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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 …;
Exercise
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.
Table: buildings (Read-only)
Table: employees (Read-only)
Query results
Exercise 8 — Tasks
- Find the name and role of all employees who have not been assigned to a building
- Find the names of the buildings that hold no employees
Stuck? Read this task's Solution.
Solve all tasks to continue to the next lesson.
Finish above Tasks
Solve all tasks to continue to the next lesson.