SQL (Structured Query Language) is a standard programming language used to interact with Relational Databases. It allows you to store, manage, and retrieve data efficiently from tables using simple commands.

SELECT column1, column2
FROM table_name
WHERE condition
GROUP BY column_name
HAVING condition
ORDER BY column_name ASC|DESC;

📝 Note: SQL keywords are not case-sensitive, but it’s a best practice to write commands in UPPERCASE for readability.

Data Manipulation Language (DML) commands are used to manage and manipulate data stored in database tables. They allow users to insert new records, retrieve existing data, update information, and delete records as needed — without altering the database structure itself.

Data Definition Language (DDL) Commands are used to define, modify, and manage the structure of database objects such as tables, schemas, and indexes. They focus on how the data is stored, not the data itself. Using DDL, you can create new tables, alter existing ones, or remove database objects entirely.

💡 In short: DDL commands deal with the blueprint of your database — its structure and schema.

Data Control Language (DCL) Commands are used to control access and permissions within a database. They help database administrators grant or revoke privileges to users or roles, ensuring that only authorized users can perform certain actions like selecting, inserting, or deleting data. Using DDL, you can create new tables, alter existing ones, or remove database objects entirely.

💡 In short: In short: DCL focuses on who can do what in the database — it manages security and user access control.

TCL commands are used to manage transactions in a database. A transaction is a sequence of one or more SQL operations that are executed as a single unit of work. TCL ensures that either all operations succeed (COMMIT) or none of them do (ROLLBACK), maintaining data integrity and consistency.

DQL commands are used to fetch data from a database. They allow users to retrieve specific information from one or more tables using different conditions, filters, and sorting options. Although DQL technically has only one main command — SELECT, it is the most used SQL command and forms the base of almost every SQL query.

JOINs are used to combine data from two or more tables based on a related column between them. They help in retrieving meaningful data spread across multiple tables by establishing relationships using keys (usually primary and foreign keys).

💡 In short: JOIN = Connect related data from multiple tables into one result set.

A Subquery (also called an inner query or nested query) is a query written inside another SQL statement. It is used to fetch data based on the result of another query, making SQL queries more dynamic and powerful.

💡 In short: A subquery allows you to use the result of one query inside another — for filtering, comparison, or validation.

• Subqueries can be used inside SELECT, FROM, or WHERE clauses.
• The inner query is executed first, and its result is used by the outer query.
• IN is used for list comparison,
  ANY for at least one match,
  and ALL for every match (all conditions must be true).

Aggregate functions perform calculations on a set of values and return a single summarized result (like total, average, count, etc.). They are commonly used with the GROUP BY clause to analyze data in groups or categories.

💡 In short: Aggregate functions help you summarize data — for example, counting total rows, finding averages, or getting minimum/maximum values.

• Aggregate functions ignore NULL values during calculations.
• You can combine them with GROUP BY to calculate summaries by category.
• They are read-only functions — they do not modify data.

String functions are used to manipulate and transform text data in SQL. They help in cleaning, formatting, and extracting parts of strings — which is especially useful for data analysis and reporting.

💡 In short: String functions make it easy to combine, format, or modify text stored in database columns.

• These functions are supported in most SQL databases (MySQL, PostgreSQL, SQL Server, Oracle).
• TRIM() is useful for removing extra spaces in text fields.
• CONCAT() and REPLACE() are especially common in data cleaning operations.

Date and Time functions in SQL are used to work with date, time, and timestamp values. They allow you to retrieve the current date/time, extract specific parts (like year or month), perform calculations, and format date outputs.

💡 In short: These functions help you analyze and manipulate time-based data, such as calculating durations, formatting timestamps, or filtering by date.

• CURRENT_DATE() and CURRENT_TIMESTAMP() are commonly used in logging and auditing.
• Use DATE_ADD() or DATE_SUB() for date calculations (like adding days/months).
• EXTRACT() and DATE_PART() are similar — their usage depends on the SQL dialect.
• TO_CHAR() is especially useful for custom date formatting in Oracle and PostgreSQL.

Conditional expressions in SQL are used to perform logic-based operations within queries. They allow you to add decision-making capability — for example, returning different results based on conditions (like an IF-ELSE statement in programming).

💡 In short: Conditional expressions make your queries dynamic and smart, letting SQL choose results based on data values.

• CASE is the most powerful and cross-database compatible conditional expression.
• IF() works mainly in MySQL, while IIF() is used in SQL Server.
• COALESCE() is commonly used for handling NULL values gracefully.
• NULLIF() helps avoid division-by-zero or redundant calculations.

Set operations are used to combine the results of two or more SELECT statements into a single result set. They help compare or merge data from multiple queries, just like mathematical set operations (UNION, INTERSECT, EXCEPT).

💡 In short: Set operations let you merge, match, or exclude data between multiple result sets.

• Each SELECT in a set operation must have the same number of columns and matching data types.
• The order of columns in each query must also match.
• UNION ALL is faster than UNION because it does not remove duplicates.
• INTERSECT and EXCEPT may not be supported in all databases (e.g., MySQL lacks them natively).

• TCL commands are used to control transactions in a database.
• Transactions allow you to ensure data integrity: either all operations succeed (commit) or none (rollback).
• SAVEPOINT is optional but helpful in complex transactions.