MySQL从多个表中选择

Question

I'm new to MySQL. I am creating a checkout page in PHP. When the users select the items they want to buy and click "Add to Cart", a temporary table gets created which has the following fields (table name is temp):

+--------------+-----------+------+-----+-------------------+----------------+
| Field        | Type      | Null | Key | Default           | Extra          |
+--------------+-----------+------+-----+-------------------+----------------+
| Cart_Item_ID | int(11)   | NO   | PRI | NULL              | auto_increment |
| Item_ID      | int(11)   | NO   |     |                   |                |
| Added_On     | timestamp | YES  |     | CURRENT_TIMESTAMP |                |
+--------------+-----------+------+-----+-------------------+----------------+

I'm only inserting to the Item_ID field which contains the ID of each item they bought (I'm populating the forms with item IDs). What I want to do is look up the item's name and price that's stored in the Inventory table. Here's how that looks:

+--------------+----------------+------+-----+-------------------+----------------+
| Field        | Type           | Null | Key | Default           | Extra          |
+--------------+----------------+------+-----+-------------------+----------------+
| Inventory_ID | int(11)        | NO   | PRI | NULL              | auto_increment |
| Item_Name    | varchar(40)    | NO   |     |                   |                |
| Item_Price   | float unsigned | NO   |     | 0                 |                |
| Added_On     | timestamp      | YES  |     | CURRENT_TIMESTAMP |                |
+--------------+----------------+------+-----+-------------------+----------------+

So how would I pull out the Item_name and Item_Price fields from the Inventory table based on the Item_ID field from the temp table so I can display it on the page? I just don't understand how to formulate the query. I'd appreciate any help. Thank you.

Answer 1

It's called JOIN - read more here

SELECT Inventory.Item_Name, Inventory.Item_Price 
FROM Inventory, temp WHERE Inventory.Inventory_ID = temp.Item_ID

Answer 2

what i understand is that the Item_ID in temp table is referencing to the Inventory_ID in inventory table. based on this assumption you can use the following query.

Select Item_Name, Item_Price from Inventory, Temp where Temp.Item_ID == Inventory.Inventory_ID

i guess this is what you want to do.

Thanks

Answer 3

As it stands, you can't (unless Inventory_ID = Item_ID)

What you need is a way of JOINing the two tables together. In this instance, if Inventory_ID = Item_ID then the following is possible:

SELECT      Item_Name,
            Item_Price

FROM        InventoryTable
INNER JOIN  TempItemTable ON (InventoryTable.Inventory_ID = ItemTable.Item_ID)

If you want to filter for a particular item you can add the constraint:

WHERE ItemTable.Item_ID = 27 --for example

That will join all the rows in your inventory table with matching rows in the Item table.

Jeff Atwood has a great (IMO) visual explanation of how JOINs work.