SQL - Looping through ever row of table in MySQL? - mysql

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 that's 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()