SQL (Structured Query Language)

SQL consists of three components :

DML (Data Manipulation Language) → for inserting, extracting, deleting and updating data in tables.
DDL (Data Definition Language) → for creating and maintaining database objects.
DCL (Data Control Language) → to control data security and authority, as well as transactions.

Data selection in tables

With SQL, zero to many rows at a time can be manipulated. This is one of the main differences between SQL and conventional file-based data. With file-based data, you generally use a program written in a language such as COBOL to process one row at a time. With SQL, on the other hand, you can process a set of rows. → the results of a SQL query can be returned to various places (depending on how the query is executed). Query output is often held in a temporary storage area, waiting to be retrieved by the display or access program or system.

SELECT <column> FROM <table>

Example, table COMPANIES

NAME	COUNTRY	CREATION
SNCF	FRANCE	1938
NS	NETHERLANDS	1938
ONCF	MOROCCO	1963
DB	GERMANY	1994
EUROSTAR	NULL ?	1994
TRENITALIA	ITALY	2000

**SELECT * FROM COMPANIES**

# all the contents of the columns

⛔ The columns to be selected are separated by a comma.

WHERE <criteria>

**SELECT NAME FROM COMPANIES
WHERE COUNTRY = 'FRANCE'

-> SNCF**

⛔ non-numeric values in WHERE are enclosed in single quotation marks