比较两个不同表中的列

Question

I am using C#, MySQL to monitor some fuel tanks I'm currently using a function to monitor the tanks that assigns one of three statuses to each tank (good, warning, critical).

One of the things this program has to do is send an email when the tank status changes. What I need help with is how to compare the tank status from 2 different table to determine if the status has changed.

Here are the two tables I'm using:

tank table

tank_number INT,
station_id int, 
ip_address varchar(30),
product varchar(30),
alert_level1 int,
alert_level2 int, 
gallons int,
date,
status

status table

ip_address varchar(30),
status varchar(5),
gallons int,
tank_number int,
date

Answer 1

This should just be a simple join on the two tables. Something along the lines of:

SELECT s.status ,
       t.tank_number
  FROM status s INNER JOIN tank t ON s.tank_number = t.tank_number
 WHERE s.status <> t.status

This assumes that tank_number is a unique identifier for the tank and retrieves just those tanks whose status has changed.