MYSQL根据值（如果存在）对SELECT行进行查询，否则从另一个表返回数据

Question

I have two tables:

CATALOG:
------------------------------------
| itemID    | itemCode  | itemName |
------------------------------------

ORDER:
----------------------------------------------
| orderID   | itemID_FK | itemPrice | etc... |
----------------------------------------------

I want to select itemName , itemPrice , itemCode etc by using a join on both tables.

So far so good; however, the row corresponding to a given itemID in CATALOG may not exist in the ORDER table. In which case, I want to fall back on using itemName , itemCode from the first table CATALOG .

Is this possible using MYSQL in a single (compound) statement?

EDIT: Here's a sample table

CATALOG:
------------------------------------
| itemID    | itemCode  | itemName |
------------------------------------
|  1        |  89873232 | Oats     |
|  2        |  32849392 | Beer     |
------------------------------------

ORDER:
----------------------------------------------
| orderID   | itemID_FK | itemPrice | etc... |
----------------------------------------------
| 213232    |  2        |  3.99     |  ...   |
----------------------------------------------

I use the following query to retrieve complete data about an item:

SELECT itemID, itemName, itemPrice FROM CATALOG
INNER JOIN ORDER
ON itemID = itemID_FK
WHERE itemID = %d

I need this join because CATALOG will not contain price which is obtained from ORDER . If I use itemID=1 there would not be a corresponding entry in ORDER table and the query would fail. I want it to retrieve at least itemName if there is no entry for the item in ORDER

Answer 1

Just use a LEFT OUTER JOIN between the two tables, so you will always have data corresponding to the table on the LEFT side of the JOIN:

SELECT C.itemName, C.itemCode, O.itemPrice, ...
FROM CATALOG as C
     LEFT OUTER JOIN ORDER AS O ON C.itemID = O.itemID_FK

All the Os in the above selection will turn up as NULL if the item from the CATALOG does not exist in ORDERS .