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
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 query builder that you don’t get when you use SQL:
- Drill-through allows people to filter, break out, zoom in, or x-rays the data on click. Drill-through is available for questions composed in the query builder and for models based on both GUI and SQL queries — as long as you’ve set the appropriate column type metadata.
- 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 simple or custom 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 (like
lowerto change text to lowercase).
- Custom filters use functions like
containsthat evaluate to either true or false.
- Custom summaries use functions like
sumto aggregate records.
Let’s say we want to know the discount percentage applied to orders based on the pre-tax 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 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
[Products.Category] which will resolve to:
[Products → 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] / [Subtotal]
Enter that expression in the field formula, then give the new column a name:
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
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:
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
Aerodynamic, which (enormous, aerodynamic products) do not exist in Metabase’s Sample Database.
To filter for products that are either
Aerodynamic, we’ll select Custom Expression from the Filter dropdown, and use the
contains function to check if the product has either
Aerodynamic somewhere in the title.
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
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
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
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
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 Database. 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
= 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 bottom-left 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.
We create a custom column named
Unit price. To compute the
Unit price, we’ll use an expression to divide the subtotal by the number of units sold (
= [Subtotal] / [Quantity]
Next, we’ll use a custom filter expression to filter for orders of
Cottonproducts (i.e. for products that contain “Wool” or “Cotton” somewhere in their
= contains([Products → Title], "Wool") OR contains([Products → Title], "Cotton")
We’ll also filter for orders between
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.Priceis $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 the
Unit priceminus half of the
Product.Price. Then we’ll summarize that data by taking the average of those numbers for each order.
= Average([Unit price] - [Products → Price] / 2)
Lastly, we’ll group those orders by
Here’s our notebook:
We’ll choose to visualize our data as a line chart, which we can click on to drill through our data:
- 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.
- Cleaning and formatting text uses custom expressions to deal with messy and missing data.
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