SQLSERVER 字符串表达式 - MS-SQL Server / 疑难问题

SQLSERVER 我有一个表，查出来的一个字段是字符串表达式
比如
a b c 5*6-3
a be ce 15*(6-3)
a b c 36-3
a be ce 1*(9-8)

我想在此后面增加一列字段，把计算结果输出来，
a b c 5*6-3 27
a be ce 15*(6-3) 45
a b c 36-3 33
a be ce 1*(9-8) 1

如何用SQL或者存储过程实现呢？有没有比较简单的方法实现呢？谢谢

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SQL code:
if object_id('[tb]') is not null drop table [tb]
go
create table [tb]([col1] varchar(1),[col2] varchar(2),[col3] varchar(2),[col4] varchar(8))
insert [tb]
select 'a','b','c','5*6-3' union all
select 'a','be','ce','15*(6-3)' union all
select 'a','b','c','36-3' union all
select 'a','be','ce','1*(9-8)'

declare @sql varchar(8000)
select
@sql=isnull(@sql+' union all ','')
+'select '''+col1+''' as col1,'''+col2+''' as col2,'''+col3+''' as col3,'''+col4+''' as col4,'+col4+' as col5 '
from tb
exec (@sql)

/**
col1 col2 col3 col4 col5
---- ---- ---- -------- -----------
a b c 5*6-3 27
a be ce 15*(6-3) 45
a b c 36-3 33
a be ce 1*(9-8) 1

(4 行受影响)
**/

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SQL code

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