[英]SQL - Looping through ever row of table in mysql?
所以我有2张桌子, communication
和movement
。
communication
具有fromID
列,具有呼叫者ID的timestamp
和呼叫时间。 然后,我进行了另一个表movement
,该表movement
具有ID
, timestamp
, x
, y
,具有一个人的ID,他们的位置(x,y)
以及他们在该位置的时间。
我想写一个看起来像这样的查询:
For every single row of communication(R)
SELECT * FROM movement m
WHERE m.ID = R.fromID && m.timestamp <= R.timestamp
ORDER BY timestamp
基本上,这是为给定的communication timestamp
找到最接近的movement timestamp
communication timestamp
。 之后,最终,我想根据movement
数据找到呼叫的位置(x,y)
。
我该怎么做? 我知道有一种基于集合的方法,但是我不想那样做。 我看着cursors
,但是我感到在那方面的表现很糟糕……所以无论如何都要循环执行此操作? 我本质上是想遍历communication
每一行,并得到结果...
我尝试过这样的事情:
DELMITER $$
CREATE PROCEDURE findClosestTimestamp()
BEGIN
DECLARE commRowCount DEFAULT 0;
DECLARE i DEFAULT 0;
DECLARE ctimestamp DEFAULT 0;
SELECT COUNT(*) FROM communication INTO commRowCount;
SET i = 0;
WHILE i < commRowCount DO
SELECT timestamp INTO ctimestamp FROM communication c
SELECT * FROM movement m
WHERE m.vID = c.fromID && m.timestamp <= R.timestamp
END$$
DELIMITER ;
但是我知道那是完全错误的……是做这种游标的唯一方法吗? 我只是在Internet上的任何地方都找不到这样的示例,而且我对SQL过程完全陌生。 任何指导将不胜感激,谢谢!!
让我们看看我是否可以使用光标将您指向正确的方向:
delimiter $$
create procedure findClosestTimeStamp()
begin
-- Variables to hold values from the communications table
declare cFromId int;
declare cTimeStamp datetime;
-- Variables related to cursor:
-- 1. 'done' will be used to check if all the rows in the cursor
-- have been read
-- 2. 'curComm' will be the cursor: it will fetch each row
-- 3. The 'continue' handler will update the 'done' variable
declare done int default false;
declare curComm cursor for
select fromId, timestamp from communication; -- This is the query used by the cursor.
declare continue handler for not found -- This handler will be executed if no row is found in the cursor (for example, if all rows have been read).
set done = true;
-- Open the cursor: This will put the cursor on the first row of its
-- rowset.
open curComm;
-- Begin the loop (that 'loop_comm' is a label for the loop)
loop_comm: loop
-- When you fetch a row from the cursor, the data from the current
-- row is read into the variables, and the cursor advances to the
-- next row. If there's no next row, the 'continue handler for not found'
-- will set the 'done' variable to 'TRUE'
fetch curComm into cFromId, cTimeStamp;
-- Exit the loop if you're done
if done then
leave loop_comm;
end if;
-- Execute your desired query.
-- As an example, I'm putting a SELECT statement, but it may be
-- anything.
select *
from movement as m
where m.vID = cFromId and m.timeStamp <= cTimeStamp
order by timestampdiff(SECOND, cTimeStamp, m.timeStamp)
limit 1;
end loop;
-- Don't forget to close the cursor when you finish
close curComm;
end $$
delimiter ;
参考文献:
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