Add Find Records That Have Multiple Associated Records as a postgres til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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@@ -529,6 +529,7 @@ _1047 TILs and counting..._
 - [Escaping String Literals With Dollar Quoting](postgres/escaping-string-literals-with-dollar-quoting.md)
 - [Export Query Results To A CSV](postgres/export-query-results-to-a-csv.md)
 - [Extracting Nested JSON Data](postgres/extracting-nested-json-data.md)
+- [Find Records That Have Multiple Associated Records](postgres/find-records-that-have-multiple-associated-records.md)
 - [Find The Data Directory](postgres/find-the-data-directory.md)
 - [Find The Location Of Postgres Config Files](postgres/find-the-location-of-postgres-config-files.md)
 - [Fizzbuzz With Common Table Expressions](postgres/fizzbuzz-with-common-table-expressions.md)

postgres/find-records-that-have-multiple-associated-records.md

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+# Find Records That Have Multiple Associated Records
+
+A common type of table association in a relational database is a one-to-many
+relationship. For instance, a database representing a bookshelf may have an
+`authors` table where each record can be associated with multiple records in
+the `books` table. That relationship is represented by a `author_id` foreign
+key column on `books` that points to `authors.id`.
+
+We can write a query to find all authors that have not zero or one, but
+multiple books by doing a join and then tacking on a `having` clause.
+
+```sql
+select authors.id, authors.name, count(books.id)
+  from authors
+  join books
+    on authors.id = books.author_id
+  group by authors.id
+  having count(books.id) >= 2;
+```
+
+This will result in a listing of author ids, author names, and their number of
+books.
+
+It does this by joining books to authors, grouping by the `authors.id` to
+produce a set of records unique to each author, and then combining multiple
+books by aggregating them with a `count`. The `having` clause is necessary
+because it is our way of _filtering_ on an aggregate value, in this case the
+`count`.