# The three valued logic

I have been written quite a few migrations and reviewed quite a few of them at [Anarock Tech](https://tech.anarock.com/). This is one of the most common mistakes I see developers making, it's simple but quite easy to overlook.

##  Context

Can you spot the issue with this simple migration?

![wrong migration](https://i.imgur.com/SHU7RSZ.png)

Kudos if you spotted it, don't worry if you didn't. Here the problem is the `complete` field can have 3 values `true`, `false`, and `NULL`.

## The three-valued logic

According to Wikipedia

> Since Null is not a member of any data domain, it is not considered a "value", but rather a marker (or placeholder) indicating the undefined value. Because of this, comparisons with Null can never result in either True or False, but always in a third logical result, Unknown.

Basically, `NULL` is not the same as `false`. 

In SQL: 
- `true AND NULL` is __NULL__ (not false)
- `false OR NULL` is __NULL__ (not false)
- `true AND NULL OR false` is also __NULL__

Having NULL values in your boolean column might break down causing a lot of issues, even if you handle it everywhere in your application it's not a good practice and it might break down inadvertently or cause a fresh pair of eyes to be confused.

Apart from this queries like

> SELECT * FROM projects WHERE complete = FALSE;

It won't give you the correct results.

## The Fix

Fortunately, the fix is extremely simple.

The migration is missing `null: false`, adding this avoids the field from storing `NULL` values apart from the usual `true` and `false`.

But that's not it! Since we are not allowing Postgres to save the `NULL` value it's a good practice to provide it with a default value in the cases where we don't specify anything. The default value is mandatory in cases where you are adding a column in a pre-existing table as Postgres doesn't know what to set the column value for the existing projects.

The correct migration should be like this:

![right migration](https://i.imgur.com/6N7k7ab.png)

> __To sum up, add a NOT NULL constraint and set a default value for boolean data types to be safe while keeping your data normalized.__