Oracle-连接三个表并获取记录

Question

I have three tables with below data

Table A

JOB_NAME    APP_NAME    START_TIME
A           ABC         00:00
C           ABC         00:00
C           ABCD        00:30

Table B

JOB_NAME    APP_NAME    SLA
A           ABC         01:00
B           ABC         01:00
C           ABC         01:00
C           ABCD        01:30

Table C

JOB_NAME    APP_NAME    PARENT
A           ABC         N/A
B           ABC         A
C           ABC         B
C           ABCD        N/A

I need to join these tables and generate below output.

JOB_NAME    APP_NAME    PARENT      START_TIME    SLA
A           ABC         N/A         00:00         01:00
B           ABC         A           N/A           01:00
C           ABC         C           00:00         01:00

But the query I wrote returns below output.

JOB_NAME    APP_NAME    PARENT  START_TIME  SLA
A           ABC         N/A     00:00       01:00
C           ABC         B       00:00       01:00

Please help to fix this issue.

SELECT C.JOB_NAME,C.APP_NAME,C.PARENT,NVL(A.START_TIME,'N/A') AS START_TIME,B.SLA FROM C
          LEFT JOIN B
           ON UPPER(C.JOB_NAME) = UPPER(B.JOB_NAME)
          LEFT JOIN A ON UPPER(C.JOB_NAME) = UPPER(A.JOB_NAME) WHERE C.APP_NAME='ABC' AND C.APP_NAME=A.APP_NAME AND C.APP_NAME=B.APP_NAME

Answer 1

You've duplicated your JOIN logic in your WHERE clause:

  ON UPPER(C.JOB_NAME) = UPPER(B.JOB_NAME)
  ...
  WHERE ... AND C.APP_NAME=A.APP_NAME AND C.APP_NAME=B.APP_NAME

thereby turning your OUTER JOINs into INNNER ones. Get rid of the superfluous WHERE clause part, and you should be fine.

BTW: for your sample data, the UPPER conversion is unnecessary; get rid of it if possible (since it will render indices on the JOB_NAME columns useless)

Answer 2

Try this:

select c.job_name
     , c.app_name
     , c.parent
     , nvl(a.start_time,'N/A') as start_time
     , b.sla
from   c
       left join b
            on  upper(b.job_name) = upper(c.job_name)
            and b.app_name = c.app_name
       left join a
            on  upper(a.job_name) = upper(c.job_name)
            and a.app_name = c.app_name
where  c.app_name = 'ABC'

You have WHERE clause conditions on A that require every row to have a value, and this excludes A.

I have always found that laying out code neatly makes issues like this easier to spot.