Timeline for What does the ST_Union of the geometry column of two tables produce?
Current License: CC BY-SA 3.0
8 events
| when toggle format | what | by | license | comment | |
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| Apr 21, 2017 at 15:44 | comment | added | tumasgiu | Quoting Wikipedia about aggregate function : In database management an aggregate function is a function where the values of multiple rows are grouped together as input on certain criteria to form a single value of more significant meaning or measurement such as a set, a bag or a list. | |
| Apr 21, 2017 at 15:41 | history | edited | tumasgiu | CC BY-SA 3.0 |
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| Apr 21, 2017 at 15:40 | comment | added | ziggy | great answer, I was never fully able to put into the words the two ways st_union can be utilized | |
| Apr 21, 2017 at 15:34 | comment | added | tumasgiu | I think it's about SQL : if the geometries were integers, ask yourself how can you sum the values of your two tables. It's the same idea. You are misunderstanding what the FROM clause of your query is doing, that is : cross joining, i.e. create a new set from the combination of the elements of two other sets. You also probably don't undestand what an aggegate function is. | |
| Apr 21, 2017 at 15:30 | comment | added | Richard | Thank you so much! That does indeed return a single giant MultiPolygon. As a relative noob I'd like to understand a bit more about what I was doing wrong: was my error a SQL one, or to do with how ST_Union works? i.e. was the Cartesian product a function of the SQL query, or ST_Union? | |
| Apr 21, 2017 at 15:28 | vote | accept | Richard | ||
| Apr 21, 2017 at 15:28 | history | edited | tumasgiu | CC BY-SA 3.0 |
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| Apr 21, 2017 at 15:22 | history | answered | tumasgiu | CC BY-SA 3.0 |