SQL Lesson 8: A short note on NULLs

As promised in the last lesson, we are going to quickly talk about NULL values in an SQL database. It's always good to reduce the possibility of NULL values in databases because they require special attention when constructing queries, constraints (certain functions behave differently with null values) and when processing the results.

An alternative to NULL values in your database is to have data-type appropriate default values, like 0 for numerical data, empty strings for text data, etc. But if your database needs to store incomplete data, then NULL values can be appropriate if the default values will skew later analysis (for example, when taking averages of numerical data).

Sometimes, it's also not possible to avoid NULL values, as we saw in the last lesson when outer-joining two tables with asymmetric data. In these cases, you can test a column for NULL values in a WHERE clause by using either the IS NULL or IS NOT NULL constraint.

Select query with constraints on NULL values
SELECT column, another_column, … FROM mytable WHERE column IS/IS NOT NULL AND/OR another_condition AND/OR …;


This exercise will be a sort of review of the last few lessons. We're using the same Employees and Buildings table from the last lesson, but we've hired a few more people, who haven't yet been assigned a building.

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Table: buildings (Read-only)
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Table: employees (Read-only)
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Query results
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Exercise 8 — Tasks
  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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