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Archives for June 2012

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

June 13, 2012 by Joey Blue Leave a Comment

SQL Jobs in Oracle and Microsoft SQL Server

I was browsing around Yahoo Answers and I started to answer a question about Jobs in SQL and Oracle.

It was interesting to me, so I decided to repost the answer here and elaborate a bit.

Here is the question:

What is difference between oracle and SQL I want to know about . . . ?

which type of jobs are available on oracle course and Sql
Oracle and Sql are same or different ? And tell me about types of jobs with oracle like oracle DBA or oracle developer like this . . . .

I had to interpret quite a bit about what they were wanting to know, so here is my attempt.

Oracle is a relational database.  There are other relational databases out there, like Microsoft SQL Server, MySQL, DB2, etc.

All of these relational databases use SQL to put data into and take data out of the relational databases.

Then, they have their own “programming” languages.

Oracle has their PL/SQL language that you use to write stored procedures and functions.

Microsoft SQL Server has their TSQL language that you use to write stored procedures and functions.

So now that we have defined some of the terms, what jobs are available?

An Oracle DBA (Database Administrator) would take care of database instances and make sure they keep running, they don’t run out of space, they are secure, they have redundancy where needed, and stuff along that line.  They have traditionally been paid a bit more than a Microsoft SQL Server DBA.  But, Oracle has many different “levers” you can pull to tune it, so their is quite a bit to learn to be a real good Oracle DBA.

An Oracle Developer would most likely need to learn PL/SQL.  PL/SQL allows you to write functions and stored procedures that end up as objects on the Oracle server.  You would also need to know basic SQL in order to be effective.  Basic SQL would include SELECT, INSERT, UPDATE, DELETE statements that allow you to put data into the database and get data out.

So the jobs would be Oracle DBA (takes a bit of experience unless you work as a Junior DBA under a Senior DBA.), Oracle Developer (this is the PL/SQL side), and then a regular developer that would work in .Net or Java.

The regular developer position would write programs that interface with the Oracle database, but you would use the .Net (C#/VB.Net) or Java along with SQL to interact with the database.

There are so many different positions I have seen throughout my years in IT.

Once you start a position, the lines end up getting blurred quite a bit.  You have your main responsibility, but you will end up crossing over and doing some scripting or programming.

Take a DBA for example:

They have the responsibility to make sure the database stays up and running and secure.  But, in order to do their job efficiently, they will most likely write some scripts that recompile indexes on some interval, or the script with defragment the disk, or check for errors in the logs.  They definitely need to know SQL in order to create and recompile objects.

DBA’s will also need to do some performance tuning of queries.  But, this job is also a developer’s job, so it crosses over and many times will need to be a collaborative effort.

So you can see that a DBA’s job has already crossed over to a Developer’s a little bit.

I’m going to cut it off there, but I could keep going on all day.

Let me know what you have experienced working in (or with) the IT industry but posting to the comments below.  Or, if you have some more questions please post them too.

Filed Under: SQL Jobs Tagged With: DBA, Jobs, Microsoft SQL Server, Oracle

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