MariaDB 10.4

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CREATE TABLE `multiqueue` (
    `ID` BIGINT(20) NOT NULL AUTO_INCREMENT,
    `CustomerID` BIGINT(20) NOT NULL,
    `Volume` INT(11) NOT NULL,
    `Content` MEDIUMTEXT NOT NULL COLLATE 'utf8mb4_unicode_ci',
    `PublishedTS` DATETIME NULL DEFAULT NULL,
    PRIMARY KEY (`ID`) USING BTREE
)
COLLATE='utf8mb4_unicode_ci'
ENGINE=InnoDB;
CREATE TABLE `customers` (
    `ID` BIGINT(20) NOT NULL AUTO_INCREMENT,
    `Priority` DOUBLE NOT NULL DEFAULT '1000',
    PRIMARY KEY (`ID`) USING BTREE
)
COLLATE='utf8mb4_unicode_ci'
ENGINE=InnoDB;
INSERT INTO multiqueue VALUES
  (1, 1, 100, 'Content1', NULL),
  (2, 1, 200, 'Content2', NULL),
  (3, 1, 300, 'Content3', NULL),
  (4, 1, 400, 'Content4', NULL),
  (5, 1, 500, 'Content5', NULL),
  (6, 2, 100, 'Content6', NULL),
  (7, 2, 200, 'Content7', NULL),
  (8, 2, 300, 'Content8', NULL),
  (9, 2, 400, 'Content9', NULL),
  (10, 2, 500, 'Content10', NULL),
  (11, 1, 600, 'Content11', NULL)
;
INSERT INTO customers VALUES
  (1, 1000),
  (2, 500),
  (3, 100000)
;
SELECT
  q.ID,
  q.Content,
  q.Volume,
  q.CustomerID,
  q.runtot
FROM
  (
    SELECT
      ID, Content, Volume, CustomerID,
      SUM(Volume)  OVER (PARTITION BY CustomerID ORDER BY ID ASC) AS runtot,
      ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY ID ASC) AS rnum
    FROM
      multiqueue
    WHERE
      PublishedTS IS NULL
  ) AS q
  INNER JOIN
  (
    SELECT
      ID, Priority,
      SUM(Priority) OVER () AS TotalPriority
    FROM
      customers AS c
    WHERE
      EXISTS
      (
        SELECT * FROM multiqueue AS q WHERE q.PublishedTS IS NULL AND q.CustomerID = c.ID
      )
  ) AS c ON q.CustomerID = c.ID
WHERE
  q.runtot < 2000 * c.Priority / c.TotalPriority
  OR q.rnum <= 1
ORDER BY
  q.ID ASC
;


ID
Content
Volume
CustomerID
runtot


1
Content1
100
1
100

2
Content2
200
1
300

3
Content3
300
1
600

4
Content4
400
1
1000

6
Content6
100
2
100

7
Content7
200
2
300

8
Content8
300
2
600

ID	Content	Volume	CustomerID	runtot
1	Content1	100	1	100
2	Content2	200	1	300
3	Content3	300	1	600
4	Content4	400	1	1000
6	Content6	100	2	100
7	Content7	200	2	300
8	Content8	300	2	600