PostgreSQL: IF EXISTS COMMENT ON TABLE

Question

I am little bit stucked with task IF EXIST.

I have several lines query with the COMMENTS ON TABLE function

COMMENT ON TABLE my_table1 IS 'Comment for table1';
COMMENT ON TABLE my_table2 IS 'Comment for table2';
COMMENT ON TABLE my_table3 IS 'Comment for table3';
COMMENT ON TABLE my_table3 IS 'Comment for table4';

I want to do, when I will execute this query, if one of the table doesn't exist query will pass that and continue.

I tried to do that task with IF EXIST action, but it doesn't works like UPDATE or other actions. Where am I wrong?

IF EXISTS (SELECT relname FROM pg_class where relname='my_table1')
then
COMMENT ON TABLE my_table1 IS 'Comment for table1';
IF EXISTS (SELECT relname FROM pg_class where relname='my_table2')
then
COMMENT ON TABLE my_table1 IS 'Comment for table2';
IF EXISTS (SELECT relname FROM pg_class where relname='my_table3')
then
COMMENT ON TABLE my_table1 IS 'Comment for table3';
IF EXISTS (SELECT relname FROM pg_class where relname='my_table4')
then
COMMENT ON TABLE my_table1 IS 'Comment for table4';

Answer 1

You could use:

DO
$do$
BEGIN
  IF EXISTS (SELECT relname FROM pg_class where relname='my_table1') THEN
    COMMENT ON TABLE my_table1 IS 'Comment for table1';
  END IF;

  IF EXISTS (SELECT relname FROM pg_class where relname='my_table2') THEN
    COMMENT ON TABLE my_table1 IS 'Comment for table2';
  END IF;
END
$do$

And check:

SELECT relname, obj_description(oid) 
FROM pg_class 
WHERE relname LIKE 'my_table%';

DBFiddle Demo