Essentially, SQL, or Structured Query Language is a language that allows you to interact with databases. With SQL, you can write queries that help you understand, filter, combine, and summarize the data stored in a database.
Once you know SQL, you’ll be able to write queries to get exactly the data you need. From there, you can import that data into a different tool or language to analyze it more deeply. Some visualization tools even have a built-in option to use SQL queries!
These SQL commands are mainly categorized into four categories as:
- DDL – Data Definition Language
- DQl – Data Query Language
- DML – Data Manipulation Language
- DCL – Data Control Language
Though many resources claim there to be another category of SQL clauses TCL – Transaction Control Language. So we will see in detail about TCL as well.
DDL – Data Definition Language
DDL or Data Definition Language actually consists of the SQL commands that can be used to define the database schema. It simply deals with descriptions of the database schema and is used to create and modify the structure of database objects in the database.
Examples of DDL commands:
- CREATE– is used to create the database or its objects (like table, index, function, views, store procedure and triggers).
- DROP– is used to delete objects from the database.
- ALTER–is used to alter the structure of the database.
- TRUNCATE–is used to remove all records from a table, including all spaces allocated for the records are removed.
- COMMENT–is used to add comments to the data dictionary.
- RENAME –is used to rename an object existing in the database.
DQL – Data Query Language
DML statements are used for performing queries on the data within schema objects. The purpose of DQL Command is to get some schema relation based on the query passed to it.
Example of DQL command:
- SELECT– is used to retrieve data from the a database.
DML – Data Manipulation Language
The SQL commands that deals with the manipulation of data present in the database belong to DML or Data Manipulation Language and this includes most of the SQL statements.
Examples of DML commands:
- INSERT– is used to insert data into a table.
- UPDATE– is used to update existing data within a table.
- DELETE– is used to delete records from a database table.
DCL – Data Control Language
DCL includes commands such as GRANT and REVOKE which mainly deals with the rights, permissions and other controls of the database system.
Examples of DCL commands:
- GRANT– gives user’s access privileges to database.
- REVOKE– withdraw user’s access privileges given by using the GRANT command.
TCL – Transaction Control Language
TCL commands deals with the transaction within the database.
Examples of TCL commands:
- COMMIT– commits a Transaction.
- ROLLBACK– rollbacks a transaction in case of any error occurs.
- SAVEPOINT–sets a savepoint within a transaction.
- SET TRANSACTION–specify characteristics for the transaction.