Update README.md

Author: cgatama

Diff for: README.md

@@ -1,2 +1,51 @@
 # Databases-and-SQL-for-Data-Science-with-Python
-Databases and SQL for Data Science with Python
+
+A database is a repository of data that provides functionality for adding, modifying, and querying the data. (A database is a logically coherent collection of data with some inherent meaning). SQL is a language used to query or retrieve data from a relational database. 
+
+The Relational Model is the most used data model for databases because it allows for data independence. 
+
+The primary key of a relational table uniquely identifies each tuple or row, preventing duplication of data and providing a way of defining relationships between tables. 
+
+SQL statements fall into two different categories: Data Definition Language (DDL) statements and Data Manipulation Language (DML) statements.
+
+
+In this lab, you will use MySQL. MySQL is a Relational Database Management System (RDBMS) designed to efficiently store, manipulate, and retrieve data
+
+You can use the WHERE clause to refine your query results.
+
+You can use the wildcard character (%) as a substitute for unknown characters in a pattern.
+
+You can use BETWEEN ... AND ... to specify a range of numbers.
+
+You can sort query results into ascending or descending order, using the ORDER BY clause to specify the column to sort on.
+
+You can group query results by using the GROUP BY clause. 
+
+Functions, Multiple Tables, and Sub-queries
+
+Most databases come with built-in functions that you can use in SQL statements to perform operations on data within the database itself.
+
+When you work with large datasets, you may save time by using built-in functions rather than first retrieving the data into your application and then executing functions on the retrieved data.
+
+You can use sub-queries to form more powerful queries than otherwise.
+
+You can use a sub-select expression to evaluate some built-in aggregate functions like the average function. 
+
+Derived tables or table expressions are sub-queries where the outer query uses the results of the sub-query as a data source.
+
+You can access a database from a language like Python by using the appropriate API. Examples include ibm_db API for IBM DB2, psycopg2 for ProstgreSQL, and dblib API for SQL Server.
+
+DB-API is Python's standard API for accessing relational databases. It allows you to write a single program that works with multiple kinds of relational databases instead of writing a separate program for each one.
+
+The DB_API  connect constructor creates a connection to the database and returns a Connection Object, which is then used by the various connection methods.
+
+The connection methods are:
+The cursor() method, which returns a new cursor object using the connection.
+The commit() method, which is used to commit any pending transaction to the database.
+The rollback() method, which causes the database to roll-back to the start of any pending transaction.
+The close() method, which is used to close a database connection. 
+
+You can use SQL Magic commands to execute queries more easily from Jupyter Notebooks. 
+Magic commands have the general format %sql select * from tablename.
+Cell magics start with a double %% (percent) sign and apply to the entire cell.
+Line magics start with a single % (percent) sign and apply to a particular line in a cell.