Why does this not work: INSERT SQL statement using "With ROW_NUMBER" . Error is Incorrect syntax near the keyword 'With'

Question

I am using this syntax which works in components separately but when I use it all together, it stops to work I get a syntax error: Incorrect syntax near the keyword 'With'. I am not sure why.

if OBJECT_ID ('tempdb..#Assignments') is not null 
   drop table #Assignments
create table #Assignments
(
SerialNumber varchar(24) not null
)
insert into #Assignments (SerialNumber)
    With Row_Number as 
        (
        select SerialNumber,row_number() 
        over (partition by serialNumber order by serialnumber) as Row_Num
        from Monitor  
        where SerialNumber IS NOT NULL
        ) 
    select SerialNumber from Row_Number where Row_Num > 1

Answer 1

It's not valid syntax. When you use a CTE the WITH is the first part of the statement.

if OBJECT_ID ('tempdb..#Assignments') is not null 
    drop table #Assignments;
create table #Assignments
(
    SerialNumber varchar(24) not null
);

    With Row_Number as 
        (
            select SerialNumber,row_number() over (partition by serialNumber order by serialnumber) as Row_Num
            from Monitor  
            where SerialNumber IS NOT NULL
        ) 
    insert into #Assignments (SerialNumber)
    select SerialNumber from Row_Number where Row_Num > 1;

Side note: make sure you include ; after your statements when you are using CTE's. The statement prior to WITH has to be terminated or you will get a syntax error.