How do I perform an IF...THEN in an SQL SELECT in snowflake?

Snowflake as for today does not support SQL-based procedural logic. The query could be rewritten as:

SELECT *
FROM (SELECT *, 1 AS priority FROM table2
      UNION ALL
      SELECT *, 2 AS priority FROM table1) sub
QUALIFY piority = MIN(priority) OVER();


-- or
SELECT * FROM table2
UNION ALL
SELECT * FROM table1 WHERE NOT EXISTS (SELECT 1 FROM table2);

Assumption: both tables have the same structure.

EDIT:

Using Snowflake scripting it is possible to use branching structures:

declare
    res RESULTSET;
    query VARCHAR;
    tab_name STRING;
begin
    if (EXISTS(select 1 from table2)) then
       tab_name := 'table2';
    else
       tab_name := 'table1';
    end if;
    
    query := 'SELECT * FROM ' || :tab_name;
   
    res := (execute immediate :query);
    return table(res);
end;