Issue with referencing column in Oracle inline view

Question

I have three tables as a master, child and grandchild. Simplified something like this:

CREATE TABLE TABLE1
(
  ID          NUMBER(10) NOT NULL,
  TIME_STAMP  DATE NOT NULL,
  COL1        VARCHAR2(64 CHAR) NOT NULL
)

CREATE TABLE TABLE2
(
  ID           NUMBER(10) NOT NULL,
  TIME_STAMP   DATE NOT NULL,
  TABLE1_ID    NUMBER(10) NOT NULL,
  COL2         VARCHAR2(64 CHAR) NOT NULL,
)

ALTER TABLE TABLE2 ADD (
  CONSTRAINT TABLE2_FK 
  FOREIGN KEY (TABLE1_ID) 
  REFERENCES TABLE1 (ID))
/

CREATE TABLE TABLE3
(
  ID          NUMBER(10) NOT NULL,
  TIME_STAMP  DATE NOT NULL,
  TABLE2_ID   NUMBER(10) NOT NULL,
  COL3        VARCHAR2(255 CHAR) NOT NULL
)

ALTER TABLE TABLE3 ADD (
  CONSTRAINT TABLE3_FK 
  FOREIGN KEY (TABLE2_ID) 
  REFERENCES TABLE2 (ID))
/

In a larger query I have these three tables join in an inline view like this:

SELECT * FROM (
  SELECT *
    FROM table3 tbl3
    JOIN table2 tbl2
      ON tbl3.table2_id = tbl2.id
    JOIN table1 tbl1
      ON tbl2.table1_id = tbl1.id
   WHERE tbl2.col2 = 'SOME_CODE'
     AND tbl1.col1 = 'SOME_CODE'
     AND tbl3.time_stamp > TO_DATE('20130901','YYYYMMDD')
) WHERE time_stamp < :query_date

My problem was that I did not specify which of the time_stamp the reference in the where clause. Surprisingly enough I didn’t get any error but instead the database decided to use column table1.time_stamp! My first question is there any reason why I don’t get ‘ORA-00918: column ambiguously defined’? Took me some time to found the problem but when located then easy corrected by specifying in the select of the inline view what columns I’m interested in in this case tbl3.time_stamp. When testing I did fount that if I included two of the time_stamp in the columns list then I as expected will get the ORA-00918.

Please help me out, am I missing something here or is there some issue (bug or feature) with the inline view in Oracle 11?

Answer 1

Interesting what if we try to get column names in the query using dbms_sql.describe procedure we can see three similar name "time_stamp" and "id":

SQL> declare
  2   c int;
  3   clm dbms_sql.DESC_TAB;
  4   c_n int;
  5  begin
  6    c := dbms_sql.open_cursor;
  7    dbms_sql.parse(c => c
  8    , statement => 'SELECT * FROM ( SELECT * '||
  9      'FROM table3 tbl3 JOIN table2 tbl2 ON tbl3.table2_id = tbl2.id '||
 10      'JOIN table1 tbl1 ON tbl2.table1_id = tbl1.id WHERE tbl2.col2 = ''SOME_CODE'''||
 11      ' AND tbl1.col1 = ''SOME_CODE''' ||
 12      ' AND tbl3.time_stamp > TO_DATE(''20130901'',''YYYYMMDD'')) where time_stamp < sysdate'
 13    , language_flag => dbms_sql.native);
 14    dbms_sql.describe_columns(c => c
 15    , col_cnt => c_n
 16    , desc_t => clm
 17    );
 18    for i in 1..c_n loop
 19      dbms_output.put_line(clm(i).col_name);
 20    end loop;
 21    dbms_sql.close_cursor(c);
 22  end;
 23  /
ID                                                                              
TIME_STAMP                                                                      
TABLE2_ID                                                                       
COL3                                                                            
ID                                                                              
TIME_STAMP                                                                      
TABLE1_ID                                                                       
COL2                                                                            
ID                                                                              
TIME_STAMP 

But if you come from JOIN syntax to joins in WHERE clause you can get ORA-00918:

SQL> SELECT * FROM (
  2    SELECT *
  3      FROM table3 tbl3
  4      , table2 tbl2
  5      , table1 tbl1
  6     WHERE tbl2.col2 = 'SOME_CODE'
  7       AND tbl1.col1 = 'SOME_CODE'
  8       AND tbl3.time_stamp > TO_DATE('20130901','YYYYMMDD')
  9       AND tbl3.table2_id = tbl2.id
 10       AND tbl2.table1_id = tbl1.id
 11  )
 12  where time_stamp < sysdate
 13  /
where time_stamp < sysdate
      *
error in line 12:
ORA-00918: column ambiguously defined

So seems it's just one of the many bugs coming from JOIN syntax.

Answer 2

point to be noted is that - you have select * in your query and i guess oracle automatically names the time_stamp columns as time_stamp, time_stamp_1, time_stamp_2.

Now in your where clause you have time_stamp= and this works as it links time_stamp with table1.time_stamp.

if you execute sql below you will be able to see above column names in output

SELECT *
    FROM table3 tbl3
    JOIN table2 tbl2
      ON tbl3.table2_id = tbl2.id
    JOIN table1 tbl1
      ON tbl2.table1_id = tbl1.id
   WHERE tbl2.col2 = 'SOME_CODE'
     AND tbl1.col1 = 'SOME_CODE'
     AND tbl3.time_stamp > TO_DATE('20130901','YYYYMMDD')