SQL & PL/SQL

CREATE TABLE tab1 (
    e_id    VARCHAR2(255),
    e_date  VARCHAR2(255),
    t_ref_num  VARCHAR2(255),
    CONSTRAINT pk_tab1 PRIMARY KEY ( e_id )
);

    INSERT INTO tab1 VALUES (
    1,
    '01-01-2000',
    11
);

INSERT INTO tab1 VALUES (
    2,
    '01-01-2001',
    12
);

INSERT INTO tab1 VALUES (
    3,
    '01-01-2002',
    13
);

CREATE TABLE tab2 (
    e_id       NUMBER(20),
    e_date     DATE,
    t_ref_num  NUMBER(20),
    CONSTRAINT pk_tab2 PRIMARY KEY ( e_id )
);

I need to insert it into the tab2 table and need to convert the data type as per the tab2 table datatype. Because in tab1 table I will get always varchar datatype but while inserting into the tab2 table I have to typecast it and have to insert in proper datatype. How can I achieve this?

MERGE INTO tab2 tt
USING (
          SELECT
              e_id,
              e_date,
              _t_ref_num
          FROM
              tab1
      )
t1 ON ( t2.e_id = t1.e_id )
WHEN MATCHED THEN UPDATE
SET tt.e_date = t1.e_date,
    tt.t_ref_num = t1.t_ref_num
WHEN NOT MATCHED THEN
INSERT (
    e_id,
    e_date,
    t_ref_num )
VALUES
    ( t1.e_id,
      t1.e_date,
      t1.t_ref_num );

I have to make the changes in the merge statement only