sql - 在这个例子中，WITH 语句做了什么？我正在尝试随机生成数据

Question

INSERT INTO files (fileUID, filename)
WITH fileUIDS(fileUID) AS
( VALUES(1) UNION ALL
  SELECT fileUID+1 FROM fileUIDS WHERE fileUID < 1000 )
SELECT fileUID,
       TRANSLATE ( CHAR(BIGINT(RAND() * 10000000000 )), 'abcdefgHij', '1234567890' )
FROM fileUIDS;

Answer 1

WITH 语法与使用本地临时表或内联 View 相同。据我所知，它仅在 SQL Server(2005+，称为通用表表达式)和 Oracle(9i+，称为子查询分解)中受支持。预期用途是创建在单个查询中多次使用(即连接)的基本 View 。

这是一个典型的例子:

WITH example AS (
     SELECT q.question_id,
            t.tag_name
       FROM QUESTIONS q
       JOIN QUESTION_TAG_XREF qtf ON qtf.question_id = t.question_id
       JOIN TAGS t ON t.tag_id = qtf.tag_id)
SELECT t.title,
       e1.tag_name
  FROM QUESTIONS t
  JOIN example e1 ON e1.question_id = t.question_id

...如果您使用以下命令，将返回相同的结果:

SELECT t.title,
       e1.tag_name
  FROM QUESTIONS t
  JOIN (SELECT q.question_id,
               t.tag_name
          FROM QUESTIONS q
          JOIN QUESTION_TAG_XREF qtf ON qtf.question_id = t.question_id
          JOIN TAGS t ON t.tag_id = qtf.tag_id) e1 ON e1.question_id = t.question_id

您提供的示例:

WITH fileUIDS(fileUID) AS ( 
     VALUES(1) 
     UNION ALL
     SELECT t.fileUID+1 
       FROM fileUIDS t
      WHERE t.fileUID < 1000 )
INSERT INTO files 
    (fileUID, filename)
SELECT f.fileUID,
       TRANSLATE ( CHAR(BIGINT(RAND() * 10000000000 )), 'abcdefgHij', '1234567890' )
  FROM fileUIDS f;

...是递归的。它从 1 开始，总共生成 999 个 fileuid(如果从 0 开始，则生成 1,000 个)。