Reading old phpfreaks threads -- what does this code do (from Barand's reply)?

[doni49]

I'm reading through some older threads on the forum -- I find that reading other people's issues and the answers they receive is a great education in itself -- and saw some code in a reply posted by Barand that I don't understand.

 

Here's a link to the post in question http://forums.phpfreaks.com/topic/299309-cumulative-sum-in-mysql-view-calculating-running-profitloss/?do=findComment&comment=1525829

 

The code in question: 

JOIN (SELECT @cum:=0) init

@cum:=@cum+profit as cum_profit

I get that the @ symbol indicates a global variable.  But what is the := for?  And is this "init" being used similar to a for loop?

 

[requinix]

A single equals sign means comparison; @cum=0 would be saying "is @cum equal to 0".

Colon+equals does assignment and will set @cum to 0 in the first part and then add profit in the second part.

 

And yes, the init part is working like the init part in a for loop.

for ($cum = 0; ; $cum = $cum + $profit) {

You could do the assignment in a separate query beforehand

SELECT 0 INTO @cum

which would be like

$cum = 0;
for (; ; $cum = $cum + $profit) {

[benanamen]

Just one thing that wasn't mentioned, it is called a user-defined variable in case you or someone else wants to look it up.