Tutorial 10- Comments And Expressions Video Lecture | Postgresql: A Complete Tutorial - Database Management

hello guys welcome to our tutorial or
PostgreSQL today I will teach you about
comments and we will learn about some of
the expressions in the PostgreSQL so
first of all we talk about comments
adding comments and SQL commands are
basically added to show the
functionality or basically tell the use
of that what we are doing at the current
moment so first of all you will be
opening the database our test database
that we created and we will click on the
SQL button above so comment in our sqs
given by double - - - - and we can see
this is a comment now as you can see
that this has turned out to be green so
this is successfully being a comment now
so if you see that I will put another
statement that is select star from
company that was our table that we
created now if you see it will fetch me
some results see I have got some result
set now to comment this out I will just
add a double - in front of it now you
could see after I run it I see empty
query no results so this is successfully
commented out so we could see the power
of comments so that we could describe
that this is a select statement so you
could see that it gives an idea to the
user that I am going to work with a
select select command so the next next
method that you put add comment is the
normal programming method that is slash
star and you could write this is command
so as you could see again it is green so
yeah this is one more method to add
comments this is basically used to add
multi-line comments when we don't want
to use a double slash in each and every
line so if you are using multiple lines
suppose you could see that it is
difficult to enter - in each and every
line so we just use this last slash star
star backslash and this is commented out
so the second thing that we're going to
learn about today is the expressions an
expression is basically a combination of
one or more values operators and
PostgreSQL functions that are used to
evaluate a particular value so the first
expression that we are going to talk
about is the as expression expression so
the as expression is basically used to
rename a particular field like suppose
if I use select name from company and
you see that I get name as the column
heading of this now suppose I do not
want to name to be the column heading of
this particular result set I would be
using the as operator that is select
name as employee underscore name suppose
I want and I don't want name I want it
to be employing him instead up in K
instead of name so I will just use this
and you see that I get it gets it gets
replaced by employee name so this is the
use of the as operator the next operator
that we'll talk about is the count
operator so the count operator is
basically used to count the number of
fields in particular data set now
suppose if I use select count star star
from company now this will result into
the count of the number of employees I
have that is 8 as you could see so this
is the use of the count Oprea count
expression and star means to calculate
the distinct values from the data set
similarly we have a maximum minimum sum
and average function so basically the
user for maximum is to give out the
maximum value in a particular data set
now suppose I want the maximum wage of
an employee working in my company so I
will you select max age from company
this will give me the maximum age of the
employee you see I have the maximum age
as 40 so next would be
the minimum age you could just replace
min by max this would give me the
minimum age this is 12 now suppose I
want the sum of all the salaries of
employees working in my company now I
will you select sum and bracket in
bracket I will use salary C this will
give me the salary of all the employees
working in my company that is select
salary from company now as you could see
that I get all the summation of the
salaries in my company so the next is
average operator that will give me the
average of the values that is select AVG
salary from company now you will see
that this will give me the average
salary in my company that would be C
this gives me the average salary so the
last operator that we talked about this
tutorial is the time stamp or the
current time like if you want to put
something like if you have a photo that
user has uploaded we want the time and
the date of the photo so we could time
stamp it so we will just do it by a
select current underscore time spam this
will give me the current timestamp see
this is 2016 6 then and the current time
with the time zone so thank you for
watching this tutorial we will be
discussing more functions in the coming
tutorials subscribe for more and stay
tuned