MySql 使用带更新的子查询

Question

我有这张 table 的病人，必须查询它

Update the database in order to provide a 3% discount to all patients in a room that has more than 2 patients.

表格如下:

CREATE TABLE patient (
    sin varchar(20) NOT NULL,
    disease varchar (20),
    bed varchar (20),
    room varchar (20),
    hospital varchar (0),
    fee varchar(20),
    entry_date date NOT NULL,
    exit_date date,
    CONSTRAINT FOREIGN KEY (sin) REFERENCES person(sin)
) 

所以我想找到所有有超过2名患者的病房，然后更新表格:

UPDATE patient C
INNER JOIN patient D ON C.sin=D.sin and D.sin IN (SELECT A.sin
                            FROM patient A
                            WHERE 2 < (SELECT COUNT(B.sin)
                                        FROM patient B
                                        WHERE A.hospital=B.hospital and A.room=B.room and A.exit_date IS NULL and B.exit_date IS NULL)
    )
SET C.fee=C.fee*0.97

问题是我收到错误:

You can't specify target table 'C' for update in FROM clause

有没有办法将子查询与更新一起使用？ 非常感谢。

Answer 1

您走在正确的道路上。 但是您想要加入房间信息，而不是患者信息。所以:

UPDATE patient p JOIN
       (select hospital, room
        from patient
        where exit_date is null
        group by hospital, room
        having count(*) > 2
       ) r
       ON p.hospital = r.hospital and p.room = r.room
    SET p.fee = p.fee * (1 - 0.03)
    WHERE exit_date is null;