ora-06502 charactor to number convertion error

今天写了一个测试的存储过程：
create or replace procedure preparedData_up
as
modSql varchar2(500); --修改sql语句
tempValue number(18,4);
begin
modSql:='update t_mms_staff_stat_test set ';
tempValue:=0;
--sm01_sales_amnt;
select sum(amount_td) into tempValue from items_sales
where store='csst1';
modSql := modSql ||' sm01_sales_amnt=' || tempValue;

--sm02_sales_amnt;
select sum(amount_td) into tempValue from items_sales
where store='csst2';
modSql :=modSql||',sm02_sales_amnt=' || to_char(tempValue,'FM9999999999999999990.00');

--sm03_sales_amnt;
select sum(amount_td) into tempValue from items_sales
where store='csst3';

modSql :=modSql||',sm03_sales_amnt=' || to_char(tempValue,'FM9999999999999999990.00');

--modSql :='update t_mms_staff_stat_test set '+modSql;

execute immediate modSql;

end preparedData_up;

编译通过，可执行的时候报ora-06502 charactor to number convertion error，
以为是数字类型该转成字符串的，所以用了to_char,但依然报错，郁闷了半天，最后终于发现是连接符的问题，我用了‘+’而不是‘||’，汗了一把，好像碰到好几回了……