MySQL加入和条件

Question

I am using this SQL code to select random entries from a question table.

I would like to add a condition to this query. Basically the table has a user id record to keep track of who added the question. This id record shall now be used on conjunction with the Join Query.

Basically: Select random question entry AND verify it belongs to the given user id.

SELECT *
  FROM Questions JOIN
       (SELECT CEIL(RAND() *
                    (SELECT MAX(Question_ID)
                       FROM Questions )) AS Question_ID
        ) AS r2
       USING (Question_ID) ;

I tried adding the condition like this:

And UID='$user_ID'

However I am getting a blank page as a result so I guess by adding the condition like this, I am messing up the entire query. Basically, I can't figure up how to put the condition...

Answer 1

SELECT *
  FROM Questions JOIN
       (SELECT CEIL(RAND() *
                    (SELECT MAX(Question_ID)
                       FROM Questions WHERE UID='$user_ID')) AS Question_ID
        ) AS r2
       USING (Question_ID) ;

Answer 2

The '' quotes in PHP denotes a literal string whereas "" input variables, they're not interchangeable. If using '' 's you can input variables by doing '{$var}' .

If your table is very large it would be slow to run a RAND() over it and would be better to pass a random id generated in PHP.

Random from PHP:

SELECT *
FROM Questions as q
WHERE q.UID = $user_ID AND q.ID = $randomID;\

Random from SQL:

SELECT *
FROM Questions as q
WHERE q.UID = $user_ID
ORDER BY RAND()
LIMIT 1;