SQLserver中两列相减并取绝对值的最大值，增加字段

select max(tt) from (SELECT (cast([c122] as float ))-(cast([c123] as float )) as tt FROM 表名) ta ;

两列相减并取绝对值的最大值

alter table 表名 add 字段名 字段类型;

sql server中如何写脚本为数据表增加一个字段