MySQL 8.0

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-- Structure
CREATE TABLE `users` (
    `uid` INT(11) NOT NULL,
    `uname` TINYTEXT NOT NULL,
    PRIMARY KEY (`uid`)
);
CREATE TABLE `groups` (
    `gid` INT(11) NOT NULL,
    `gname` TINYTEXT NOT NULL,
    PRIMARY KEY (`gid`)
);
CREATE TABLE `entities` (
    `eid` INT(11) NOT NULL,
    `ename` TINYTEXT NOT NULL,
    PRIMARY KEY (`eid`)
);
CREATE TABLE `entity_user_access` (
    `eid` INT(11) NOT NULL,
    `uid` INT(11) NOT NULL,
    PRIMARY KEY (`eid`, `uid`),
    CONSTRAINT `FK_eua_entity` FOREIGN KEY (`eid`) REFERENCES `entities` (`eid`),
    CONSTRAINT `FK_eua_user` FOREIGN KEY (`uid`) REFERENCES `users` (`uid`)
);
CREATE TABLE `entity_group_access` (
    `eid` INT(11) NOT NULL,
    `gid` INT(11) NOT NULL,
    PRIMARY KEY (`eid`, `gid`),
    CONSTRAINT `FK_ega_entity` FOREIGN KEY (`eid`) REFERENCES `entities` (`eid`),
    CONSTRAINT `FK_ega_group` FOREIGN KEY (`gid`) REFERENCES `groups` (`gid`)
);
CREATE TABLE `group_members` (
    `gid` INT(11) NOT NULL,
    `uid` INT(11) NOT NULL,
    PRIMARY KEY (`gid`, `uid`),
    CONSTRAINT `FK_gm_group` FOREIGN KEY (`gid`) REFERENCES `groups` (`gid`),
    CONSTRAINT `FK_gm_user` FOREIGN KEY (`uid`) REFERENCES `users` (`uid`)
);
CREATE TABLE `group_of_groups` (
    `gid` INT(11) NOT NULL,
    `parent_gid` INT(11) NOT NULL,
    PRIMARY KEY (`gid`, `parent_gid`),
    CONSTRAINT `FK_gog_group` FOREIGN KEY (`gid`) REFERENCES `groups` (`gid`),
    CONSTRAINT `FK_gog_parent_group` FOREIGN KEY (`parent_gid`) REFERENCES `groups` (`gid`)
);

-- Data
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
INSERT INTO `users` VALUES
    (101, 'Alice'),
    (102, 'Bob'),
    (103, 'Charlie');
INSERT INTO `groups` VALUES
    (201, 'Administrators'),
    (202, 'Users'),
    (203, 'Operators'),
    (204, 'Guests'),
    (205, 'X-Mas event');
INSERT INTO `entities` VALUES
    (301, 'Foo'),
    (302, 'Bar'),
    (303, 'Qux'),
    (304, 'Snow');
INSERT INTO `entity_user_access` VALUES
    (301, 101),
    (301, 102),
    (302, 103),
    (303, 103);
INSERT INTO `entity_group_access` VALUES
    (301, 201),
    (302, 201),
    (302, 203),
    (303, 201),
    (304, 201),
    (304, 205);
INSERT INTO `group_members` VALUES
    (201, 102),
    (202, 103),
    (203, 102),
    (204, 101);
INSERT INTO `group_of_groups` VALUES
    (201, 203),
    (202, 205),
    (204, 205);
/*!40014 SET FOREIGN_KEY_CHECKS=IFNULL(@OLD_FOREIGN_KEY_CHECKS, 1) */;
SET @TEST_EID = 302;
SET @TEST_UID = 103;
SELECT COUNT(*)
FROM
(    SELECT
        DISTINCT
        entities.eid,
        entity_user_access.uid
    FROM
        entity_user_access
        JOIN entities
            ON entity_user_access.eid = entities.eid
UNION
    SELECT
        DISTINCT
        entities.eid,
        users.uid
    FROM
        `groups`
        JOIN group_members
            ON group_members.gid = groups.gid
        JOIN users
            ON users.uid = group_members.uid
         JOIN 
            entity_group_access
            ON groups.gid = entity_group_access.gid
         JOIN entities
            ON entity_group_access.eid = entities.eid) t1
WHERE eid = @TEST_EID and uid = @TEST_UID


COUNT(*)


1