Add Counting Records With Ecto as an elixir til

elixir/counting-records-with-ecto.md

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+# Counting Records With Ecto
+
+Sometimes you want to know how many records there are in a table. Ecto gives
+us a couple ways to approach this.
+
+We can use the
+[`count\1`](https://hexdocs.pm/ecto/Ecto.Query.API.html#count/1) function
+that the Ecto query API provides.
+
+```elixir
+> Repo.one(from p in "people", select: count(p.id))
+
+16:09:52.759 [debug] QUERY OK source="people" db=1.6ms
+SELECT count(p0."id") FROM "people" AS p0 []
+168
+```
+
+Alternatively, we can use the
+[`fragment/1`](https://hexdocs.pm/ecto/Ecto.Query.API.html#fragment/1)
+function to use PostgreSQL's `count` function.
+
+```elixir
+> Repo.one(from p in "people", select: fragment("count(*)"))
+
+16:11:19.818 [debug] QUERY OK source="people" db=1.5ms
+SELECT count(*) FROM "people" AS p0 []
+168
+```
+
+Lastly, `Ecto.Repo` has the
+[`aggregate/4`](https://hexdocs.pm/ecto/Ecto.Repo.html#c:aggregate/4)
+function which provides a `:count` option.
+
+```elixir
+> Repo.aggregate(from(p in "people"), :count, :id)
+
+16:11:23.786 [debug] QUERY OK source="people" db=1.7ms
+SELECT count(p0."id") FROM "people" AS p0 []
+168
+```