Help in interpreting data - Deficit I was looking at the deficit for different countries 
http://appsso.eurostat.ec.europa.eu/nui/show.do?dataset=gov_10q_ggnfa&amp;lang=en

How shall they be read?

For instance Greece had a -15% in Q4 of 2015 and +0.5% in Q2 of 2016.
Does this datum refer only in that specific period? I mean in Q4 of 2015 Greece had a 15% unbalance, but this is not on annual basis. 
If we want the yearly data we shall sum up the 4 quarters?
Following this path in the entire 2015 Greece had a total annual deficit of 30%? Isn't it too high?
It is the difference between total general government revenue and total general government expenditure defined in the National Accounts system.  You can find some sort of explanation at http://ec.europa.eu/eurostat/statistics-explained/index.php/Government_finance_statistics_-_quarterly_data#Government_revenue_and_expenditure 

For Greece, Eurostat's underlying figures look like this 

UNIT    NA_ITEM/TIME            2015Q1  2015Q2  2015Q3  2015Q4  2016Q1  2016Q2  2016Q3
M euro  Total GGov revenue      16,945  19,711  19,970  27,484  18,430  20,598  23,780
M euro  Total GGov expenditure  20,925  21,336  20,796  34,290  20,222  20,368  20,982
M euro  Net lend(+)/borrow(-)   -3,980  -1,625    -826  -6,806  -1,792     230   2,798
% GDP   Total GGov revenue        42.0    44.5    43.3    61.3    46.2    46.3    50.9
% GDP   Total GGov expenditure    51.8    48.1    45.1    76.5    50.7    45.8    44.9
% GDP   Net lend(+)/borrow(-)     -9.9    -3.7    -1.8   -15.2    -4.5     0.5     6.0


The percentages are of quarterly GDP.  If you wanted exact annual percentages then you should add up the four quarterly figures in Euros and divide by the sum of the four quarterly figures for GDP in Euros (from a different Eurostat table), but you will get a close approximation if you take the average of the four quarterly percentages.  So for Greece it might represent a government deficit of around 7.7% in 2015 

Be aware that the numbers are not seasonally adjusted: it looks as if a lot of Greek government revenue and expenditure comes in Q4.