sql 联合查询

SELECT NZD_Products.Prd_typename a FROM  dbo.NZD_Products where dbo.NZD_Products.Prd_barcodenumber = 'PD20090413'
union
select NZD_Seed.Seed_Category a from dbo.NZD_Seed where dbo.NZD_Seed.id = '13'