create table `tblVariables` ( 
	`Variable` VARCHAR( 255 ),
	`VariableValue` VARCHAR( 255 ), 
	constraint `tblVariables_PK` primary key (`Variable`) ); 

create table `tblVariables1` ( 
	`Variable` VARCHAR( 255 ),
	`VariableValue` VARCHAR( 255 ), 
	constraint `tblVariables_PK` primary key (`Variable`) ); 

UPDATE tblVariables SET tblVariables.[Value] = ( SELECT a.Value FROM  tblVariables1 a WHERE  tblVariables.Variable = a.Variable )
WHERE ( SELECT a.Value FROM  tblVariables1 a WHERE  tblVariables.Variable = a.Variable ) is NOT NULL;

Operation must use an updateable query