一段代码看懂日期格式在 PL/SQL 中的作用

DECLARE
  l_Cursor_Num INTEGER;
  l_Date       DATE;
  l_Date_Char  VARCHAR2(20);
BEGIN
  EXECUTE IMMEDIATE q"?ALTER SESSION SET NLS_DATE_FORMAT = "YYYY-MM-DD"?";
  l_Date       := "2020-06-17"; -- PLSQL中的隐式转换：字符->日期
  l_Date_Char  := SYSDATE; -- PLSQL中的隐式转换：日期->字符
  l_Cursor_Num := Dbms_Sql.Open_Cursor();
  Dbms_Sql.Parse(l_Cursor_Num,
                 "SELECT :T_SELECT_DATE D FROM DUAL WHERE TRUNC(SYSDATE) = :T_WHERE_DATE", -- SQL 条件中的隐式转换：字符->日期
                 Dbms_Sql.Native);
  Dbms_Sql.Bind_Variable(l_Cursor_Num, ":T_SELECT_DATE", l_Date);
  Dbms_Sql.Bind_Variable(l_Cursor_Num, ":T_WHERE_DATE", l_Date_Char);
  Dbms_Sql.Define_Column(l_Cursor_Num, 1, l_Date_Char, 20);
  IF Dbms_Sql.Execute_And_Fetch(l_Cursor_Num) > 0 THEN
    Dbms_Sql.Column_Value(l_Cursor_Num, 1, l_Date_Char); -- 游标提取的转换：日期->字符
    Dbms_Output.Put_Line(l_Date_Char);
  END IF;
  Dbms_Sql.Close_Cursor(l_Cursor_Num);
END;