MySQL:使用CASE/ELSE值作为连接参数

Question

我正在尝试根据谁是付款人或收款人,将USERS表中的NAME和PHOTO加入TRANSACTIONS表.它一直告诉我找不到桌子this- 我做错了什么？

SELECT `name`,`photo`,`amount`,`comment`,
(
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
) AS `this`
FROM `transactions`
RIGHT JOIN `users` ON (`users`.`id`=`this`)
WHERE `payee_id`=72823 OR `payer_id`=72823

Answer 1

从有关别名的文档:

别名用作表达式的列名,可用于GROUP BY,ORDER BY或HAVING子句.

您不能在联接中使用别名.您只能在上面列出的位置使用它.原因是别名位于连接结果中的字段上.如果允许连接在其定义中使用这些别名,它将(或可能)导致递归定义.

要解决您的问题,您可以CASE在两个地方重复该条款:

SELECT `name`,`photo`,`amount`,`comment`,
(
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
) AS `this`
FROM `transactions`
RIGHT JOIN `users` ON `users`.`id`= (
    CASE `payer_id`
    WHEN 72823 THEN `payee_id`
    ELSE `payer_id`
    END
)
WHERE `payee_id`=72823 OR `payer_id`=72823

但是,我可能会将此查询重写为两个选择和UNION它们:

SELECT name, photo, amount, comment, payer_id AS this
FROM transactions
JOIN users ON users.id = payer_id
WHERE payee_id = 72823
UNION ALL
SELECT name, photo, amount, comment, payee_id AS this
FROM transactions
JOIN users ON users.id = payee_id
WHERE payer_id = 72823

结果:

'name3', 'photo3', 30, 'comment3', 3
'name1', 'photo1', 10, 'comment1', 1
'name2', 'photo2', 20, 'comment2', 2

测试数据:

CREATE TABLE users (id INT NOT NULL, name NVARCHAR(100) NOT NULL, photo NVARCHAR(100) NOT NULL);
INSERT INTO users (id, name, photo) VALUES
(1, 'name1', 'photo1'),
(2, 'name2', 'photo2'),
(3, 'name3', 'photo3'),
(4, 'name4', 'photo4');

CREATE TABLE transactions (amount INT NOT NULL, comment NVARCHAR(100) NOT NULL, payer_id INT NOT NULL, payee_id INT NOT NULL);
INSERT INTO transactions (amount, comment, payer_id, payee_id) VALUES
(10, 'comment1', 72823, 1),
(20, 'comment2', 72823, 2),
(30, 'comment3', 3, 72823),
(40, 'comment4', 4, 5);