尝试此查询

SELECT full_name FROM user AS U 
left join approver as A on U.id = a.subordinate_id 
where A.id is null AND U.is_admin = 0

您需要将AND U.is_admin = 0放在EXISTS之外

create table `user`(
 id int,
 full_name varchar(20),
 is_admin int
);

create table `approver`(
 id int,
 subordinate_id int,
 approver_id int
);

insert into `user`(id,full_name,is_admin) values
(1,'jane',0),
(2,'Helio',0),
(3,'fran',0),
(4,'mila',0),
(5,'admin',1);

insert into `approver`(id,subordinate_id,approver_id) values
(1,1,2),
(2,3,4);

SELECT 
    full_name
FROM user AS U 
 WHERE NOT EXISTS(SELECT A.* FROM approver AS A WHERE A.subordinate_id = U.id )
AND U.is_admin = 0

DB Fiddle Demo

你可以尝试这个查询。我们可以在第一个表中选择admin=“0”，然后在第二个表中进行左连接。在这之后，我们可以进行内部连接的逆操作，以获得你所期望的结果。

SELECT fullname
FROM
(SELECT * FROM user WHERE is_admin="0") U
LEFT JOIN Approver A
ON U.id=A.subordinate_id
WHERE A.subordinate_id IS NULL;