Trigger function and trigger to change one column after an update

Question

I'm new to psql and triggers. I am trying to create a trigger function and a trigger such that on an update of a single column the value of that column for the rest of table should be updated. Here is an example

id	col1	col2
1	text	true
2	text	false
3	text	false
4	text	false
5	text	false

With the table above, assume that there is an update to the first row, where the value of col2 has been set to true . I want to create a trigger function and trigger such that when there is an another update on the table, for example we set col2 for id=3 to true , the value of col2 for id=1 should be set to false .

Basically, what I want the trigger to do is to ensure that only one row in the entire table can have col2 set as true

Answer 1

You can try a function like this:

CREATE OR REPLACE FUNCTION test_set_false()
  RETURNS "pg_catalog"."trigger" AS $BODY$
    DECLARE
BEGIN

if new.col2 = true then 
update test_table set col2 = false where col2 = true and id != new.id;
end if;

return new;
END;

$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

and call it in trigger (before update)

CREATE  TRIGGER "test_set_false_update_trigger" BEFORE update ON test_table
                FOR EACH ROW
                EXECUTE PROCEDURE test_set_false();