同一个别名使用两次时，后续引用将指向哪一个？

Question

有一个查询使用相同的别名（但在不同的条件下）两次连接同一个表。随后将引用这两个数据集中的哪一个别名点？

SELECT ...
  FROM table_1
  JOIN table_2 A ON ...
  JOIN table_3 A ON ...
  JOIN table_4 B ON A.col_1 = B.col_1

我使用以下查询进行了一个简单的实验：

  WITH
    test_1 AS (SELECT 1 AS col_1 FROM dual UNION ALL SELECT 2 AS col_1 FROM dual)
  , test_2 AS (SELECT 2 AS col_1 FROM dual UNION ALL SELECT 3 AS col_1 FROM dual)
  , test_3 AS (SELECT 1 AS col_1 FROM dual UNION ALL SELECT 2 AS col_1 FROM dual)
  , test_4 AS (SELECT 1 AS col_1 FROM dual UNION ALL SELECT 2 AS col_1 FROM dual)
SELECT *
  FROM test_1 b
  LEFT JOIN test_2 a ON a.col_1 = b.col_1
  LEFT JOIN test_3 a ON a.col_1 = b.col_1
  LEFT JOIN test_4 c ON a.col_1 = c.col_1;

结果：

col_1	col_1	col_1	col_1
1	空	1	空
2	2	2	2

建议表 test_4 的联接与别名的第一次出现相关联。同时，所有聊天机器人都确信应该使用最后提到的别名。因此，需要一个“刻在石头上”的规则来证明错误修复的合理性（因为实验可能无法涵盖所有可能的行为场景）。那么，根据“规则”，Oracle中两次引用别名时应该使用哪个别名呢？

Answer 1

如果为列提供明确的标识符，那么表别名将引用与具有适当列标识符的表（或内联视图）相对应的别名。例如，如果您有示例数据：

CREATE TABLE test_1 (col_1) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL UNION ALL
SELECT 4 FROM DUAL;

CREATE TABLE test_2 (col_2) AS
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL;

CREATE TABLE test_3 (col_3) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 4 FROM DUAL;

CREATE TABLE test_4 (col_4) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL;

然后：

SELECT *
  FROM test_1 b
  LEFT JOIN test_2 a ON a.col_2 = b.col_1
  LEFT JOIN test_3 a ON a.col_3 = b.col_1
  LEFT JOIN test_4 c1 ON a.col_2 = c1.col_4
  LEFT JOIN test_4 c2 ON a.col_3 = c2.col_4;

等同于：

SELECT *
  FROM test_1 b
  LEFT JOIN test_2 a1 ON a1.col_2 = b.col_1
  LEFT JOIN test_3 a2 ON a2.col_3 = b.col_1
  LEFT JOIN test_4 c1 ON a1.col_2 = c1.col_4
  LEFT JOIN test_4 c2 ON a2.col_3 = c2.col_4;

并且两个输出：

COL_1	COL_2	COL_3	COL_4	COL_4
1	空	1	空	1
3	3	空	3	空
2	2	空	2	空
4	空	4	空	空

在这种情况下，使用的

a

别名取决于后续列标识符，并且

a.col_2

和

a.col_3

指不同的

a

别名。

---

如果您有不明确的列：

CREATE TABLE test_1 (col_1) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL UNION ALL
SELECT 4 FROM DUAL;

CREATE TABLE test_2 (col_1) AS
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL;

CREATE TABLE test_3 (col_1) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 4 FROM DUAL;

CREATE TABLE test_4 (col_1) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL;

然后查询：

SELECT *
  FROM test_1 b
  LEFT JOIN test_2 a ON a.col_1 = b.col_1
  LEFT JOIN test_3 a ON a.col_1 = b.col_1
  LEFT JOIN test_4 c ON a.col_1 = c.col_1;

等同于：

SELECT *
  FROM test_1 b
  LEFT JOIN test_2 a1 ON a1.col_1 = b.col_1
  LEFT JOIN test_3 a2 ON a2.col_1 = b.col_1
  LEFT JOIN test_4 c ON a1.col_1 = c.col_1;

并且两个输出：

COL_1	COL_1	COL_1	COL_1
2	2	空	2
3	3	空	3
4	空	4	空
1	空	1	空

其中

a.col_1

不明确，但似乎已解析为包含

a

列标识符的

col_1

表别名的第一个实例。

Oracle 小提琴：11gR2、18、21、23