How to add values of a column for each ID only if they meet a condition?

[ercksh8]

Hello all. So currently I am trying to create a table that will merge data from 4 datasets (say Y1, Y2, Y3, Y4).

The 4 datasets are panel data from year 1, 2, 3, and 4 respectively. I have merged the four data sets using the data merge by ID. When I viewed the resulting data, I realized that there were some IDs that are not present in all four datasets. I would like to be able to make a dataset that merges only the IDs that are present in all four years.

After doing so, I would like to create a new variable that adds all expenditures that were made by each ID in all 4 years, if they meet a separate condition. (i.e. if their zip code starts with the number 5). 

Y1

ID	Expenditure	Zipcode
1	50	20000
2	140	30000
3	30	51000
4	40	52000
5	50	50000

Y2

ID	Expenditure	Zipcode
6	100	50000
2	110	30000
3	120	51000
4	130	52000
5	140	50000

Y3

ID	Expenditure	Zipcode
1	50	20000
2	140	30000
3	30	51000
4	40	52000
5	50	50000

Y4

ID	Expenditure	Zipcode
1	50	20000
2	140	30000
3	30	51000
4	40	52000

If the datasets look something like this, the first step would create a data table with IDs 2, 3, 4 since they are the only ones present in all 4 datasets. Then the second part would create a new table with only IDs 3, 4 and would have an additional column sum_expenditure.

I'm sorry my question is so all over the place. Thank you all in advance!

[Shmuel · Accepted solution]

data all;

merge y1(in=in1)

           y2(in=in2)

           y3(in=in3)

           y4(in=in4)

; by ID;

   if in1 and in2 and in3 and in4;

run;