Introduction to SQL Concepts

Introduction to SQL

• SQL (Structured Query Language) is a domain-specific language used for managing and manipulating databases.
• It's used to create, read, update, and delete data in a database relational model.
• SQL is standardized, meaning commands and syntax are generally consistent across various database systems.
• Common database systems utilizing SQL include MySQL, PostgreSQL, Oracle, and SQL Server.

Core SQL Concepts

• Database: A structured collection of data organized into tables.
• Table: A collection of related data organized into rows (records) and columns (attributes).
• Row (Record): A single entry in a table representing information about a specific entity.
• Column (Attribute): A field containing data about a specific property of an entity.
• Data Types: Define the kind of data a column can store (e.g., integer, text, date).

Data Definition Language (DDL)

• CREATE TABLE: Creates a new table.
  • Specifies column names and data types.
  • Optionally defines constraints like primary keys and foreign keys.
• ALTER TABLE: Modifies an existing table.
  • Adds, modifies, or deletes columns.
  • Alters constraints.
• DROP TABLE: Deletes a table.

Data Manipulation Language (DML)

• SELECT: Retrieves data from one or more tables.
  • Specifies which columns to retrieve.
  • Filters data using WHERE clause.
  • Orders the results using ORDER BY.
  • Groups data using GROUP BY.
• INSERT: Adds new rows to a table.
• UPDATE: Modifies existing rows in a table.
• DELETE: Removes rows from a table.

Data Querying using SQL

• SELECT statements: fundamental for retrieving data from databases.
• WHERE Clause: Used to filter records that match specified criteria. (e.g., WHERE age > 30).
• JOIN Clause: Combines rows from two or more tables based on a related column. (e.g., INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN).
• Aggregate Functions: Perform calculations on sets of values. (e.g., COUNT, SUM, AVG, MAX, MIN).
• GROUP BY Clause: Groups rows that have the same values in specified columns.
• ORDER BY Clause: Sorts the result set in ascending or descending order based on one or more columns.
• Subqueries: A query embedded within another query.

Constraints and Keys

• Primary Key: Uniquely identifies each row in a table.
• Foreign Key: Establishes a link between two tables.
• Unique Constraint: Ensures that all values in a column are unique.
• Not Null Constraint: Ensures that a column does not contain null values.

SQL Data Types

• Data types define the type of data that can be stored in a column of a table.
• Common data types include INT, VARCHAR, DATE, FLOAT, BOOLEAN.

Transaction Management

• Transactions: A sequence of database operations treated as a single logical unit of work.
• COMMIT: Saves changes made during a transaction.
• ROLLBACK: Reverts changes made during a transaction if something goes wrong.

Advanced SQL Concepts

• Views: Virtual tables based on queries.
• Stored Procedures: Precompiled SQL code for reusable functionalities.
• Indexes: Speed up data retrieval by creating a lookup table.
• Triggers: Automatically execute SQL code in response to certain events (like inserting, updating, or deleting data).

SQL Security

• User accounts and permissions are essential for access control.