Lesson
Custom expressions in the notebook editor
When you should use custom expressions and why you should take advantage of them in Metabase's notebook editor.
In mathematics, expressions are collections of symbols that together express a value. If you’ve ever worked with spreadsheet software before, expressions are formulas, like =SUM(A1, B1)
.
Custom expressions in Metabase’s Notebook Editor are powerful tools that can cover the vast majority of analytics use cases without the need to take SQL out of the toolbox. In fact, there are big advantages to using the Notebook Editor that you don’t get when you use SQL:

[Drillthrough] allows people to filter, break out, zoom in, or xrays the data on click. Drillthrough is not available for questions composed in the SQL editor.

Extensibility: Using the Notebook Editor to build queries allows those with whom you share your questions to learn from and build on your questions without needing to know any SQL.
And you can always switch to SQL at any point during your development by converting an existing Notebook Editor question to a native SQL question.
There are three places in the Notebook Editor where we can use custom expressions:
 Custom columns use functions to compute values (like
+
to add numbers) or manipulate text (likelower
to change text to lowercase).  Custom filters use functions like
contains
that evaluate to either true or false.  Custom summaries use functions like
count
orsum
to aggregate records.
Custom columns
We can add a custom column to our data using an expression to compute a new column. Let’s see an expression in action. Here is the Orders
table from the Sample dataset included with Metabase.
Let’s say we want to know the discount percentage applied to orders based on the pretax subtotal. For example, if we gave a $1 discount on a $10 order, we’d want to see a column show that we had discounted that order by 10%.
Unfortunately, a quick scan of the columns in the preview tells us that the database does not store that computation (i.e. there is no “discount percentage” column). We only have the subtotal of the order, and the discount total.
Thanks to math, however, we can use the discount total and order subtotal to compute the percentage. Here’s where expressions come into play: we can use an expression to compute the discount percentage for each row, and store that computed value in a new column.
Let’s walk through how to create a custom column.
When in the Notebook Editor, we select Add custom column in the Data section.
To calculate the discount percentage, we’ll need the original total (i.e., the subtotal + the discount), and then we’ll need to divide the discount by the original total to get the discount percentage.
In expressions, we reference columns using brackets. For example, we can refer to the Discount column in the Orders
table as [Discount]
. If we need to reference a column from another table linked by a foreign key, we can use a .
between the table and the column, as in [Table.Column]
(alternatively you can select [Table → Column]
from the dropdown menu that appears when you type an open bracket ([
). For example, we could enter [Product.Category]
which will resolve to: [Product → Category]
.
For now, we’re just interested in columns in the Orders
table, so there’s no need to reference another table. Here’s the expression (or formula), we’ll use to compute our custom discount percentage column:
= [Discount] / ([Discount] + [Subtotal])
Enter that expression in the field formula, then give the new column a name: Discount Percentage
.
Click done, then click the Visualization button to see your new column.
Since the value in our new Discount percentage
column concerns discounts, let’s move the column next to the Discount
column. You can move columns around on tables by clicking on the column header and dragging the column to your target location, like so:
Since we’re computing a percentage, let’s fix the formatting so it’s easier to read. Click on the Discount percentage
heading to bring up the Action Menu for the column, then click on the gears icon to format the column.
Metabase will slide out a Formatting sidebar with options. Let’s change the style to Percent
, and bump up the number of decimal places to 2
. And since the title Discount Percentage
takes up a lot of space, let’s rename the column to Discount %
.
There’s an option to add a mini bar chart as well. This bar chart won’t show the percentage with respect to 100%; instead the mini bar chart will show us the discount percentage relative to the discount percentage given to other orders. Let’s leave the mini bar chart off for now.
Here’s the finished question with the added Discount %
column:
Custom filters
Metabase comes with a lot of filtering options out of the box, but you can design more sophisticated filters using custom filter expressions. These are particularly useful for creating filters that use OR
statements, and that’s what we’ll be covering here.
Normally in the Notebook Editor, when we add multiple filters to our question, Metabase implicitly combines the filters with an AND
operator. For example, if we add a filter for products that are Enormous
and a filter for products that are Aerodynamic
, our question will only return products that are both Enormous
AND Aerodynamic
, which (enormous, aerodynamic products) do not exist in Metabase’s Sample Dataset.
To filter for products that are either Enormous
OR Aerodynamic
, we’ll select Custom Expression from the Filter dropdown, and use the contains
function to check if the product has either Enormous
or Aerodynamic
somewhere in the title.
contains(string1, string2)
contains
checks to see if string1 contains string2 within it. So string1
is the string to check (the haystack), and string2
is the text to look for (the needle). And since we want to look for either Enormous
or Aerodynamic
products, we can write two contains
expressions with an OR operator in between:
= contains([Title], "Enormous") OR contains([Title], "Aerodynamic")
The resulting data set will contain products that are either Enormous
or Aerodynamic
:
Note that custom filter expressions must always resolve to either true or false. In the case of the contains
function used above, the expression evaluates as true if the title has either Enormous
or Aerodynamic
in it, otherwise the expression evaluates as false.
You can, however, nest expressions that do not resolve to true or false within statements, like:
= contains(concat([First Name], [Last Name]), "Wizard")
because the outermost function (contains
) resolves to either true or false. Whereas you couldn’t use concat([First Name], [Last Name])
as a filter, as it would resolve to a string of text (though you could use concat to create a custom column like Full Name
).
Custom summaries
Custom expressions unlock many different ways to aggregate our data. Let’s consider the Share
function, which returns the percent of rows in the data that match the condition, as a decimal. For example, say we want to know the total percentage of paper products in our product line, i.e. what share of our product line is composed of paper products?
To start, we’ll select the Products
table from the Sample Dataset. Next, we’ll click the Summarize button in the Notebook Editor and select Custom Expression. Then, we’ll select Share
from the dropdown menu, which will prompt us for a condition. In this case, we want to know which products have “Paper” in their title, so we’ll use the contains
function to search through Title
.
= Share(contains([Title], "Paper"))
Then we name our expression (e.g., Percentage of Paper Products
) and click Done. Click the Visualize button and Metabase will compute the share of paper products.
To change the formatting, select the Settings button in the bottomleft to bring up the Settings sidebar and change Number options → Style
to Percent
.
Putting it all together
Let’s create a fairly complex (contrived) question using expressions. Say we’ve been tasked to find the average net inflow for wool and cotton products by month in 2019, with net inflow being the selling price minus the cost we paid for the product. In other words: for each wool and cotton product unit sold, how much money on average did we make (or lose) per unit each month in 2019?
To the get these fascinating numbers, we’ll need to use expressions to:
 Compute the selling price per unit (custom column).
 Filter results to only include wool or cotton products (custom filter), and limit those results to 2019.
 Compute the average net inflow (custom summary), and group by month.
Let’s go:

