Presto equivalent of CONCAT_WS

I’m looking for a function in Presto to concat two columns with a separator like underline.

Answer

Your are looking here for the array_join function, see docs.

array_join(x, delimiter, null_replacement) → varchar

Concatenates the elements of the given array using the delimiter and an optional string to replace nulls.

Example:
columns are c1,c2 you can add more of course:

WITH  demo_table (c1,c2) AS 
    (SELECT * FROM  (VALUES  (1,2),(3,4),(5,null),(7,8) ))
SELECT array_join(array[c1,c2], '_', 'NA')
FROM demo_table

Results will be:
1_2
3_4
5_NA
7_8