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ORA-02014: cannot select FOR UPDATE from view

Oracle Tips by Laurent Schneider

 

Laurent Schneider is considered one of the top Oracle SQL experts and he is the author of the book "Advanced Oracle SQL Programming" by Rampant TechPress.


 

Note:  The select for update is not a good locking strategy because there are many things that can go wrong.  Instead of select for update, savvy Oracle developers will adopt alternatives mechanisms like a re-read upon update commit where the transaction re-read the rows and ensure that it has not changed since the original read.

 

Question:  Suppose we have a table A, whose primary key has 3 columns, X, Y and Z.  The table is to be joined to itself:
 

SELECT * FROM
  a src INNER JOIN a dst ON src.x=dst.x AND src.y = dst.y
WHERE
  src.z = 'source' AND
  dst.z = 'dest'


For any given row that has a Z column of text "source"
There can only be one other row having matching X and Y columns and a Z column of dest.

The whole of the PK is accounted for, and there either exists a pair of rows, or there not exists a pair of rows.  But this view is not updatable and I get the error "ORA-02014: cannot select FOR UPDATE from view"

Answer:  You get the ORA-02014 error because you have no foreign key to preserve your rows, and you cannot define a foreign key a(decode(z,'source',x),decode(z,'source',y))
references a(decode(z,'dest',x),decode(z,'dest',y)).

However, you could create 2 tables SRC and DEST, and a view A to make your view updatable:

SQL> create table a(
   x number, y number, z varchar2(7),
   constraint a_pk primary key (x,y,z))
Table created.
SQL> insert into a(x,y,z) values (1,1,'source')
1 row created.
SQL> insert into a(x,y,z) values (1,1,'dest')
1 row created.
SQL> commit
Commit complete.
SQL> SELECT * FROM a

         X          Y Z      
---------- ---------- -------
         1          1 dest  
         1          1 source


2 rows selected.
SQL> create table src (
   x number, y number, z varchar2(7),
   constraint src_pk primary key (x,y))
Table created.
SQL> create table dest (
   x number, y number, z varchar2(7),
   constraint dst_pk primary key (x,y),
   constraint dst_fk foreign key (x,y) references src(x,y))
Table created.
SQL> insert into src(x,y,z) select x,y,z from a where z='source'
1 row created.
SQL> insert into dest(x,y,z) select x,y,z from a where z='dest'
1 row created.
SQL> drop table a
Table dropped.
SQL> create view a as select x,y,z from src
union all
select x,y,z from dest
View created.
SQL> select * from a

         X          Y Z      
---------- ---------- -------
         1          1 source
         1          1 dest  


2 rows selected.
SQL> update (
   select src.x, src.y, src.z, dest.x destx, dest.y desty, dest.z destz
   from src join dest on (src.x=dest.x and src.y=dest.y))
set x=9 where y=10
0 rows updated.


 

 

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