Trying to understand the output of a few similar queries

Question

I'm using the standard world schema that comes with MySQL Workbench. I initially tried this query to give me all of the cities in the United States; however, I kept getting one row of null entries at the bottom.

select * 
from city 
where CountryCode = 'USA';

I looked up a solution and found MySQL SELECT only not null values. So, next I tried this:

select * 
from city 
where Name is not null and CountryCode = 'USA';

which gave me the exact same thing as the first query. So, I decided to take a different approach. Instead of getting multiple columns, I wanted to see if I could just get the Name column:

select Name 
from city 
where CountryCode = 'USA';

Much to my surprise, this worked fine, and without me having to check for null.

So my question is two-fold:

• Why did my second approach give me the row of nulls at the bottom (also how to fix it), and
• Why did the last approach not give me a null at the bottom?

EDIT: Additional Info
Also, not sure if this is important, but despite the fact that the first two queries returned a row with nulls at the bottom, for all three queries, I got the same number of returned rows.

Here's the output from the first two queries:

Here's the output from the last one:

Answer 1

I do not know that much about MySQL, but your two first queries are updatable queries, so you might be allowed by default to modify or add records to it. The last 'null' line is then displayed to allow the addtion of data, but does not correspond to any record in the table.

Your last query is not updatabale, so it will not be displayed with a last 'empty' line.

Answer 2

about your second query.. When you cheking for your field havent any value you should cheking for that field not empty LIKE THIS:

SELECT * FROM `city` WHERE name <> '' AND CountryCode  = 'USA'

in the last query when you select a field Singly then you have Set and Set in mathematics not get the null or empty value LIKE this in a set in mathematics there is no different between

a= {0,1,2} AND b= { , 0 , 1 , 2} so when you select a column Singly you have a set so you haven't empty value or null value because its not defined in set