相关子查询中没有这样的列

Question

所以我写了一个与 this answer 中描述的查询几乎相同的查询。但是我收到一条错误消息：没有这样的列 b.when.

我的select声明：

SELECT
    mileage,
    (SELECT b.mileage FROM MileageEvents as b WHERE `b.when` < `a.when` ORDER BY `b.when` DESC LIMIT 1) as last_mileage,
    gallons,
    cost_per_gallon,
    `when`
FROM MileageEvents as a

我知道我以前写过这样的查询，但我似乎无法弄清楚这个查询是怎么回事。我做错了什么？

我的数据库转储：

PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE IF NOT EXISTS "MileageEvents" (
    "mileage"   INTEGER,
    "when"  TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    "cost_per_gallon"   INTEGER,
    "gallons"   INTEGER,
    "id"    INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
    "car_id"    INTEGER NOT NULL
);
INSERT INTO MileageEvents VALUES(150000,'2019-03-12',3.3500000000000000888,12,1,1);
INSERT INTO MileageEvents VALUES(150300,'2019-03-19',3.25,12,2,1);
INSERT INTO MileageEvents VALUES(150693,'2019-03-22',3.4500000000000001776,12,3,1);
INSERT INTO MileageEvents VALUES(151000,'2019-03-25',3.3900000000000001243,12,4,1);
INSERT INTO MileageEvents VALUES(151600,'2019-04-01',2.25,12,5,1);
INSERT INTO MileageEvents VALUES(151883,'2019-06-10 23:01:43',2.4500000000000001776,11.695999999999999729,6,1);
DELETE FROM sqlite_sequence;
INSERT INTO sqlite_sequence VALUES('MileageEvents',6);
COMMIT;

Answer 1

（首先，我会使用正常的双引号转义而不是 MySQL 样式的反引号，或者更好的是，一个不是保留关键字的标识符）

您需要使用b."when"。您所做的是将整个字符串 b.when 视为标识符，table 名称和列名称之间没有分隔。

重写：

SELECT
    mileage,
    (SELECT b.mileage FROM MileageEvents as b WHERE b."when" < a."when" ORDER BY b."when" DESC LIMIT 1) as last_mileage,
    gallons,
    cost_per_gallon,
    "when"
FROM MileageEvents as a;

注意：如果使用 sqlite 3.25 或更新版本，您可以完全避免子查询：

SELECT
    mileage,
    lag(mileage) OVER (ORDER BY "when") AS last_mileage,
    gallons,
    cost_per_gallon,
    "when"
FROM MileageEvents;

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