来自另一个数据库的MySQL子查询，其中表名取决于主查询

Question

Hi I am trying to sub query a table(name: table_guest_id) from another database where id from the table name depends on the main query. Here is the set up of my databases:

• Three tables: events, profile, table_guest_id
• Two databases: main and guest(dbName below just to make it clear)
• main contains: events and profile
• guest contain: table_guest_id(where id = e.ID)

And this is my sql code:

    public function getListAttendee($user)
    {
        $sql = "SELECT p.name e.user, e.ID, e.date 
                FROM events e, profile p
                WHERE p.user_id=e.user AND e.event_date >= CURDATE() 
                AND (SELECT COUNT(*) 
                     FROM dbName.table_guest_[??note: need to insert e.ID??] a 
                     WHERE a.event_id=e.ID AND a.user_id='{$user}' AND
                     a.status='invited') > 0";
     }

I have put a note where ID OR e.ID will be inserted, but it is not working yet. Please let me know if you have a different solution on how to sub query a table where the name of the table depends on the main query values. IS THIS IDEA EVEN POSSIBLE?

Answer 1

It seems you are trying to perform variable substitution on a SQL table name.

  FROM dbName.table_guest_[??note: need to insert e.ID??] a 

You Can't Do That™ directly in SQL. You'll need to write your php code to generate that table name, or use the string-processing feature that the MySQL team calls prepared statements.

If your different databases are hosted on the same MySQL server, you can write queries that refer to more than one of them at a time. Simply give the database name as well as the table name. For example, if you have databases db1 and db2 you can do this.

 SELECT whatever
   FROM db1.events e
   JOIN db2.user_118 u ON whatever = whatever