How to get cumulative daily sales from a table

I have 2 simple tables, one is a calendar listing of all days in a month and the other is a table showing sales amounts on specific days:

Calendar:

-- ----------------------------
-- Table structure for `calendar`
-- ----------------------------
DROP TABLE IF EXISTS `calendar`;
CREATE TABLE `calendar` (
  `Date` date DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

-- ----------------------------
-- Records of calendar
-- ----------------------------
INSERT INTO `calendar` VALUES ('2011-03-01');
INSERT INTO `calendar` VALUES ('2011-03-02');
INSERT INTO `calendar` VALUES ('2011-03-03');
INSERT INTO `calendar` VALUES ('2011-03-04');
INSERT INTO `calendar` VALUES ('2011-03-05');
INSERT INTO `calendar` VALUES ('2011-03-06');
INSERT INTO `calendar` VALUES ('2011-03-07');
INSERT INTO `calendar` VALUES ('2011-03-08');
INSERT INTO `calendar` VALUES ('2011-03-09');
INSERT INTO `calendar` VALUES ('2011-03-10');
INSERT INTO `calendar` VALUES ('2011-03-11');
INSERT INTO `calendar` VALUES ('2011-03-12');
INSERT INTO `calendar` VALUES ('2011-03-13');
INSERT INTO `calendar` VALUES ('2011-03-14');
INSERT INTO `calendar` VALUES ('2011-03-15');
INSERT INTO `calendar` VALUES ('2011-03-16');
INSERT INTO `calendar` VALUES ('2011-03-17');
INSERT INTO `calendar` VALUES ('2011-03-18');
INSERT INTO `calendar` VALUES ('2011-03-19');
INSERT INTO `calendar` VALUES ('2011-03-20');
INSERT INTO `calendar` VALUES ('2011-03-21');
INSERT INTO `calendar` VALUES ('2011-03-22');
INSERT INTO `calendar` VALUES ('2011-03-23');
INSERT INTO `calendar` VALUES ('2011-03-24');
INSERT INTO `calendar` VALUES ('2011-03-25');
INSERT INTO `calendar` VALUES ('2011-03-26');
INSERT INTO `calendar` VALUES ('2011-03-27');
INSERT INTO `calendar` VALUES ('2011-03-28');
INSERT INTO `calendar` VALUES ('2011-03-29');
INSERT INTO `calendar` VALUES ('2011-03-30');
INSERT INTO `calendar` VALUES ('2011-03-31');

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and:

DROP TABLE IF EXISTS `sales`;
CREATE TABLE `sales` (
  `Date` date NOT NULL,
  `A` int(11) NOT NULL DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

-- ----------------------------
-- Records of sales
-- ----------------------------
INSERT INTO `sales` VALUES ('2011-03-03', '20');
INSERT INTO `sales` VALUES ('2011-03-07', '15');
INSERT INTO `sales` VALUES ('2011-03-14', '25');
INSERT INTO `sales` VALUES ('2011-03-15', '10');
INSERT INTO `sales` VALUES ('2011-03-30', '5');
INSERT INTO `sales` VALUES ('2011-03-31', '10');

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I'm trying to end up with a table which shows for each day of the month the cumulative sales amounts. I've LEFT JOINed the tables with

SELECT calendar.Date, sales.A
FROM calendar LEFT JOIN sales using(Date)

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which gives:

2011-03-01  null
2011-03-02  null
2011-03-03  20
2011-03-04  null
2011-03-05  null
2011-03-06  null
2011-03-07  15
2010-03-08  null
etc
etc

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but what I want is for this to become:

2011-03-01  0
2011-03-02  0
2011-03-03  20
2011-03-04  20
2011-03-05  20
2011-03-06  20
2011-03-07  35
2010-03-08  35
etc
etc

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How can I do this?