Snowflake (LEFT JOIN) LATERAL：无法评估不支持的子查询类型

Question

Lateral Join

In a FROM clause, the LATERAL keyword allows an in-line view to reference columns from a table expression that precedes that in-line view.

A lateral join behaves more like a correlated subquery than like most JOINs.

让我们稍微调整文档中提供的代码：

CREATE TABLE departments (department_id INTEGER, name VARCHAR);
CREATE TABLE employees (employee_ID INTEGER, last_name VARCHAR,
                        department_ID INTEGER, project_names ARRAY);

INSERT INTO departments (department_ID, name) VALUES 
    (1, 'Engineering'), 
    (2, 'Support'),
    (3, 'HR');  -- adding new row

INSERT INTO employees (employee_ID, last_name, department_ID) VALUES 
    (101, 'Richards', 1),
    (102, 'Paulson',  1),
    (103, 'Johnson',  2);

查询：

SELECT * 
FROM departments AS d,
LATERAL (SELECT * FROM employees AS e 
         WHERE e.department_ID = d.department_ID 
         ORDER BY employee_id DESC LIMIT 1) AS iv2  -- adding ORDER BY ... LIMIT ...
ORDER BY employee_ID;

SQL compilation error: Unsupported subquery type cannot be evaluated

是的，我知道我可以用 ROW_NUMBER() 或其他方式重写此查询。

1) 为什么在这种特定情况下无法使用 TOP/LIMIT？

2) 有没有实现LEFT JOIN LATERAL/OUTER APPLY的语法？

我也希望能够获取结果集中的所有源行，即使 LATERAL 子查询没有为它们生成任何行。得到最终结果：

┌────────────────┬──────────────┬──────────────┬────────────┬────────────────┬───────────────┐
│ department_id  │    name      │ employee_id  │ last_name  │ department_id  │ project_names │
├────────────────┼──────────────┼──────────────┼────────────┼────────────────┼───────────────┤
│             1  │ Engineering  │ 102          │ Paulson    │ 1              │ null          │
│             2  │ Support      │ 103          │ Johnson    │ 2              │ null          │
│             3  │ HR           │ null         │ null       │ null           │ null          │
└────────────────┴──────────────┴──────────────┴────────────┴────────────────┴───────────────┘

db<>fiddle demo

Answer 1

因此，尽管我们之前已经讨论过您知道可以重写它，但这里是重写

WITH departments AS (
    SELECT * FROM VALUES
        (1, 'Engineering'), 
        (2, 'Support'),
        (3, 'HR')
        v(department_ID, name)
), employees AS (
    SELECT * FROM VALUES 
        (101, 'Richards', 1),
        (102, 'Paulson',  1),
        (103, 'Johnson',  2)
        v(employee_ID, last_name, department_ID)
), dep_emp AS (
  SELECT * 
  FROM employees 
  QUALIFY ROW_NUMBER() OVER (PARTITION BY department_ID ORDER BY employee_id) = 1
)
SELECT * 
FROM departments AS d
LEFT JOIN dep_emp AS e ON d.department_ID = e.department_ID
ORDER BY employee_ID;

如你所愿：

DEPARTMENT_ID    NAME           EMPLOYEE_ID    LAST_NAME    DEPARTMENT_ID
1                Engineering    101            Richards     1
2                Support        103            Johnson      2
3                HR             null           null         null

通过从 LATERAL 移动到具有 QUALIFY 的 CTE 来实现 LIMIT/TOP，然后使用 LEFT JOIN 来获取空匹配，您就有了所需的步骤。

对于为什么会这样的未问问题。 Snowflake 并不是真正的每行数据库，它更像是一个 Map/Reduce/MergeJoin 进程，它可以将简单的相关子查询重写为多步骤（也称为 CTE/joins），但它不能重写复杂的东西。他们一直在改进它。但是如果你了解你的数据和你的模型，那么在批量操作中表达事物并让 MergeJoin 的力量给你带来胜利是最有意义的。

Is there a syntax to achieve LEFT JOIN LATERAL/OUTER APPLY? this is done via parameter , OUTER => TRUE in the FLATTEN command

Snowflake (LEFT JOIN) LATERAL：无法评估不支持的子查询类型

Snowflake (LEFT JOIN) LATERAL: Unsupported subquery type cannot be evaluated

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