We create a custom column named
Unit price
. To compute theUnit price
, we’ll use an expression to divide the subtotal by the number of units sold (Quantity
):= [Subtotal] / [Quantity]

Next, we’ll use a custom filter expression to filter for orders of
Wool
andCotton
products (i.e. for products that contain “Wool” or “Cotton” somewhere in theirProduct.Title
).= contains([Products → Title], "Wool") OR contains([Products → Title], "Cotton")

We’ll also filter for orders between
01/01/2019
and12/31/2019
. 
We’ll use a custom expression to create a custom summary. Let’s assume the standard retail markup of 50% (the keystone markup). So if the
Product.Price
is $2, we’ll assume the product cost us $1 to acquire per unit. Given this assumption, we can simply define net inflow per unit sold to be theUnit price
minus half of theProduct.Price
. Then we’ll summarize that data by taking the average of those numbers for each order.= Average([Unit Price]  [Product → Price] / 2)

Lastly, we’ll group those orders by
Orders.Created_At
by month.
Here’s our notebook:
And our chart:
Since we composed this question using the Notebook Editor we can drill through our data:
Further reading
 Docs on writing expressions in the Notebook Editor. There are a lot more functions that you can use in expressions that we didn’t cover, so check out the docs for the full list of functions.
 Time series comparisons shows how to use custom expressions to visualize and compare time series.