This should produce the following table: id SELECT id, date, row_number : row_number + 1 row_num FROM attendance CROSS JOIN ( SELECT row_number : = 0 ) r To transform the data into this table it’s important to know user variables can hold a value from one row to the next. We want to end up with a table like below. The goal is to convert these data into a start_date and stop_date which would greatly reduce the storage needs.įor id 1 the start_date and stop_date equivalents would look like: id You can insert it into a MySQL or MariaDB database and follow along. The TL DR version: Order of operations matter a lot in user variables and wrap the user variable in a subquery or function to force evaluation.Īt this bottom of the article I’ve included the data used in this article.
If you don’t know how to force the user variables to be evaluated when you need them, then the results are unpredictable. One word of warning, the above article is not really a suggessted reading before this article–it’s more of required reading. This article is going to stick with the same philosophy–I don’t need to necessarily understand them, however, I want to be able to predict their behavior. Which focuses on getting the desired behavior from user variables rather than understanding them.
I’ve spent some time researching on the interweb and about the best article I’ve found on the subject is: Slightly tweaking Dante, “Abandon all normal, ye who enter here.” User variables are weird. Out of desperation, I found myself researching the possibility of creating my own functions using MySQL User Variables. On top of it, I only had read access to the database without the ability to create a stored procedure. I was surprised to find MySQL 10.2 and less does not support some common Windowing Functions, specifically, Value Functions and Rank Functions. Recently I was required to work with MySQL 10.0. Now we are at the Raspberry Pi command prompt we are going to run a script which will pull down the latest version of NodeJS built for ARM and install it to the Raspberry Pi.īut before we can do that we need to install a few helper programsĪt the command prompt type and say “yes” when prompted. Luckily, there have been people smarter than me who’ve already done some heavy lifting for us.Īlright, start by SSH’ing into your Raspberry Pi. Now you’ve setup the Arch Linux card and SSH’ed lets go through setting up a NodeJS environment on the rp0w.
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