既然已经用了isnull(),就不需要case ... when ... end了:
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'update t_storage set willoutnum=isnull(willoutnum,0)+'+S_num+' where cinvgg='''+e_cinvgg.Text+''' and cinvname='''+cmb_cinvname.text+''' and cinvpp='''+cmb_cinvpp.Text+''' and address='''+cmb_address.Text+''''
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'update t_storage set willoutnum=isnull(willoutnum,0)+'+S_num+' where cinvgg='''+e_cinvgg.Text+''' and cinvname='''+cmb_cinvname.text+''' and cinvpp='''+cmb_cinvpp.Text+''' and address='''+cmb_address.Text+''''
' where cinvgg='''+e_cinvgg.Text+''' and cinvname='''+cmb_cinvname.text+''' and cinvpp='''+
cmb_cinvpp.Text+''' and address='''+cmb_address.Text+'''' 询问 when 0 then 0 else willoutnum end 含义