Hands on Learning Postgresql

这是记录我学习的Open Full Stack上的一门小课程Part 13 - Using relational databases。

这本课程是用PostgreSQL。

类似Mongoose，这门课程用到了Sequelize。官网的描述是：

Sequelize is a modern TypeScript and Node.js ORM for Oracle, Postgres, MySQL, MariaDB, SQLite and SQL Server, and more. Featuring solid transaction support, relations, eager and lazy loading, read replication and more.

这里首先讲到之前课程用到的MongoDB。

Mongo is a document database and one of its most characteristic features is that it is schemaless, i.e. the database has only a very limited awareness of what kind of data is stored in its collections. The schema of the database exists only in the program code, which interprets the data in a specific way, e.g. by identifying that some of the fields are references to objects in another collection.

The \d command, which tells us what tables are in the database.

postgres=# \d
            List of relations
 Schema |     Name     |   Type   |  Owner
--------+--------------+----------+----------
 public | notes        | table    | username
 public | notes_id_seq | sequence | username
(2 rows)

With the command \d notes, we can see how the notes table is defined:

postgres=# \d notes
                                     Table "public.notes"
  Column   |          Type          | Collation | Nullable |              Default
-----------+------------------------+-----------+----------+-----------------------------------
 id        | integer                |           | not null | nextval('notes_id_seq'::regclass)
 content   | text                   |           | not null |
 important | boolean                |           |          |
 date      | time without time zone |           |          |
Indexes:
    "notes_pkey" PRIMARY KEY, btree (id)

In practice, a migration is a single JavaScript file that describes some modification to a database. A separate migration file is created for each single or multiple changes at once.