- Based on Uje Indo suggestion, I used field calculator to edit existing field "class" with this statement. It works perfectly.
WHEN "meanB0" < 12.5 AND "meanB3" < 85 THEN 'water'
WHEN "meanB0" > 12.5 AND "meanB0" <18 THEN 'nypa'
WHEN "meanB0" >= 17 AND "meanB7" > 170 THEN 'deadtree'
WHEN "meanB0" >=17 AND "meanB7" < 100 THEN 'rhizopora'
WHEN "meanB0" >=17 AND "meanB7" >= 100 AND "meanB7" <= 180 THEN 'bruguiera'
WHEN "meanB0" <=12.5 AND "meanB3" >= 85 THEN 'land'
- Based on Jochen Schwarze plus trial and error, I finally used saga dissolve polygon by attributes, it creates a shapefile with the class name only, and all other band information is gone.
Afterward, to calculate mean,max,min etc for each class polygon, i have to:
- split the raster image into each band using raster calculator
- run saga grid statistic for polygon where for grids, choose all the raster band that split before, and for the shapefile choose the dissolve polygon. Run it, and the result will give a shp file with cell number, sum, range, min, max, mean, var, standard deviation for each band and each polygon.