不知道哪里有错误？谢谢！

[dyeerp]

提示：从字符串转换为 datetime 时发生语法错误。下面哪里有错误，如何改？谢谢！或者这个SQL语句怎么写？满足下面的条件同时把@Fd转换成日期型、并月份永远是01月。
declare @m int,@F int,@Fd datetime
declare @FYear int,
@FPeriod int

select @FYear=2008,@FPeriod=8 --@FYear、,@FPeriod是可变的值，现在假设@FYear=2008,@FPeriod=8

if(@FPeriod-9)<0
begin
select @F=@FYear-1,@m=@FPeriod+3
end
else
begin
select @F=@FYear,@m=@FPeriod-9
end
select @Fd=cast((cast(@F as varchar(50))+cast(0 as varchar(50))+cast(@m as varchar(50))+cast('01' as varchar(50))) as datetime)

[lussnailatnet]

declare @m int,@F int,@Fd datetime 
declare @FYear  int, 
        @FPeriod int 

select @FYear=2008,@FPeriod=8  --@FYear、,@FPeriod是可变的值，现在假设@FYear=2008,@FPeriod=8 

if(@FPeriod-9) <0 
begin 
select @F=@FYear-1,@m=@FPeriod+3 
end 
else 

begin 
select @F=@FYear,@m=@FPeriod-9 
end 

select @Fd=cast((cast(@F as varchar(50))+'-'+cast(@m as varchar(50))+'-'+cast('01' as varchar(50))) as datetime)

select @Fd

[hyqwan11112]

declare @m int,@F int,@Fd datetime 
declare @FYear  int, 
        @FPeriod int 

select @FYear=2008,@FPeriod=8  --@FYear、,@FPeriod是可变的值，现在假设@FYear=2008,@FPeriod=8 

if(@FPeriod-9) <0 
begin 
select @F=@FYear-1,@m=@FPeriod+3 
end 
else 

begin 
select @F=@FYear,@m=@FPeriod-9 
end 

select @Fd=cast((cast(@F as varchar(50))+'-'+cast(@m as varchar(50))+'-'+cast('01' as varchar(50))) as datetime)

select @Fd

[-晴天]

declare @m int,@F int,@Fd datetime 
declare @FYear  int, 
        @FPeriod int 

select @FYear=2008,@FPeriod=8
set @fd=convert(datetime,cast(@fyear as varchar(4))+'01'+right('0'+cast(@fperiod as varchar),2))
select @fd

/*
-----------------------
2008-01-08 00:00:00.000

(1 行受影响)

*/

[dawugui]

declare @m int,@F int,@Fd datetime 
declare @FYear  int, 
        @FPeriod int 

select @FYear=2008,@FPeriod=8  --@FYear、,@FPeriod是可变的值，现在假设@FYear=2008,@FPeriod=8 

if (@FPeriod-9) < 0 
begin 
  set @F = @FYear - 1
  set @m = @FPeriod + 3 
end 
else 
begin 
  set @F = @FYear
  set @m = @FPeriod - 9 
end 
set @Fd = cast(cast(@F as varchar) + '-01-' + right('00'+cast(@m as varchar),2) as datetime)