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)
SELECT q1.ID, Content, Volume, CustomerID, runtot
FROM (
  SELECT ID, Content, Volume, CustomerID,
    SUM(Volume) OVER (PARTITION BY CustomerID ORDER BY ID) AS runtot,
    ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY ID) AS rnum
  FROM multiqueue
  WHERE PublishedTS IS NULL
) AS q1
JOIN customers AS c1 ON q1.CustomerID=c1.ID
WHERE
  runtot < 2000 * Priority / (SELECT SUM(Priority) FROM customers WHERE ID = CustomerID)
  OR rnum <= 1


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

5
Content5
500
1
1500

6
Content6
100
2
100

7
Content7
200
2
300

8
Content8
300
2
600

9
Content9
400
2
1000

10
Content10
500
2
1500


SELECT q1.ID, Content, Volume, CustomerID, runtot
FROM (
  SELECT ID, Content, Volume, CustomerID,
    SUM(Volume) OVER (PARTITION BY CustomerID ORDER BY ID) AS runtot,
    ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY ID) AS rnum
  FROM multiqueue
  WHERE PublishedTS IS NULL
) AS q1
JOIN customers AS c1 ON q1.CustomerID=c1.ID
WHERE
  runtot < 2000 * Priority / (SELECT SUM(Priority) FROM customers)
  OR rnum <= 1


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
5	Content5	500	1	1500
6	Content6	100	2	100
7	Content7	200	2	300
8	Content8	300	2	600
9	Content9	400	2	1000
10	Content10	500	2	1500