The differences between variables in each of the RDMSes are outlined as follows: 

• Oracle: In Oracle, there is a different syntax for using variables, as shown in the following query: 

DECLARE
   var_parkname varchar2(100); 
   var_parkkey varchar2(5) := 'ALB01'; 
BEGIN
SELECT parkname INTO var_parkname FROM parks_copy
WHERE parkkey = var_parkkey; 
DBMS_OUTPUT.PUT_LINE(var_parkname);
END;

The query has you declare a variable for parkkey (var_parkkey), as well as for the output of the query with the variable in it, which is named var_parkname. Then, you output var_parkname to get the results. 

• PostgreSQL: You can't use variables outside of functions. 
• SQL Server: In SQL Server, you can't use := to set a variable, only =. Also, you need to declare the variable before setting it, as shown in the following code snippet: 

DECLARE @varname varchar(5); 

You can also declare and set the variable in one line, as shown in the following code snippet: 

DECLARE @varname varchar(5) = 'ALB01';

The first DECLARE statement will require you to use a SET statement to set the value, but the second DECLARE statement won't.