DECLARE @expr nvarchar(4000),
@sql nvarchar(4000),
@res decimal(18, 6)
set @expr = '(1+2)+6'
SELECT @sql = N'SELECT @res = CONVERT(decimal(18, 6), ' + @expr + ')'
EXEC master.dbo.sp_executesql @sql, N'@res decimal(18, 6) OUTPUT', @res OUTPUT
PRINT @res
