MySQL查询：存在先前值时，使用表中的先前值。 当表中没有以前的值时需要修复

Question

$query = "INSERT INTO temperature (city, change, curr_temp) SELECT  city, '".$cleaned_change."',  curr_temp+'".$cleaned_change."' FROM temperature where city='".$chosen_city."' order by Time desc, id desc limit 1";

So, each row displays city, changes of temperature and current temperature. If we have $cleaned_change=2 (Forengheits) change for San Francisco and its curr_temp was 47F (taken from the previous row using SELECT), we insert ('San Francisco', '2', '49'). (was 47F, change is +2F, now it is 49F).

The problem is: if that's the first row it doesn't work, because we have no 47F to be selected (there are no previous values). As result, it doesn't insert a row. How to fix it?

The good idea is to insert the very first row "manually" (with a temperature of 0F or 32F or whatever). And that works fine. but is there any possible way to check if curr_temp has a previous value? I want it work from the scratch (when the table is truncated, it stops working) and using just one query.

PS There are NO nulls. If row exists, curr_temp has a numeric value in the Table. Thank you.

PSS I forgot to notice that I have slightly simplified the query. Yes, it cleans the POST values from possible injections. But, thank you for saying that.

Answer 1

Try making just a SELECT query, then do your PHP logic, then use an INSERT query. Then you can build in protection against SQL injection and have a simpler time with your logic.