SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in a relational database. SQL is the standard language for Relational Database System. All the Relational Database Management Systems (RDMS) like MySQL, MS Access, Oracle, Sybase, Informix, Postgres and SQL Server use SQL as their standard database language.

Before starting any ML/DS project, getting the data is a step that can't be skipped. Some of these data reside in different database and you will need to find ways to communicate with them to get the data. SQL is the language of DB and you can easily use SQL to get the data you want. After completing a DS/ML project, there might be need to keep collecting more data or store the cleaned data into a DB, SQL gives you the easy access to store such data as well. It also allow you to perform some analysis directly from the DB and derive insights that can impact the business.

Understanding SQL is crucial if you want to communicate with DB, get your data from the Warehouse as well as store your data in a DB. It also gives you an edge in the marketplace during job search. It allow you understand where the data is coming from, how it stored, the kind of values that should be in each column and how different tables relate to each other.

The major problem knowlegde of SQL solve is how to communicate to the database. With this knowledge, data practitioner can get the data they want, how they want it and when they want it. They can also save the clean data back to the DB. SQL can also be used to perform data analysis and derive useful insights. It helps you move data from a data warehouse (where data resides) to a jupyter notebook environment (where analysis is carried out)

As humans, we communicate with eachother via the language we understand. This has enable us to ask questions and get answers. For instance you ask your friend for something using a language that you both understand that is communication. Now in context of SQL your friend is the database that has something and you are the data practitioner that needs that something. You then communicate with the database by using the language that it understand (SQL).

1. Database: A database is an organized collection of structured information, or data, typically stored electronically in a computer system. A database is usually controlled by a database management system (DBMS). Together, the data and the DBMS, along with the applications that are associated with them, are referred to as a database system, often shortened to just database.Data within the most common types of databases in operation today is typically modeled in rows and columns in a series of tables to make processing and data querying efficient. The data can then be easily accessed, managed, modified, updated, controlled, and organized. Most databases use structured query language (SQL) for writing and querying data. Read More here
2. Tables: Tables are database objects that contain all the data in a database. In tables, data is logically organized in a row-and-column format similar to a spreadsheet. Each row represents a unique record, and each column represents a field in the record. Read More here
3. Relationship: It specifies how tables are associated in a DB. Without relationship, there will be no relational DB. This link explains how relationships are formed within a database. For a more indepth explanation click here.

4. Queries: A query is a way of requesting information from the database, a query is a question. A database query is a request to access data from a database to manipulate it or retrieve it. click here to learn more

• Begineers

  • knows how to create tables
  • can use select statement
  • understand the use of joins
  • knows how to use alias

• Experts

  • can use sql subquery
  • can create sql script to run procedures
  • understand the use of views

1. SQL Basics for Beginners: a 27 minute Youtube Video by Edureka on 'SQL Basics for Beginners' which will help you understand the basics of SQL and also sql queries which are very popular and essential..
2. Learn and practice SQL with mode: introduction to intermediate sql
3. Advance SQL Course Kaggle: with the Kaggle SQL course you will master how to use:

• Join and Union
• Analytic Functions
• Nested and Repeated Data
• Writing Efficient Queries

4.SQL for DataScience: Well curated SQL course specifically for data science. Here you'll learn how SQL relate to data science and how to use SQL to make your work easier.

5. Practice with SQL Interview Questions

6. Types of Relationship in SQL: One of the most important things in databases is to understand the types of relations in the databases. That stands for both – a process of designing a database model as well as when you’re analyzing your data. Understanding these relations is somehow natural and not so complex but is still essential in the database theory (and practice). Read More