employees who have not been assigned to a building
☐ 2. Find the names of the buildings that hold no employees
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