SQL-在MySQL中遍历表的每一行？

Question

So I have 2 tables, communication ,and movement .

communication has columns fromID , timestamp that has ID of caller, and time the call was made. Then I have another table movement that has ID , timestamp , x , y , that has the ID of a person, their location (x,y) , and the time that they are at that location.

I want to write a query that looks something like this:

For every single row of communication(R)
    SELECT * FROM movement m
    WHERE m.ID = R.fromID && m.timestamp <= R.timestamp
    ORDER BY timestamp 

Basically, what this is doing is finding the closest movement timestamp for a given communication timestamp . After that, eventually, I want to find the location (x,y) of a call, based on the movement data.

How would I do this? I know there's a set based approach, but I don't want to do it that way. I looked into cursors , but I get the feeling that the performance is terrible on that... so is there anyway to do this with a loop? I essentially want to loop through every single row of the communication , and get the result...

I tried something like this:

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 ; 

But I know thats completely wrong... Is the only way to do this cursors?? I just can't find an example of this anywhere on the internet, and I'm completely new to procedures in SQL. Any guidance would be greatly appreciated, thank you!!

Answer 1

Let's see if I can point you in the right direction using cursors:

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 ;

References:

• MySQL Reference: Cursors
• MySQL Reference: Date and time functions - timestampdiff()