如何CONNECT BY被执行的查询和评估-步步（通过实施例）。

假设我们有下表和按查询连接：

select * from mytable;

         X
----------
         1 
         2 
         3 
         4

SELECT level, m.* 
FROM mytable m
START with x = 1
CONNECT BY PRIOR x +1 = x  OR  PRIOR x + 2 = x 
ORDER BY level;

步骤1：

从表mytable中选择满足START WITH条件的行，将LEVEL = 1分配给返回的结果集：

 CREATE TABLE step1 AS
 SELECT 1 "LEVEL", X from mytable
 WHERE x = 1;

 SELECT * FROM step1;

         LEVEL          X
    ---------- ----------
             1          1

第2步

等级提高1：

LEVEL = LEVEL + 1

mytable使用CONNECT BY条件作为联接条件联接上一步返回的结果集。

此子句PRIOR column-name中指的是上一步返回的结果集，而简单column-name指的是mytable表：

CREATE TABLE step2 AS
SELECT 2 "LEVEL", mytable.X from mytable
JOIN step1 "PRIOR"
ON "PRIOR".x +1 = mytable.x or  "PRIOR".x + 2 = mytable.x;

select * from step2;

     LEVEL          X
---------- ----------
         2          2 
         2          3

步骤x + 1

重复＃2，直到最后一个操作返回空结果集。

第三步

CREATE TABLE step3 AS
SELECT 3 "LEVEL", mytable.X from mytable
JOIN step2 "PRIOR"
ON "PRIOR".x +1 = mytable.x or  "PRIOR".x + 2 = mytable.x;

select * from step3;

     LEVEL          X
---------- ----------
         3          3 
         3          4 
         3          4

第4步

CREATE TABLE step4 AS
SELECT 4 "LEVEL", mytable.X from mytable
JOIN step3 "PRIOR"
ON "PRIOR".x +1 = mytable.x or  "PRIOR".x + 2 = mytable.x;

select * from step4;

     LEVEL          X
---------- ----------
         4          4

第5步

CREATE TABLE step5 AS
SELECT 5 "LEVEL", mytable.X from mytable
JOIN step4 "PRIOR"
ON "PRIOR".x +1 = mytable.x or  "PRIOR".x + 2 = mytable.x;

select * from step5;

no rows selected

第5步未返回任何行，因此现在我们完成查询

最后一步

UNION ALL 所有步骤的结果，并将其作为最终结果返回：

SELECT * FROM step1
UNION ALL
SELECT * FROM step2
UNION ALL
SELECT * FROM step3
UNION ALL
SELECT * FROM step4
UNION ALL

SELECT * FROM step5;

     LEVEL          X
---------- ----------
         1          1 
         2          2 
         2          3 
         3          3 
         3          4 
         3          4 
         4          4

现在，将以上过程应用于您的查询：

SELECT * FROM dual;

DUMMY
-----
X

SELECT LEVEL FROM DUAL CONNECT BY rownum>5;

步骤1

由于查询不包含该START WITH子句，因此Oracle从源表中选择所有记录：

CREATE TABLE step1 AS
SELECT 1 "LEVEL" FROM dual;

select * from step1;

     LEVEL
----------
         1

第2步

CREATE TABLE step2 AS
SELECT 2 "LEVEL" from dual
JOIN step1 "PRIOR"
ON rownum > 5

select * from step2;

no rows selected

由于最后一步没有返回任何行，因此我们将完成查询。

最后一步

SELECT * FROM step1
UNION ALL

SELECT * FROM step2;

     LEVEL
----------
         1

最后一个查询的分析：

select level from dual connect by rownum<10;

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