SQL Tips, Tricks, and Training - SQL Training Online

June 12, 2012 by Joey Blue Leave a Comment

SQL Wildcard

SQL Wildcard filters allow you to search within a string to do partial matches.

So you can use them to search for all names that begin with the letter ‘A’.

Or you could search for all addresses that contain the name ‘OAK’.

Let’s take a look at an example. I will start with a basic employee table and run the following SQL query:

select * from employee;

The results look like this:

I want to find all of the employee’s with a name starting with ‘A’.

In order to do this, I need to use a SQL Where clause along with a LIKE wildcard filter.

The SQL will look like this:

select * from employee where employee_name like 'A%';

And here are the results:

I filtered out the 2 rows that have an employee_name starting with ‘A’. (ALLEN and ADAMS).

Let’s look at the where clause of that statement in more detail:

where employee_name like 'A%';

Where – Starts the filter section of SQL

employee_name – Is the column we want to look at.

like – The comparison operator. This is how we want to compare. Some other options would be =, <= ,>=, between, in, etc..

‘A%’ – This is the value we are looking for. We have to enclose it in ‘ ‘ (called ticks). Since the first letter inside of the ticks is an A, it will match the first character. Then we have the %. This means we don’t care how many characters come after the A. The name could contain just the A with no letters after it. It could contain the A with 1 letter after it, or it could contain the A with 100 letters after it. The % doesn’t care.

Now, there is one other wildcard that is often used. It is the ‘_’ (underscore). This is a single character wildcard.

Let’s look at an example. I want to find all names where the second letter is ‘L’:

select * from employee where employee_name like '_L%';

Here are the results of the query:

In each of the 3 results, the second letter of the employee_name is an L.

That is how you use the wildcard filter in the SQL language.

Please leave any questions and comments below.

June 11, 2012 by Joey Blue Leave a Comment

SQL Between

The SQL Between is a way of looking for a range of values when you are trying to filter in a SQL Select statement.

So let’s take at an Employee table with the following query:

select * from employee;

Here are the results:

This employee table has 14 rows. I want to filter this table to find all of the salaries that are greater than or equal to 1100.00 and less than or equal to 1600.00.

I will use the following query to do that:

select * from employee where salary >= 1100 and salary <= 1600;

Here are the results:

Notice how we get the salary of 1100 and the salary of 1600. This is because I used the greater than or equal to and the less than or equal to.

I could reword what I want with the following sentence. I want to filter this table to find all of the salaries that are between 1100.00 and 1600.00.

Then I can write the query like this:

select * from employee where salary between 1100 and 1600;

This query will give you the same results as before, but it uses the BETWEEN clause.

Some things to note about the BETWEEN clause.

First, you only put the column name in there 1 time vs. 2 times with the first query.

Also notice that the BETWEEN clause includes the Endpoints. So, if you look at the query above, we are including in the results a Salary of 1100 and a Salary of 1600.

Now, if you are wondering if one way is better than the other? Whether you should use the BETWEEN clause or the ‘>=’ and ‘<=’?

I would have to say that it is really just a preference. I prefer to use the BETWEEN clause when I can because it helps with the readability of the query.

As you get into more complex queries, you will start to find ways to keep your query as simple and readable as possible so that you don’t loose your mind.

(Trust me, queries can become really complex fast!)

That’s it for the BETWEEN clause.

Drop me a note in the comments section if you have any questions, comments, or suggestions.

June 10, 2012 by Joey Blue Leave a Comment

SQL IN

The SQL IN clause is used when you are filtering data and you want to look for more than one value.

Let’s take a look at the following query:

select * 
from customer 
where customer_company = 'Dell' 
 or customer_company='HP' 
 or customer_company='Walmart';

This query says that we want all of the columns, from the company table, where the customer_company is either Dell, or HP, or Walmart.

Here are the results:

This query works great, but it means you have to type a lot more than you need to.

That is where the “IN” clause comes into play. Instead of typing “customer_company” 3 times like we did in the previous SQL Query, we can write the query as follows:

select * 
from customer 
where customer_company IN ('Dell','HP','Walmart');

The results of this query are the exact same as the results above.

Notice how much less typing I had to do. And, notice the syntax.

It goes column name (in this case customer_company), then you use the “IN” word, then you list all of the values in side of parenthesis and separated by commas.

That’s it. I often use the IN clause instead of a bunch of OR’s, because it is neater and easier for me to read.

As you start putting more complex queries together, it will be a lot easier to keep track of an IN clause instead of a bunch of OR statements.

Remember to put any question, comments, or other topics you would like for me to explain into the comment section below.

June 9, 2012 by Joey Blue Leave a Comment

SQL AND OR

The SQL AND OR is used when filtering with the SQL Where clause. More specifically, we are talking about the “AND” and the “OR” clauses.

Let’s start by looking at a basic SQL statement:

select * from customer

The customer table has the following records:

Now if we were to filter to just the “Dell” records, we would use the following query:

select * 
from customer 
where customer_company = 'Dell';

But, I want to get the “Dell” records and the “HP” records. To do that we would use the following query:

select * 
from customer 
where customer_company = 'Dell' 
 or customer_company='HP';

And we get the following results:

Now, you are probably asking, “You said we want Dell AND HP records, why did we use the OR clause?”

It can be a little confusing, but here is how I think of it.

In the case where we want the company of Dell and HP, we are actually saying we want all of the records where the customer_company is Dell or the customer company is HP.

There is a never a case where the customer_company is both Dell and HP.

So let’s look at an example where we use the AND statement.

If, I wanted to get all of the Dell customers that have a name of Bill, I would use the following query:

select * 
from customer 
where customer_company = 'Dell' 
 and customer_name='Bill';

In this case, I am actually looking for records where both criteria is met. Here is the result:

Those are the basics of using the SQL AND and OR clauses.

Please leave and questions or comments below and let me know if you want me to talk about any other topics.

June 8, 2012 by Joey Blue Leave a Comment

SQL Where

The SQL Where clause in the SQL Language allows you to filter data when you query a database.

Let’s look at the following query that gets data from the customer table:

select * from customer;

Here are the results:

Now, I only need to get the “Dell” customers. In order to do that, I have to use the where clause.

Let’s add that to the query:

select * from customer where customer_company='Dell';

When I execute this query, I get the following results:

So I have filtered the records from 6 down to 2.

Notice the following part of the query:

customer_company='Dell';

The red part is how Microsoft SQL Server represents a string. It knows it is a string, because of the single tick (‘) marks.

All strings need to be enclosed in these.

Let’s show an example where we want everyone except for Dell customers:

select * from customer where customer_company != 'Dell';

Instead of using the “=” sign, we use the “!=” which says “Not Equal”.

Here are the results:

We have filtered out the 2 Dell records.

That is the basics of the where clause.

Please leave any comments or questions below.