Introduction to SQL

tags: #sql

What is SQL?

SQL is a Data Manipulation Language used to manipulate database (access, modify or retrieve the data).

This is different from Data Definition Language for specifying database schema structure.

SQL was originally coined as "SEQUEL"

S - Structure
E - English
QUE - Query
L - Language

Now popularly known as SQL - Structured Query Language

Executing SQL Statements

In SQL, a semicolon (;) is used to mark the end of a SQL statement.

It is used as a separator between multiple SQL statements in a batch, and it tells the database engine where each statement ends and where the next one begins.

This also makes your SQL code more portable and more consistent across different database platforms (not all DBMS require ; at the end of each statement).

Meaning of `NULL`

Meaning of NULL differs by context and data you are working with, but can be a result of:

Unknown value due the value being absent
Unavailable or withheld value
Not applicable attribute
Cannot do comparison with NULL values (incl. NULL = NULL)

Logical Values in SQL

SQL uses a three-valued logic involving: TRUE, FALSE, and UNKNOWN

What is UNKNOWN and how is it different from NULL?

UNKNOWN is a special value in SQL that represents missing, unknown, or inapplicable data. It is typically used in situations where the actual value of a data item is not known or cannot be determined.

NULL is a value that represents the absence of a value, whereas UNKNOWN represents a missing or unknown value that cannot be determined.

Logical Connectives

AND CONNECTIVE	TRUE	FALSE	UNKNOWN
TRUE	TRUE	FALSE	UNKNOWN
FALSE	FALSE	FALSE	FALSE
UNKNOWN	UKNOWN	FALSE	UNKNOWN

OR CONNECTIVE	TRUE	FALSE	UNKNOWN
TRUE	TRUE	TRUE	TRUE
FALSE	TRUE	FALSE	UNKNOWN
UNKNOWN	TRUE	UNKNOWN	UNKNOWN

NOT CONNECTIVE
TRUE	FALSE
FALSE	TRUE
UNKNOWN	UNKNOWN