Text & math functions (Concat, Upper, Length, Round)

# ======================================================================
# Database *functions* run inside PostgreSQL, not in Python. `annotate()` adds a
# computed column to every row: `Concat` joins values (SQL `||`), `Upper` upper-
# cases, `Length` counts characters, and `Round(field, n)` rounds to n decimals.
# `values_list()` then returns plain tuples instead of model instances.
# ======================================================================

# ----------------------------------------------------------------------
# annotated books
# ----------------------------------------------------------------------

# Django:
Book.objects.annotate(
    label=Concat(Upper("title"), Value(" by "), "author__name"),
    title_len=Length("title"),
    price_rounded=Round("price", 0),
).values_list("label", "title_len", "price_rounded")

# SQL:
#   SELECT (COALESCE(UPPER("examples_book"."title"), '') || COALESCE((COALESCE(' by ', '') || COALESCE("examples_author"."name", '')), '')) AS "label", LENGTH("examples_book"."title") AS "title_len", ROUND("examples_book"."price", 0) AS "price_rounded"
#   FROM "examples_book"
#   INNER JOIN "examples_author" ON ("examples_book"."author_id" = "examples_author"."id")

# Result:
#   HARRY POTTER by J. K. Rowling  (len=12, ~$20)
#   THE HOBBIT by J. R. R. Tolkien  (len=10, ~$15)
#   THE LORD OF THE RINGS by J. R. R. Tolkien  (len=21, ~$29)
#   POSTGRES FOR AUTHORS by Karl Müller  (len=20, ~$0)