SQL Basics and Applications

SQL and its Importance

• SQL (Structured Query Language) is used to manage and manipulate relational databases
• It provides commands for creating, reading, updating, and deleting data
• SQL is universal for database interaction with most relational database management systems
• It's essential for data analysis, allowing users to extract meaningful insights from datasets
• SQL is designed for efficiently handling large volumes of data, even with little technical knowledge
• SQL is relatively easy to learn and use

Real-World Applications

• Business intelligence: companies use SQL to generate reports and analyze business performance
• Finance: banks and financial institutions use SQL for transaction processing and risk management
• Healthcare: medical records and patient data management
• E-commerce: inventory management, customer data, and sales tracking

Main Data Types

• INT: whole numbers (e.g., 42)
• FLOAT: floating-point numbers (approximate) (e.g., 3.14)
• DOUBLE: double-precision floating-point numbers (e.g., 2.7182818284)
• DECIMAL (p,s): fixed-point numbers with precision (p) and scale (s) (e.g., 123.45, DECIMAL(5.2))
• VARCHAR (n): variable-length text up to n characters (e.g., John Doe, VARCHAR(100))
• CHAR (n): fixed-length text of n characters (e.g., A, CHAR(1))
• TEXT: large amount of text (e.g., This is a long text′)
• DATE: date value (YYYY-MM-DD) (e.g., 2024-07-09)

Main Structures

• Creating Tables: tables consist of columns (defining data type) and rows (containing data)
  • Basic syntax (CREATE TABLE TableName (Column1 DataType1, Column2 DataType2, ...))
• Selecting Records: retrieving data from a table
  • Basic syntax (SELECT column1, column2, ... FROM table1)
  • Using JOIN (combining rows from 2 or more tables based on a related column), with clauses like WHERE, ON
• Inserting Records: inserting data into a table
  • Basic syntax (INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...))
• Updating Records: updating data in a table
  • Basic syntax (UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition)
• Deleting Records: deleting data from a table
  • Basic syntax (DELETE FROM table_name WHERE condition)
• Altering Tables: adding columns to a table
  • Basic syntax (ALTER TABLE table_name ADD column_name datatype)
• Dropping Tables: deleting a table
  • Basic syntax (DROP TABLE table_name)

Understanding Joins In SQL

• Joins: combine rows from two or more tables based on related columns
• Inner Join: returns only rows with matching values in both tables
• Left Outer Join: returns all rows from the left table and matching rows from the right table; if no match is found, NULL values are returned
• Right Outer Join: returns all rows from the right table and matching rows from the left table; if no match is found, NULL values are returned

Using Simple Queries for Data Analysis

• Retrieving data: basic SELECT query
• Performing aggregations: using aggregate functions (COUNT, SUM, AVG, MIN, MAX)
• Filtering results: using WHERE clause with conditions
• Using simple joins: combining data from multiple tables

Using Aggregations for Deeper Analysis

• Aggregations: summarizing data, deriving insights, performing calculations
• Using functions like count, sum, max, and average

Filtering Conditions

• WHERE: filtering results based on specified conditions
• Using comparison and logical operators

Operators

• *Arithmetic (+, -, , /): used for mathematical operations
• Comparison (=, <, >, <=, >=, <>): used to compare values
• LIKE: pattern matching in strings
• Logical (AND, OR, NOT): combining multiple conditions
• NULL: representing missing or unknown values

String Functions

• Concat: joining strings
• Substring: extracting parts of strings
• Length: getting the length of a string
• Upper/Lower: converting strings to uppercase/lowercase
• Trim/Ltrim/Rtrim: removing leading/trailing/spaces in strings
• Replace: replacing characters in the string

Using Subqueries

• Subqueries: queries nested inside another query
• Used in SELECT, FROM, or WHERE clauses
• Returning one/many rows
• Helpful for complex queries