SQL-在MySQL中遍历表的每一行？

小编典典

SQL-在MySQL中遍历表的每一行？

sql

所以我有2张桌子communication，和movement。

communication具有列fromID，timestamp具有呼叫者的ID和进行呼叫的时间。然后，我有另一台movement已ID，timestamp，x，y，，有一个人，他们的位置的ID
(x,y)，而且它们在该位置的时间。

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，这是在寻找movement timestamp给定值的最接近值communication timestamp。之后，最终，我想根据数据查找(x,y)呼叫的位置movement。

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。我调查了一下cursors，但是我感到在那方面的表现很糟糕……所以循环中是否有这样做的方法呢？我本质上是想遍历的每一行communication并获取结果…

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！！

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2021-03-23

共1个答案

小编典典

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 ;

2021-03-23