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SQL Commands - Basics

 

  • SQL stands for Structured Query Language
  • SQL lets you access and manipulate databases

 A SQL database is a collection of tables that stores a specific set of structured data.

Database Management Systems (DBMS) are software systems used to store, retrieve and run queries on data. A DBMS serves as an interface between an end-user and a database, allowing users to create, read, update, and delete data in the database.

eg : MySQL, Oracle DB, etc. 

RDBMS stands for Relational Database Management System. RDBMS is a program used to maintain a relational database. RDBMS uses SQL queries to access the data in the database.

Types of Commands Available in SQL : 

  • Data Definition Language 
  • Data Manipulation Language 
  • Data Query Language 
  • Data Control Language 
  • Transactional Control Language 

 Data Definition Language : Set of commands used to create and modify the structure of database objects in a database.

  • create, alter, drop, truncate

Create :  To create a database and table. 


Alter : To add, change, modify or drop columns in an existing table. 


   

 

 


 

 

 

 

 


 

 

 

 

 

Truncate: To delete the data inside a table but not the table itself (retains structure of the table)


 


 

 

Drop:  To remove/delete an entire table. 


 

 Data Manipulation Language :  A set of commands used to add (insert), delete (delete), and alter (update) data in a database. 

  • insert, update, delete

Insert: To add new records/rows in a table 

Syntax


 

 

Update:  Modifies existing records in a table based on a condition specified by where clause



 

 

Delete: Deletes existing rows in a table


 

 

Data Query Language :   Commands used to make various queries in the table

  • select
Select: To retrieve/query an SQL table

Basic Syntax

 

 

Data Control Language:  Allows to set up and control BigQuery resources using GoogleSQL query syntax.

  • grant, revoke  
Grant: To provide access or privileges on the database objects to the users. 
 

 
 
 
 
 
 
An entity contained within a database is an object, eg: tables 
 
Revoke: To revoke granted permissions/privileges 

 
 
 
 
Transactional Control Language :  Used to allow/administer changes to DML commands, in effect governing transactions. 
  • commit, rollback 
Commit : Ends the transaction to save the transaction changes to  physical data files. There is no option to undo the transaction after a commit is issued. 
                 Syntax: 
                                 
                                  ----------SQL statements------------
                                  commit; 
 
Rollback: In a realtime transaction, up until a commit is executed there is an option to rollback a transaction so that the actual data can be retained.
              Syntax: 
                             
                             ----------------SQL statements------------
                             rollback; 
 
[DCL and TCL  query languages are used in advanced/realtime applications]
 
 
 
                                                                                                                                                             
  

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