The language SQL (Structured Query Language) is a database language that allows you to access databases.
This language is used to access the main databases existing today such as SQL Server Microsoft or Oracle database, of course the syntax between these two databases may have some differences, but the basic commands are practically the same.
DDL (Data Definition Language) Instructions
Data Definition Language, defines the structure of data and tables. The most common DDL commands are CREATE, ALTER, DROP, RENAME and TRUNCATE.
CREATE – Command used to create a BANK (CREATE DB) is TABLE (CREATE TABLE).
CREATE DB – Creates a database. See an example of use:
- CREATE DATABASE database_name
CREATE TABLE – Creates a table. See an example of use:
CREATE TABLE doctor
primary key code,
- CREATE TABLE is the create command
- doctor – the name you want to give the table
- code, name, age, crm – are the columns you insert in this table
- primary key – define what the primary key will be
NOTE: after the column names, you can define the parameters of each column, eg data type, size, if required.
ALTER TABLE – Command used to add, delete or modify columns in an existing table.
- ALTER TABLE table_name ADD column_name datatype
- Example usage: ALTER TABLE doctor ADD Address VARCHAR(80)
Delete a column
- ALTER TABLE table_name DROP COLUMN column_name
- Example usage: ALTER TABLE doctor DROP Address
DROP – Command used to delete data. DROP can be used in three ways:
- DROP INDEX – Delete an index in a table
- Usage example: ALTER TABLE table_name DROP INDEX index_name
- DROP TABLE – Delete a table
- Usage example: DROP TABLE table_name
- DROP DATABASE – Delete a database
- Usage example: DROP DATABASE database_name
RENAME – Command used to rename tables. See below an example of use:
- RENATE TABLE doctor TO doctors
TRUNCATE – Command to delete data from a table, not the table itself. See usage example:
- TRUNCATE TABLE doctors
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