如何在使用INNER JOIN时让MySQL选择随机获胜者

Question

SELECT * FROM `surveys` INNER JOIN `surveyusers` ON `surveyusers`.`survID` = `surveys`.`survID` WHERE `surveyusers`.`hasWon` = 0 ORDER BY RAND();
UPDATE surveys SET winner = email;

This was a school assignment that required us to select and update the survey winner.

I use PHP to manage and show surveys, I want the SQL to pick the winner. PHP sets survID

I want to pick a random winner and set the winner coloumn in surveys table to the email in the surveyusers table. But I really have a lot of trouble with it.

Answer 1

Updates can take the same syntax as a Select.

You can combine these two queries into one.

UPDATE `surveys` `s` SET `s`.`winner` = (
    SELECT email FROM `surveyusers`
    WHERE `surveyusers`.`hasWon` = 0
    AND `survID` = 1
    ORDER BY RAND()
    LIMIT 1
) WHERE `survID` = 1;

The survID you would have to set from PHP of course.

Answer 2

You can nest a SELECT query within your UPDATE query:

UPDATE surveys SET winner = (
    SELECT `surveyusers`.email FROM `surveys` 
    INNER JOIN `surveyusers` ON `surveyusers`.`survID` = `surveys`.`survID` 
    WHERE `surveyusers`.`hasWon` = 0 ORDER BY RAND() LIMIT 1
) WHERE `survID` = xxx

where xxx is the survey to update.