How do you replace a SUMIF function (from excel) into Access?

[Excelnoobie2013]

I currently have a SUMIF function (in Excel) that looks like this (=IF(R1061=0,SUMIF($S$99:$S$19626,S1061,$P$99:$P$1 9626),.
If it helps any it represents this (=IF(WeekEndDate=0,SUMIF($(PO#)$99:$(PO#)$19626,(P O#)1061,$(Amount)$99:$(Amount)$19626),)
So what it does is sum the amounts for every line that has that PO# if there is no week ending date listed, otherwise it leaves a null value.

I'm a beginner at best to Access, and am much more versed with Excel so forgive me. I am trying to import my excell database (which has grown much too large to continue using Excel) and this is the biggest barrier to entry for me.

From reading around the web (this site included) all I came up with was a function around the lines of IFF([Week End]=0,SUM([PO #],[Amount]),null) however, that doesn't A.) work and B.) exactly duplicate my previous function at all. Any help would be greatly appreciated

[Rabbit]

You're doing this in an aggregate query? No need for a sum if, just filter out the records you don't want to sum on.