getting started with loops and iteration in sequel pro - mysql

Question

I'm using sequel pro to select data from several tables. There are two things I need to do that seem to need a loop of some kind. I have never used any form of iteration in sql and can't find a beginners-level resource to learn from.
Can anyone suggest how to do the following two tasks, or suggest a tutorial where I can learn the fundamentals and figure it out from there:
Task 1: Go through a version history table, find the relevant history record for a given id that applied at a given date, and select the value from that record. The form of the history table is:
id, Item_id, version-created_at, value
eg
1, 123, 2014-05-01, 754
2, 456, 2014-05-10, 333
3, 123, 2014-05-27, 709
and I need to find what the value of item 123 was on the date 2014-05-25 (ie I need to find record id=1 and value = 754 because that is the most recent version for item 123 created prior to my target date.
So I figure I need to run through the table looking for item 123 and comparing dates of those records. But I don't know how to deal with the iteration of moving from one record to the next and comparing them.
Task 2: Go through a single text field that contains a number of product id and matching product prices in a string, and find the id of the product with the lowest price. Form of the string is a series of pairs of price "p" and id "i", in random order, like this:
"
- :p: 99.8
:i: 3
- :p: 59.0
:i: 5
- :p: 109.8
:i: 18
- :p: 82.45
:i: 46
"
and in this example I need to find "5", being the id of the product with the lowest price $59.
So I figure I need to step through each of the p/i sets, maybe by counting characters, but I have no idea how to iterate through and compare to find the best price.
A little help would go a long way.
Thanks.

Answer 1

For first answer you can do something like this:-
SELECT value FROM history where id = 123 AND version-created_at = '2014-05-01';
and for another task you must try this at front end rather than at back end.