In spring 2013, Barron added MasteringChemistry prelecture homework assignments: short, untimed assignments due before lecture designed to help increase student awareness about new topics and to identify student misconceptions and areas of misunderstanding. Barron tells students it is okay to be confused after doing the homework because the concepts will become clear during class.
In SQL, anything you evaluate / compute with NULL results into UNKNOWN This is why SELECT * FROM MyTable WHERE MyColumn != NULL or SELECT * FROM MyTable WHERE MyColumn <> NULL gives you 0 results.
sql - Not equal <> != operator on NULL - Stack Overflow
I have seen SQL that uses both != and <> for not equal. What is the preferred syntax and why? I like !=, because <> reminds me of Visual Basic.
Should I use != or <> for not equal in T-SQL? - Stack Overflow
Microsoft SQL Server is one of the exceptions: it doesn't support , and requires .
The @CustID means it's a parameter that you will supply a value for later in your code. This is the best way of protecting against SQL injection. Create your query using parameters, rather than concatenating strings and variables. The database engine puts the parameter value into where the placeholder is, and there is zero chance for SQL injection.
The other answers are correct if you're dealing with SQL Server, and it's clear that you are. But since the question title just says SQL, I should mention that there are some forms of SQL such as MySQL where a pound sign is used as an alternative commenting symbol.