Database Engineer/DBA - (MySQL, MongoDB, SQLite) (Udemy.com)

The MySQL is the world's most popular open source database.This course will teach you  a lot to  start using this incredibly popular MySQL database and gain useful skills in the process.  In learning about MySQL you will also gain an understanding of relational databases in general.  Furthermore, you will harness that power by learning SQL and using it to build databases, populate them with data and query that data through extensive hands-on practices.

MongoDB is a document-oriented DBMS, with JSON-like  objects comprising the data model, rather  than RDBMS tables. MongoDB does not support joins nor transactions. However, it features  secondary indexes, an expressive query language,atomic writes on a per-document level, and fully-consistent reads. MongoDB uses JSON, a binary object format similar to, but more expressive than JSON.
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MongoDB uses dynamic schema. We can create collections without defining the structure,i.e. the fields or the types of their values, of the documents. You can change the structure of documents simply by adding new fields or deleting existing ones. Documents in a collection need unique set of fields.

MongoDB database stores its data in collections  not in tables The collections are the rough equivalent of RDBMS tables. 
A collection holds one or more documents, which corresponds to a record or a row in a relational database table, and each document  has one or more fields, which corresponds  to a column in a relational database table.

SQLite is a software library that provides a relational database management system. The lite in SQLite means light weight in terms of setup, database administration, and required resource.  SQLite does NOT require a server to run.

Because of the serverless architecture, you don’t need to “install” SQLite before using it. There is no server process that needs to be configured, started, and stopped. SQLite uses dynamic types for tables. It means you can store any value in any column, regardless of the data type.

SQLite is self-contained means it requires minimal support from the operating system or external library. This makes SQLite usable in any environments especially in embedded devices like iPhones, Android phones, game consoles, handheld media players, etc.

SQLite is capable of creating in-memory databases which are very fast to work with.

SQLite database is integrated with the application that accesses the database. The applications interact with the SQLite database read and write directly from the database files stored on disk.