合并两个不同的表

Question

I have 3 tables. One contains details of items, Second contains selling details and the third one contains the purchase details. I want to create query which displays a ledger type result. I have tried it but the following query is giving a warning like: Current selection does not contain a unique column.

The result table will also contain a column to identify from which table it is picking the values. Like, it will put "P" for purchase and "S" for sale.

I have searched everything but I am not able to find a solution in my case. Here is the query:

SELECT date,
       itemcode,
       qty,
       billno,
       price,
       "p" AS SP
FROM   purchase
WHERE  itemcode = 1007
UNION ALL
SELECT date,
       itemcode,
       qty,
       billno,
       rate,
       "s"
FROM   billing
WHERE  itemcode = 1007
ORDER  BY date ASC 

Answer 1

SELECT date，itemcode，qty，billno，price，null AS rate，“ p” AS SP FROM Purchase WHERE itemcode = 1007 UNION ALL选择date，itemcode，qty，billno，null AS price，rate，“ s” AS SP FROM billing项目代码= 1007订单日期日期ASC