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CHAPTER 10 DATABASE TABLES
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the EMP relational table, but in this nested table model we can only access the EMP data via the DEPT data. We must always join; we cannot query the EMP data alone. Well, we can t do it in a supported, documented method, but we can use a trick (more on this trick later). If we needed to update every row in the EMPS_NT, we would have to do four updates: one each for the rows in DEPT_AND_EMP to update the virtual table associated with each row. Another thing to consider is that when we updated the employee data for department 10, we were semantically updating the EMPS column in the DEPT_AND_EMP table. We understand that physically there are two tables involved, but semantically there is only one. Even though we updated no data in the department table, the row that contains the nested table we did modify is locked from update by other sessions. In a traditional parent/child table relationship, this would not be the case. These are the reasons why I tend to stay away from nested tables as a persistent storage mechanism. It is the rare child table that is not queried stand-alone. In the preceding example, the EMP table should be a strong entity. It stands alone, so it needs to be queried alone. I find this to be the case almost all of the time. I tend to use nested tables via views on relational tables. So, now that we have seen how to update a nested table instance, inserting and deleting are pretty straightforward. Let s add a row to the nested table instance department 10 and remove a row from department 20: ops$tkyte@ORA11GR2> insert into table 2 ( select emps from dept_and_emp where deptno = 10 ) 3 values 4 ( 1234, 'NewEmp', 'CLERK', 7782, sysdate, 1200, null ); 1 row created. ops$tkyte@ORA11GR2> delete from table 2 ( select emps from dept_and_emp where deptno = 20 ) 3 where ename = 'SCOTT'; 1 row deleted. ops$tkyte@ORA11GR2> select d.dname, e.empno, ename 2 from dept_and_emp d, table(d.emps) e 3 where d.deptno in ( 10, 20 ); DNAME EMPNO ENAME -------------- ---------- ---------ACCOUNTING 7782 CLARK ACCOUNTING 7839 KING ACCOUNTING 7934 MILLER RESEARCH 7369 SMITH RESEARCH 7566 JONES RESEARCH 7876 ADAMS RESEARCH 7902 FORD ACCOUNTING 1234 NewEmp 8 rows selected. That is the basic syntax of how to query and modify nested tables. You will find that you often need to un-nest these tables as we just did, especially in queries, to make use of them. Once you conceptually visualize the virtual table per row concept, working with nested tables becomes much easier. Previously I stated, We must always join; we cannot query the EMP data alone, but then I followed that up with a caveat: You can if you really need to. It is not documented heavily; use this approach only as a last ditch method. Where it will come in most handy is if you ever need to mass update the nested table (remember, you would have to do that through the DEPT table with a join). There is an
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CHAPTER 10 DATABASE TABLES
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underdocumented hint (it is mentioned briefly and not fully documented), NESTED_TABLE_GET_REFS, which is used by various tools such as EXP and IMP to deal with nested tables. It is also a way to see a little more about the physical structure of the nested tables. If you use this hint, you can query to get some magical results. The following query is what EXP (a data unload utility) uses to extract the data from this nested table: ops$tkyte@ORA11GR2> SELECT /*+NESTED_TABLE_GET_REFS+*/ 2 NESTED_TABLE_ID,SYS_NC_ROWINFO$ 3 FROM "OPS$TKYTE"."EMPS_NT" 4 / NESTED_TABLE_ID SYS_NC_ROWINFO$(EMPNO, EN -------------------------------- ------------------------F60DEEE0FF7D7BC1E030007F01001321 EMP_TYPE(7782, 'CLARK', ' MANAGER', 7839, '09-JUN-8 1', 2450, 100) F60DEEE0FF7D7BC1E030007F01001321 EMP_TYPE(7839, 'KING', 'P RESIDENT', NULL, '17-NOV81', 5000, 100) Well, this is somewhat surprising, if you describe this table: ops$tkyte@ORA11GR2> desc emps_nt Name Null ----------------------------- -------EMPNO ENAME JOB MGR HIREDATE SAL COMM Type -------------------NUMBER(4) VARCHAR2(10) VARCHAR2(9) NUMBER(4) DATE NUMBER(7,2) NUMBER(7,2)
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These two columns don t even show up. They are part of the hidden implementation of nested tables. The NESTED_TABLE_ID is really a foreign key to the parent table DEPT_AND_EMP. DEPT_AND_EMP actually has a hidden column in it that is used to join to EMPS_NT. The SYS_NC_ROWINF$ column is a magic column; it is more of a function than a column. The nested table here is really an object table (it is made of an object type), and SYS_NC_INFO$ is the internal way Oracle references the row as an object, instead of referencing each of the scalar columns. Under the covers, all Oracle has done for us is implement a parent/child table with system-generated primary and foreign keys. If we dig a little deeper, we can query the real data dictionary to see all of the columns in the DEPT_AND_EMP table: sys@ORA11GR2> select name 2 from sys.col$ 3 where obj# = ( select 4 from 5 where 6 and 7 /
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object_id dba_objects object_name = 'DEPT_AND_EMP' owner = 'OPS$TKYTE' )
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