Good job! The last function we'll briefly discuss in this section is ST_Union. It returns a new geometry a new geometry that is the sum (or union-hence the name) of input geometries.

Interestingly, there are two versions of the function:

• Non-aggregate: provide two or more input geometries in the brackets to combine them: ST_Union(geometry1, geometry2). The result will be a new geometry created from all input geometries. This version works in a very similar way to ST_Intersection, which we discussed in the previous exercise.
• Aggregate: in this version of ST_Union, we only need a single argument: a column name. ST_Union will then work similarly to aggregate functions such as SUM(...). It will create a new geometry by adding up ALL geometries in the given column that were returned by the query. This can be very convenient. Take a look at the example:

  SELECT 
    ST_Union(boundaries)
  FROM sf_planning_districts;

  This query will return a single geometry representing the union of all planning districts of San Francisco.