postGIS SQL request with aggregating ST_Union()

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I have a postGIS table that looks like this :

id | value | geometry(MultiPolygon)1 10 ...1 20 ...1 30 ...1 10 ...1 20 ...1 30 ...What I wish to do is to create a new geometry column containing a Multipolygon which is the union of all the other polygons of rows having the same id and a smaller or equal 'value'.

So :

• first row 'new_geometry' would be identical to 'geometry'.
• Second row 'new_geometry' would be the union of the first and second row 'geometry'.
• Third row : union of rows 1,2,3
• equal to row 4
• etc

I have written something like this:

SELECT t.id, t.val, St_Union(t.geometry)FROM (SELECT cs.id, cs.value as val, cs.geometry FROM mytable cs INNER JOIN (SELECT DISTINCT value from myTable) v ON (v.value