Calculated Field Using Data From Another Table

问题

I have a table set up that needs to have a field that calculates the price of an order. To do so, I have a 3-letter code set up for each item, but the price of these items isn't included in that table, but another table. To calculate the price, I need to multiply the quantity of the item by the price of the item. So basically, how do I associate the 3-letter code with the price of the item in the other table. The 3 letter code field is also included in the table with the price of the item

回答1:

What you need is a join:

   SELECT table1.quantity * table2.price AS result
     FROM table1
LEFT JOIN table2
       ON table1.code = table2.code

Assuming table1 and table2 have the following fields (you might want to adjust as necessary):

• code: the 3-letter code
• quantity: the quantity of an item
• price: the price of the item

Also see here for more background information: SQL JOIN and different types of JOINs

来源：https://stackoverflow.com/questions/37507531/calculated-field-using-data-from-another-table