Archives for June 2012

The SQL Count is an important function that I use when I am exploring a new table.

I often use it to see how big a table is, but I also use it to determine counts within a group.

Let’s take a look at it’s uses.

Here is the employee table:

select * 
from employee; 

Results:

You can tell this has 14 records, but imagine that it had 1,000,000 records.

You wouldn’t be able to just select * without it taking a while to bring all of the records back.

That is where the SQL Count comes in:

select count(*) 
from employee; 

Here you can see it gave us one column with the count of the whole table.

Pretty simple.

Now, what if we wanted to find out how many people had a commission?

If we look above, we can simply count those people.  But, we have this new SQL Count function.

So, let’s use it:

select count(commision) 
from employee; 

Will we get 14 records, or 4 records?

Well, here are the results:

We only got 4 records.  Why is that?…

That is because the count function doesn’t count NULL records.  Again, if you look above, you will see a bunch of commissions that have the value of NULL.

This NULL value means we don’t have a value.  So, when the SQL Count  function looks at those rows, it ignores them.

But, look at the 10th row above.  It has a “0.00” value.  Why does it count that one?

Because, even though the value is 0, it is still a value (it isn’t null).

An that’s it.  That’s how you use the SQL Count function.

Today I want to talk about SQL Functions and how they are used in the SQL language.

Let’s start by looking at a simple employee table.

select * from employee; 

This employee table has 14 rows.

Now, there are some built-in SQL Functions that allow you to average, sum, get the max and minimum and count. (There are others too, but I am only mentioning a few.)

These functions will look at a group of rows and perform the required action.

So, what if I want to get the average salary of all 14 of my employees?

Then I would do the following:

select AVG(salary) from employee;

The average function is actually just AVG and you put the column in between the parenthesis.

Here are the results:

So the average salary is 2073.21…

That was pretty easy.  But, what if you want the column to have a name?

select AVG(salary) as average_salary from employee; 

You do what is called a column alias.  After you put the column in (in this case AVG(salary) ) you put in the “as” keyword, then you put whatever you want to name the column. (In this case I called the column average_salary)

And here is the result:

And now your column has a name.

So now let me go ahead and use the rest of the functions I mentioned.

select 
AVG(salary) as average_salary 
,sum(salary) as sum_salary 
,min(salary) as min_salary 
,max(salary) as max_salary 
,count(salary) as count_salary 
from employee; 

And here are the results:

Not too difficult?  But, I want to try one more thing.

I want to use the SUM function and the COUNT function to create my own average function.

If you remember from Algebra, the average is calculated by summing all of the values and dividing by the number of values.

So let’s give it a try:

select sum(salary)/count(salary) as my_new_average 
from employee; 

Notice how I took the Sum/Count.

Here is the result:

Notice that the value is the same as when we used the built in average function.  That is what we would expect.

So that is an introduction into SQL Functions.

Leave any question, comments, or topic suggestions bellow.