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;
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
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
重复＃2，直到最后一个操作返回空结果集。
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
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
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
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;
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
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。
SELECT * FROM step1
UNION ALL
SELECT * FROM step2;
LEVEL
----------
1
select level from dual connect by rownum<10;
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