learn SQL: introduction

Let’s start with some definitions

Database: a place where you store data. It’s a collection of tables in terms of relational database.

Table: A data structure to store data in form of columns and rows. AKA, a relation.

A row is also called a tuple which contains a record and a column name can be called an attribute. A cell is cross section of a row and a column.

A Table

Each column stores some specific kind of data according to its datatype (ex: varchar, number…)

SQL: (Structured Query Language)

initially called SEQUEL (Structured English Query Language). SQL is a domain-specific language to interact with database.

• SQL keywords are case insensitive.

Classification of Commands

SQL Commands Classification

Constraints: Rules enforced on data of a table, column wise.

https://www.tutorialspoint.com/sql/sql-constraints.htm

Create Table:

• Constraints on the attributes can be specified in create command itself or later after creating a table, using Alter table command.
• We can specify multiple foreign key constraints on single table.

create table table_name(
               col_1 datatype constraint,
               col_2 datatype constraint);

Or we can specify constraints after column names.

create table table_name(
               col_1 datatype ,
               col_2 datatype ,
               constraint(col_1),
               constraint (col_2) );

Select Clause:

The minimal select statement goes like this-

select <column_name>
from <table_name>

It selects the column called column_name from table called table_name.

To query multiple columns, we use

select <column1, column2 ...>
from <table_name>

To query all the columns, we use * aka asterisk

select *  from emp;

Select Distinct values:

select job  from emp;

Here we have many jobs repeating and to get only unique records we can use the keyword DISTINCT

select distinct job  from emp;

here we get only 5 rows using Distinct. It returns unique values only.

• Distinct will return NULL as one value so If you have null values than it’ll return an empty space as well.

select distinct comm
from emp;

The ‘-’ here in the output represents a null value.