Question

I am currently successfully using a CASE expression to update an empty column based on attributes from other columns. For example

UPDATE table SET cat = CASE 
WHEN term = '{"Boulder"}' then 'Boulder'

However, I need to do the same but on an text array and particularly when an element is in a specific position within that array.

For example if the data looks like

{"Boulder, Tree, Bush"}

WHEN position('Tree' in term) > 0 then 'Boulder'

But I receive an error message

function pg_catalog.position(character varying[], unknown) does not exist

I have used position in a function before so I am not sure why PostgreSQL does not like it in this situation.

Is there a way to using a CASE expression whilst determining the position of a text element within an array.

Answer 1

Apparently your term column is defined as an array, e.g. varchar[]. The position function only works with scalar values, not with arrays.

If you want to test if an element is contained in an array you need to use a different operator: @>

update foobar
   set cat = 'Boulder'
where term @> '{"Boulder"}'

The expression '{"Boulder"}' creates an array with a single element. It’s equivalent to array['Boulder'] (which I find more readable). So the above where condition updates all rows where the array term contains all elements of the array on the right hand side of the operator. In this case it’s only a single element you are testing for.

More details about the array functions and operators can be found in the manual: http://www.postgresql.org/docs/current/static/functions-array.html

Edit after the requirements have changed

To find and update only those where boulder is in the first, second or third place, you can use this:

update foobar
   set cat = 'Boulder'
where 'Boulder' in (term[1], term[2], term[3]);