how to create an array from two columns?

[Rex X]

How to make the following work?

1. combine columns A and B to make one array as a new column AB. Both
column A and B are string types.

  select
string_columnA,
string_columnB,
*array(string_columnA, string_columnB) *as AB
from Table1;

2. append columnA to an existing array-type column B

select
string_columnA,
array_columnB,
array_flatmerge(string_columnA, array_columnB) as AB
from Table2;

In fact, I should say "set" instead of "array" above, since I expect no
duplicates.

Any idea?