DECLARE @TableA TABLE
(
   user_name VARCHAR(10) NOT NULL,
   transaction_date DATETIME NOT NULL,
   transaction_id INT NOT NULL,
   PRIMARY KEY(user_name, transaction_date)
);

INSERT INTO @TableA(user_name, transaction_date, transaction_id)
   VALUES('User1', '20100101', 1),
         ('User1', '20100103', 2),
         ('User2', '20100102', 1),
         ('User2', '20100131', 1);

DECLARE @TableB TABLE
(
   date DATETIME NOT NULL PRIMARY KEY
);

INSERT INTO @TableB(date)
   SELECT DATEADD(DAY, V.number, '20100101')
     FROM master..spt_values AS V
    WHERE V.type = 'P'
      AND V.number BETWEEN 0 AND 30;

SELECT A1.user_name,
       B.date,
       A2.transaction_id
  FROM (SELECT DISTINCT user_name
          FROM @TableA) AS A1
       CROSS JOIN
       @TableB AS B
       LEFT OUTER JOIN
       @TableA AS A2
       ON A2.user_name = A1.user_name
          AND A2.transaction_date >= B.date
          AND A2.transaction_date < DATEADD(DAY, 1, B.date);