How to specify the parent query field from within a subquery in mySQL?

Question

Is there a way to specify the parent query field from within a subquery in mySQL?

For Example:
I have written a basic Bulletin Board type progra

Answer 1

Parent query field within a subquery in MySQL 8.

I'm selecing games scores on the basis of username from tblgamescores using nested query.

SELECT 
    GameScoresID, 
    (SELECT Username FROM tblaccounts WHERE AccountID = FromAccountID) AS FromUsername, 
    (SELECT Username FROM tblaccounts WHERE AccountID = ToAccountID) AS ToUsername,
    (SELECT Username FROM tblaccounts WHERE AccountID = WinAccountID) AS WinUsername,
    (SELECT Username FROM tblaccounts WHERE AccountID = LossAccountID) AS LossUsername,
    FromUserScore,
    ToUserScore 
FROM tblgamescores a 
WHERE FromAccountID = (SELECT AccountID FROM tblaccounts WHERE Username = "MHamzaRajput");

Answer 2

Thanks Don. I had a nested query as shown below and a WHERE clause in it wasn't able to determine alias v1. Here is the code which isn't working:

Select 
    teamid,
    teamname
FROM
    team as t1
INNER JOIN (
    SELECT 
        venue_id, 
        venue_scores, 
        venue_name 
    FROM venue 
    WHERE venue_scores = (
        SELECT 
            MAX(venue_scores) 
        FROM venue as v2 
        WHERE v2.venue_id = v1.venue_id      /* this where clause wasn't working */
    ) as v1    /* v1 alias already present here */
);

So, I just added the alias v1 again inside the JOIN. Which made it work.

Select 
    teamid,
    teamname
FROM
    team as t1
INNER JOIN (
    SELECT 
        venue_id, 
        venue_scores, 
        venue_name 
    FROM venue as v1              /* added alias v1 here again */
    WHERE venue_scores = (
        SELECT 
            MAX(venue_scores) 
        FROM venue as v2 
        WHERE v2.venue_id = v1.venue_id   /* Now this works!! */
    ) as v1     /* v1 alias already present here */
);

Hope this will be helpful for someone.

Answer 3

You could try something like this

SELECT  pt.id,
        CountTable.Cnt
FROM    post_table pt LEFT JOIN
        (
            SELECT  parent_id,
                    COUNT(1) Cnt
            FROM    post_table
            WHERE   parent_id <> 0
            GROUP BY parent_id
        ) CountTable ON pt.id = CountTable.parent_id
WHERE   pt.parent_id = 0

To get back to your example, use the alias of the main table in the sub select

SELECT  pt.id,
        (SELECT COUNT(1) FROM post_table WHERE parent_id = pt.id) 
FROM    post_table pt
WHERE   pt.parent_id = 0

Answer 4

The following syntax works in Oracle. Can you test if the same works in MYSQL too? It is called scalar subquery in Oracle.

You would just need to alias the two tables differently to distinguish between them if you are using the same table twice.

sql> select empno,
  2         (select dname from dept where deptno = emp.deptno) dname
  3    from emp 
  4    where empno = 7369;

     EMPNO DNAME
---------- --------------
      7369 RESEARCH

sql> select parent.empno,
  2         (select mgr from emp where empno = parent.empno) mgr
  3    from emp parent
  4    where empno = 7876;

     EMPNO        MGR
---------- ----------
      7876       7788

Answer 5

How about:

$query = "SELECT p1.id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";

or if you put an alias on the p1.id, you might say:

$query = "SELECT p1.id as p1_id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";

Answer 6

Give the tables unique names:

$query = "SELECT a.id, (SELECT COUNT(1) FROM post_table b WHERE parent_id = a.id) as num_children FROM post_table a WHERE a.parent_id = 0";