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Databases / db / sqlite / MongoDB / MySQL

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Databases / db / sqlite / MongoDB / MySQL

Apr 25, 2024

Table of contents

 

  • http://coding-geek.com/how-databases-work/

    • Highly rec'd by soham

 

 

 

 

Tools

 

 

Best practices

How/when to use particular data types

How/when to use ER Diagrams

Specific DBs

SQLite

When to use SQLite

  • https://www.sqlite.org/whentouse.html

    • Websites

      • SQLite works great as the database engine for most low to medium traffic websites (which is to say, most websites). The amount of web traffic that SQLite can handle depends on how heavily the website uses its database. Generally speaking, any site that gets fewer than 100K hits/day should work fine with SQLite. The 100K hits/day figure is a conservative estimate, not a hard upper bound. SQLite has been demonstrated to work with 10 times that amount of traffic.

      • The SQLite website (https://www.sqlite.org/) uses SQLite itself, of course, and as of this writing (2015) it handles about 400K to 500K HTTP requests per day, about 15-20% of which are dynamic pages touching the database. Dynamic content uses about 200 SQL statements per webpage. This setup runs on a single VM that shares a physical server with 23 others and yet still keeps the load average below 0.1 most of the time.

      • See also: Hacker New discussion from 2022-12-13.

    • Also:

      • Client/server database engines are designed to live inside a lovingly-attended datacenter at the core of the network.

      • A good rule of thumb is to avoid using SQLite in situations where the same database will be accessed directly (without an intervening application server) and simultaneously from many computers over a network.

  • Ask HN: Have you used SQLite as a primary database?

    • The sqlite docs page has a nice article on when to use an embedded database such as sqlite and when to go with a client/server model (postgres, mysql or others)

      When not to use sqlite:

      - Is the data separated from the application by a network?

      - Many concurrent writers?

      - Data size > 280 TB

    • It is exceptionally great if you don't need parallel writes or have many terabytes of data - ie: for most services out there.
      When embedding natively, like in a Rust app, the performance is better than any other RDBMs because no network/serialization overhead and being able to use pointers in-process if needed.

      The DevOps story also is a dream: typically it is just a single file (optionally + some more for journaling) and setup is automated away (most language libs bundle it already), plus it is widely known since smartphone SDKs and all webbrowsers include/expose it.

      A subtile advantage: the supported SQL subset is so small, that "if it works in sqlite, it will also work with $RDBMS" in most cases, but not the other way around. I always use it when getting started when in need of relational data, and only had to swap it out for postgres once, but not due to technical/scaling reasons (IT policy change & stuff).

      Having said that, it is mind-boggling what kind of load you can handle with a small VPS that runs a Rust microservice that embeds it's own SQLite natively... that would be an expensive cluster of your typical rails/django servers and still have worse performance.

Misc tips

  • Installation

    • Go to SQLite3 download page

      1. Go down to the “Precompiled Binaries For Windows” section.

      2. Download “sqlite-shell” and “sqlite-dll” archive files.

        • sqlite-dll-win64-x64-3160200.zip

      3. Download the sqlite tools .zip

        • sqlite-tools-win32-x86-3160200.zip

      4. Unpack everything in C:\sqlite folder.

      5. Add that folder to your PATH.

  • Creating the empty .db file:

    1. Hit the Windows key, type 'cmd', then right-click the 'Command Prompt' result and select 'Run as Administrator'.

    2. Navigate to the folder where you want the db to be.

    3. Type 'sqlite3 name_of_the_db.db' and hit 'Enter'.

  • Simple example of a schema file:

    • CREATE TABLE salespeople ( id INTEGER PRIMARY KEY, first_name TEXT NOT NULL, last_name TEXT NOT NULL, commission_rate REAL NOT NULL );

Good to know

  • SQLite will allow you to insert a string into an integer field. (Source)

MySQL

  • How to get your database to allow emojis and other unusual characters:

    • ALTER DATABASE dbname CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci

PostgreSQL

Websites

Tutorials

Extensions

  • PostGIS

    • This was one of the major reasons Krieger and Systrom went with Postgres over MySQL. (Source)

Books

Snippets

  • Get the number of rows in all tables in a database.

    • SELECT schemaname,relname,n_live_tup
      FROM pg_stat_user_tables
      ORDER BY n_live_tup DESC;

MongoDB

Using MongoDB:

  • Import a bson file into your Mongo /data/ folder: mongorestore -d db_name -c collection_name "C:\path\to\export.bson"

    • Make sure to add your MongoDB's 'bin' folder to the PATH to get mongorestore to work from a command prompt.

  • Run Mongod: "C:\Program Files\MongoDB\Server\4.0\bin\mongod.exe" --dbpath="c:\data\db"

  • Run MongoDB shell: "C:\Program Files\MongoDB\Server\4.0\bin\mongo.exe"

  • Show list of dbs: 'show dbs'

  • Switch to using a particular db: 'use <db>'

  • Show list of collections for the active db: 'show collections'

Queries

  • Get all records from a particular collection once you're switched to a particular db

    • 'db.<collection_name>.find()'

  • Check if a field contains a string

    • db.getCollection('collection_name').find({"Party1": {$regex : ".*similarly situated.*"}})

  • Find records by non-existence of a field:

    • db.mycollection.find({ "price" : { "$exists" : false } })

  • Find a record by its id:

    • db.collection_name.find({"_id": ObjectId("587862a88362593254464c69")})

      • So you can find it by the id's string value, but you need to wrap the string with ObjectId()

Neo4j

General

Tutorials

Tools

Pros and cons of graph databases