Types of SQL Commands

Here are the main categories of SQL commands:

1. Data Definition Language (DDL): These commands are used to define and manage the structure of the database and its objects. This includes creating, altering, and deleting database schemas, tables, indexes, and other database elements.

• CREATE: Creates new database objects (e.g., CREATE TABLE, CREATE DATABASE, CREATE INDEX, CREATE VIEW).
• ALTER: Modifies the structure of existing database objects (e.g., ALTER TABLE ADD COLUMN, ALTER TABLE MODIFY COLUMN).
• DROP: Deletes existing database objects (e.g., DROP TABLE, DROP DATABASE, DROP INDEX, DROP VIEW).
• TRUNCATE: Removes all rows from a table, but the table structure remains. It’s faster than DELETE as it doesn’t log individual row deletions.
• RENAME: Renames a database object (e.g., RENAME TABLE old_name TO new_name).

2. Data Manipulation Language (DML): These commands are used to manipulate the data within the database tables. This includes inserting, updating, deleting, and retrieving data.

• SELECT: Retrieves data from one or more tables. This is the most frequently used DML command.
• INSERT: Adds new rows (records) into a table.
• UPDATE: Modifies existing rows in a table.
• DELETE: Removes rows from a table based on specified conditions.
• MERGE: Performs insert, update, or delete operations on a target table based on the results of a join with a source table. (Not supported in all SQL dialects).
• CALL: Executes a stored procedure.
• EXPLAIN PLAN: Describes the execution plan that the database will use for a query (helps in query optimization).
• LOCK TABLE: Controls the concurrency of access to tables.

3. Data Control Language (DCL): These commands are used to control access to the data and database objects. This involves managing user privileges and permissions.

• GRANT: Gives specific privileges to database users on certain objects (e.g., GRANT SELECT, INSERT ON employees TO user1).
• REVOKE: Removes privileges previously granted to database users (e.g., REVOKE SELECT ON employees FROM user1).

4. Transaction Control Language (TCL): These commands are used to manage transactions within the database. Transactions are logical units of work that may involve multiple SQL statements. TCL commands ensure data integrity by managing how changes are committed or rolled back.

• COMMIT: Saves all changes made within the current transaction permanently to the database.
• ROLLBACK: Undoes all changes made within the current transaction since the last COMMIT or the beginning of the transaction.
• SAVEPOINT: Sets a point within a transaction to which you can later roll back.

5. Data Query Language (DQL): Although sometimes considered a subset of DML (specifically the SELECT statement), DQL is often listed as a separate category because its primary function is to query and retrieve data.

• SELECT: (As described in DML)

Beyond these main categories, it’s also important to understand the concept of SQL Dialects:

While the core SQL commands are standardized to a large extent (ANSI SQL), different Database Management Systems (DBMS) such as MySQL, PostgreSQL, SQL Server (T-SQL), Oracle (PL/SQL), and SQLite have their own extensions and variations in syntax, functions, and features. These are known as SQL dialects.

For example, the way you might limit the number of rows returned in a query differs between MySQL (LIMIT), SQL Server (TOP), and PostgreSQL/SQLite (LIMIT). Similarly, functions for date manipulation or string operations can vary significantly.

Therefore, while the fundamental types of SQL commands remain consistent, the specific syntax and available features can differ depending on the particular database system you are using.