MySQL - 如果为空，则选择默认值

Question

Here's my query that I'm working with:

SELECT (COUNT(`pass` = 0) / COUNT((`pass` = 0) + (`pass` = 1))) AS `PassRate` from `quiz_results`

Where I'm trying to get a percentage of people who have passed the quiz, however the issue is that if nobody as passed the quiz, then NULL is returned. I'm trying to figure out how to return 0 in the event of NULL .

I've tried using ISNULL however I didn't have any luck.

The other issue I'm running into is that if nobody has failed the quiz, the value is always zero. So my two problems are as follows:

• Convert NULL to 0.
• Return 100 when COUNT( pass =0) returns 0.

Unfortunately I'm not all that great with SQL and I'm a bit stumped. I'm accessing MySQL through PHP. I added that in here, because I don't know if it changes anything.

I'm using The SQL installed with XAMPP (Comes with PhpMyAdmin)

Answer 1

First, you don't want COUNT() , you want SUM() :

SELECT (SUM(`pass` = 0) / SUM((`pass` = 0) + (`pass` = 1))) AS `PassRate` 
FROM `quiz_results`

The expressions COUNT(pass = 0) and COUNT(pass = 1) always return exactly the same number, and this is equivalent to COUNT(pass) . COUNT() is just looking at whether or not the expression is NULL .

Then, you can replace this with AVG() , probably:

SELECT AVG(pass = 0)  -- Should this be `pass = 1` ?
FROM quiz_results;

And, if you still get unwanted NULL values:

SELECT COALESCE(AVG(pass = 0), 0)  -- Should this be `pass = 1` ?
FROM quiz_results;