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October 19, 2012 by Joey Blue Leave a Comment

How to use Datetime in SQL Server – SQL Training Online

This video is a quick overview of using the SQL Server Datetime function.

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The datetime is actually made up of two different pieces:

1) Date

2) Time

This can be demonstrated using the SQL Cast function on a string to convert it to a datetime.

Here is some SQL for the Date portion:

select cast('20121019' as datetime)

Here is some SQL for the Time portion:

select cast('10:05:00' as datetime)

Now, you can also simply use the built-in getdate() function to bring back the current system time.

select getdate()

And finally, I want to bring back a datetime from an actual table. time

In this case, we will use the employee table from the SQL Training Online Simple DB:

select hire_date from employee

If we want to convert the result to a more formatted string, we will use the TSQL CONVERT function:

select convert(varchar,hire_date,104)
from employee

In this example, we are converting from a datetime to a varchar (string). The result will be in the format of 104.

To translate 104 to a datetime format, you will want to look at the MSDN Documentation.

And, that is how you use the Datetime SQL.

Let me know what you think by commenting or sharing on twitter, facebook, google+, etc.

Filed Under: SQL Training Tagged With: datetime, SQL

June 19, 2012 by Joey Blue Leave a Comment

SQL Count

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:

Select 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; 

SQL Count * Results

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:

SQL Count Column 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.

Filed Under: SQL Training Tagged With: Beginning SQL, SQL Function, SQL Select

June 18, 2012 by Joey Blue Leave a Comment

SQL Average

The SQL Average function is an easy function to use.

Let’s take a look at the Employee table:

select * from employee; 

Results:

SQL Average Results

If I want to find the average of the salary column, I would write the following query:

select avg(salary) as average_salary from employee; 

With the following results:

SQL Avg Results

Notice how we only get one column returned because that is all we asked for.

You can use the SQL Average on any numeric column using this technique.

When we get to the Group By clause, you will see how we can average across different groups of columns.

Filed Under: SQL Training Tagged With: Beginning SQL, Function, SQL Average, SQL Select

June 15, 2012 by Joey Blue Leave a Comment

SQL Functions

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; 

SQL Functions Result

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:

SQL Avg Result

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:

SQL Alias 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:

SQL Functions Result

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:

SQL Average 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.

Filed Under: SQL Training Tagged With: Beginning SQL, SQL Function, SQL Select

June 14, 2012 by Joey Blue Leave a Comment

SQL Order By

The SQL Order By allows you to sort your results when you run a SQL Query.

Let’s look at an example:

select * from employee order by employee_name; 

This will sort by the employee_name column.

Here are the results:

image

If you want to change the sort from Ascending (which is the default) to Descending, you would write your query as follows.

select * from employee order by employee_name desc; 

With the following results:

image

You can also sort by more than one column.

select * from employee order by job asc,employee_name desc; 

Notice how job and employee_name columns are both sorted.

image

And the last trick is that you can use the column number.  In this case I will sort by the 3rd column (job) then the 2nd column (employee_name).

select * from employee order by 3 asc,2 asc; 

Notice that I changed the employee_name column to sort ascending.

image

And that’s it for the SQL Order By clause.

Leave any comments or questions below.

Filed Under: SQL Training Tagged With: Beginning SQL, SQL Order By, SQL Where

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