MySQL中符号@的作用

SELECT (@i:=@i+1),t.* FROM table_name t,(SELECT @i:=0) AS j

SELECT
	( @i := @i + 1 ),
	p.* 
FROM
	( SELECT * FROM sys_region ORDER BY create_time DESC ) p, ( SELECT @i := 0 ) k

SELECT
	@r := CASE WHEN @type = a.LEVEL THEN 
			@r + 1 
		ELSE 
			1 
		END AS rowNum,
	@type := a.`level` AS type,
	a.id 
FROM
	sys_region a,( SELECT @r := 0, @type := '' ) b;

INSERT INTO t_top ( col_account, col_date, p_account, p_name )

SELECT b.col_account, b.col_date, b.p_account, b.p_name FROM
(
	SELECT
		@num := CASE WHEN @account = a.col_account THEN 
				@num + 1 
			ELSE 
				1 
			END AS rownum,
		@account := a.col_account AS account,
		a.* 
	FROM
		( SELECT * FROM zb_top  ORDER BY col_account, p_avg DESC ) a,
	( SELECT @num := 0, @account := '' ) j
) b where 6 > b.rownum

CREATE TABLE `pd` (
	`imei` varchar(32) NOT NULL DEFAULT '',
	`date` datetime DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `pd` VALUES ('1', '2013-07-25 00:00:01');
INSERT INTO `pd` VALUES ('1', '2013-07-26 00:00:02');
INSERT INTO `pd` VALUES ('2', '2013-07-23 00:00:04');
INSERT INTO `pd` VALUES ('2', '2013-07-26 00:00:03');
INSERT INTO `pd` VALUES ('3', '2013-07-26 00:00:01');

SELECT * FROM
(
	SELECT
		imei user_id,
		max(max_dd),
		max(max_dd_2),
		to_days(max(max_dd)) - to_days(max(max_dd_2)) days 
	FROM
		(
			SELECT
				imei,
				max_dd,
				max_dd_2 
			FROM
				(
					SELECT
						tmp.imei,
						tmp.date,
						IF(@imei = tmp.imei, @rank := @rank + 1, @rank := 1) AS ranks,
						IF(@rank = 1, @max_d := tmp.date, @max_d := NULL) AS max_dd,
						IF(@rank = 2, @max_d_2 := tmp.date, @max_d_2 := NULL) AS max_dd_2,
						@imei := tmp.imei 
					FROM
						(SELECT imei, date FROM pd ORDER BY imei ASC, date DESC) tmp,
						(SELECT @rownum := 0, @imei := NULL, @rank := 0, @max_d := NULL, @max_d_2 := NULL) a 
				) result 
		) t 
	GROUP BY
		imei 
	HAVING
		count(*) > 1 
) x WHERE x.days >= 1 AND EXISTS (SELECT 'x' FROM pd WHERE date > '2013-07-26 00:00:00')