Some spatial data manipulation acts in Postgres/PostGis

Load only the table names, which have spatial references

        //tables with spatial references are store in geometry_columns table in postgres (after postgis installation)
        DataTable loadTableNames() 
        {
            DataTable dt = new DataTable();
            using (NpgsqlConnection cnn = new NpgsqlConnection(cnsb.ConnectionString))
            {
                NpgsqlDataAdapter da = new NpgsqlDataAdapter("select f_table_name from geometry_columns", cnn);
                try
                {
                    da.Fill(dt);
                }
                catch (ArgumentException asd)
                { MessageBox.Show("Error: " + asd.Message); }
                catch (NpgsqlException fgh)
                { MessageBox.Show("Error: " + fgh.Message); }
            }
            return dt;
        }

Look at some spatial queries

Select settlemants, which are in county named 'Pest megye' and sort the result by the name of county and settlemants

SELECT county.name, settlemants.name, settlemants.geom FROM county, settlemants 
WHERE  county.name='PEST MEGYE' 
AND (SELECT _st_contains(county.geom, settlemants.geom)) 
ORDER BY county.name, settlemants.name asc 

Select county name and county geometry from tables county and rivers, where the desired name is 'Duna', and county overleps the 'Duna' river

SELECT county.name, county.geom FROM county, rivers WHERE rivers.name = 'Duna' AND (SELECT _st_overlaps(county.geom, rivers.geom))

Select settlemants, which are in county named 'Pest megye' and sort the result by the name of county and settlemants, and save the result into

SELECT county.name as cname, settlemants.name as bnev, settlemants.geom INTO pest_county FROM county, settlemants 
WHERE  county.name = 'PEST MEGYE' 
AND (SELECT _st_contains(county.geom, settlemants.geom)) 
ORDER BY county.name, settlemants.name asc 

Polygon geometry column creation for parcel database

CREATE TABLE IF NOT EXISTS (id BIGSERIAL PRIMARY KEY, codeNumber VARCHAR, geom GEOMETRY(POLYGON,23700))", "table name") 
             // Explanation: ID column (PK), codeNumber column and geometry column (type, code number of the projection system)

Insert a record, which contains polygon geometry data too

INSERT INTO (codeNumber, geometry) SELECT ST_GeomFromText('SRID=23700;POLYGON(({2}))') ", "table name", codeNumber, geometry)); 
                        //Explanation: codeNumber-> string, geomerty->poligon string