added data aggregation example for postgres (#520)

Author: KappaDistributive

src/database.md

@@ -6,10 +6,12 @@
 | [Insert and Query data][ex-sqlite-insert-select] | [![rusqlite-badge]][rusqlite] | [![cat-database-badge]][cat-database] |
 | [Create tables in a Postgres database][ex-postgres-create-tables] | [![postgres-badge]][postgres] | [![cat-database-badge]][cat-database] |
 | [Insert and Query data][ex-postgres-insert-query-data] | [![postgres-badge]][postgres] | [![cat-database-badge]][cat-database] |
+| [Aggregate data][ex-postgres-aggregate-data] | [![postgres-badge]][postgres] | [![cat-database-badge]][cat-database] |
 
 [ex-sqlite-initialization]: database/sqlite.html#create-a-sqlite-database
 [ex-sqlite-insert-select]:  database/sqlite.html#insert-and-select-data
 [ex-postgres-create-tables]: database/postgres.html#create-tables-in-a-postgres-database
 [ex-postgres-insert-query-data]: database/postgres.html#insert-and-query-data
+[ex-postgres-aggregate-data]: database/postgres.html#aggregate-data
 
 {{#include links.md}}

src/database/postgres.md

@@ -4,4 +4,6 @@
 
 {{#include postgres/insert_query_data.md}}
 
+{{#include postgres/aggregate_data.md}}
+
 {{#include ../links.md}}

src/database/postgres/aggregate_data.md

@@ -0,0 +1,44 @@
+## Aggregate data
+
+[![postgres-badge]][postgres] [![cat-database-badge]][cat-database]
+
+This recipe lists the nationalities of the first 7999 artists in the database of the [`Museum of Modern Art`] in descending order.
+
+```rust,no_run
+extern crate postgres;
+use postgres::{Connection, Error, TlsMode};
+
+struct Nation {
+    nationality: String,
+    count: i64,
+}
+
+fn main() -> Result<(), Error> {
+    let conn = Connection::connect(
+        "postgresql://postgres:[email protected]/moma",
+        TlsMode::None,
+    )?;
+
+    for row in &conn.query 
+	("SELECT nationality, COUNT(nationality) AS count 
+	FROM artists GROUP BY nationality ORDER BY count DESC", &[])? {
+        
+        let (nationality, count) : (Option<String>, Option<i64>) 
+		= (row.get (0), row.get (1));
+        
+        if nationality.is_some () && count.is_some () {
+
+            let nation = Nation{
+                nationality: nationality.unwrap(),
+                count: count.unwrap(),
+        };
+            println!("{} {}", nation.nationality, nation.count);
+            
+        }
+    }
+
+    Ok(())
+}
+```
+
+[`Museum of Modern Art`]: https://github.com/MuseumofModernArt/collection/blob/master/Artists.csv