Filtering rows

In the previous page we learned how select, derive, and join change the columns of a table.

Now we will explore how to manipulate the rows of a table using filter and take.

filter transform

The filter transform picks rows to pass through based on their values:

1. from invoices
2. filter billing_city == "Berlin"

The resulting table contains all the rows that came from Berlin.

PRQL converts the single filter transform to use the appropriate SQL WHERE or HAVING command, depending on where it appears in the pipeline.

take transform

The take transform picks rows to pass through based on their position within the table. The set of rows picked can be specified in two ways:

• a plain number x, which will pick the first x rows, or
• an inclusive range of rows start..end.

1. from invoices
2. take 4

1. from invoices
2. take 4..7

Of course, it is possible combine all these transforms into a single pipeline:

1. from invoices

3. # retain only rows for orders from Berlin
4. filter billing_city == "Berlin"

6. # skip first 10 rows and take the next 10
7. take 11..20

9. # take only first 3 rows of *that* result
10. take 3

We did something a bit odd at the end: first we took rows 11..20 and then took the first 3 rows from that result.

Note

Note that a single transform take 11..13 would have produced the same SQL. The example serves an example of how PRQL allows fast data exploration by “stacking” transforms in the pipeline, reducing the cognitive burden of how a new transform with the previous query.