Handling null (empty array) while converting json to csv with jq?

Question

When trying to convert json to csv, I get an error about null with the following jq query:

printf "[]" | jq -r '(map(keys) | add | unique) as $cols | map(. as $row | $cols | map($row[.])) as $rows | $cols, $rows[] | @csv'

It gives the following error,

jq: error (at <stdin>:0): Cannot iterate over null (null)

but works fine if the json array is non-empty and has an object in it:

$ printf '[{"a":1}]' | jq -r '(map(keys) | add | unique) as $cols | map(. as $row | $cols | map($row[.])) as $rows | $cols, $rows[] | @csv'
"a"
1

The jq grammar looks great, but I am not familiar with it as I just started to use this tool.

Can someone please explain how to patch the query to output nothing when the array is empty? (first example).

Answer 1

You can use a Error Suppression / Optional Operator: ? block to counter against an empty array invocation and return without throwing any errors to the console as

printf '[]' | jq -r '.? |(map(keys) | add | unique) as $cols | map(. as $row | $cols | map($row[.])) as $rows | $cols, $rows[] | @csv'

where .? can also be written explicitly using a try catch block as try .? and have rest of the filter following it. The part of the filter after the optional operator is skipped if the expression fails.