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Nested tables are part of the object-relational extensions to Oracle. A nested table, one of the two collection types in Oracle, is very similar to a child table in a traditional parent/child table pair in the relational model. It is an unordered set of data elements, all of the same data type, which could be either a built-in data type or an object data type. It goes one step further, however, since it is designed to give the illusion that each row in the parent table has its own child table. If there are 100 rows in the parent table, then there are virtually 100 nested tables. Physically, there is only the single parent and the single child table. There are large syntactic and semantic differences between nested tables and parent/child tables as well, and we ll look at those in this section. There are two ways to use nested tables. One is in your PL/SQL code as a way to extend the PL/SQL language. The other is as a physical storage mechanism for persistent storage of collections. I use them in PL/SQL all of the time, but I have never used them as a permanent storage mechanism. In this section, I ll briefly introduce the syntax to create, query, and modify nested tables. Then we ll look at some of the implementation details and what is important to know about how Oracle really stores nested tables.
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CHAPTER 10 DATABASE TABLES
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Nested Tables Syntax
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The creation of a table with a nested table is fairly straightforward it is the syntax for manipulating them that gets a little complex. Let s use the simple EMP and DEPT tables to demonstrate. We re familiar with that little data model that is implemented relationally as follows ops$tkyte@ORA11GR2> create table dept 2 (deptno number(2) primary key, 3 dname varchar2(14), 4 loc varchar2(13) 5 ); Table created. ops$tkyte@ORA11GR2> create table emp 2 (empno number(4) primary key, 3 ename varchar2(10), 4 job varchar2(9), 5 mgr number(4) references emp, 6 hiredate date, 7 sal number(7, 2), 8 comm number(7, 2), 9 deptno number(2) references dept 10 ); Table created. with primary and foreign keys. We ll do the equivalent implementation using a nested table for the EMP table: ops$tkyte%ORA11GR2> create or replace type emp_type 2 as object 3 (empno number(4), 4 ename varchar2(10), 5 job varchar2(9), 6 mgr number(4), 7 hiredate date, 8 sal number(7, 2), 9 comm number(7, 2) 10 ); 11 / Type created. ops$tkyte%ORA11GR2> create or replace type emp_tab_type 2 as table of emp_type 3 / Type created. To create a table with a nested table, we need a nested table type. The preceding code creates a complex object type, EMP_TYPE, and a nested table type of that, EMP_TAB_TYPE. In PL/SQL, this will be treated much like an array would. In SQL, it will cause a physical nested table to be created. Here is the simple CREATE TABLE statement that uses it:
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CHAPTER 10 DATABASE TABLES
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ops$tkyte%ORA11GR2> create table dept_and_emp 2 (deptno number(2) primary key, 3 dname varchar2(14), 4 loc varchar2(13), 5 emps emp_tab_type 6 ) 7 nested table emps store as emps_nt; Table created. ops$tkyte%ORA11GR2> alter table emps_nt add constraint 2 emps_empno_unique unique(empno) 3 / Table altered. The important part of this CREATE TABLE statement is the inclusion of the column EMPS of EMP_TAB_TYPE and the corresponding NESTED TABLE EMPS STORE AS EMPS_NT. This created a real physical table, EMPS_NT, separate from and in addition to the table DEPT_AND_EMP. We add a constraint on the EMPNO column directly on the nested table to make the EMPNO unique as it was in our original relational model. We cannot implement our full data model; however, there is the self-referencing constraint: ops$tkyte%ORA11GR2> alter table emps_nt add constraint mgr_fk 2 foreign key(mgr) references emps_nt(empno); alter table emps_nt add constraint mgr_fk * ERROR at line 1: ORA-30730: referential constraint not allowed on nested table column This will simply not work. Nested tables do not support referential integrity constraints, as they cannot reference any other table even themselves. So, we ll just skip that requirement for this demonstration (something you cannot do in real life!). Next, we ll populate this table with the existing EMP and DEPT data: ops$tkyte%ORA11GR2> insert into dept_and_emp 2 select dept.*, 3 CAST( multiset( select empno, ename, job, mgr, hiredate, sal, comm 4 from SCOTT.EMP 5 where emp.deptno = dept.deptno ) AS emp_tab_type ) 6 from SCOTT.DEPT 7 / 4 rows created. There are two things to notice here: Only four rows were created. There are really only four rows in the DEPT_AND_EMP table. The 14 EMP rows don t exist independently. The syntax is getting pretty exotic. CAST and MULTISET are syntax most people have never used. You will find lots of exotic syntax when dealing with object-relational components in the database. The MULTISET keyword is used to tell Oracle the subquery is expected to return more than one row (subqueries in a SELECT list have previously been limited to returning one row). The CAST is used to instruct Oracle to treat the returned set as a collection type. In this case, we CAST the MULTISET to be a EMP_TAB_TYPE. CAST is a general-purpose routine not limited to use in collections. For example, if we wanted to fetch the EMPNO column from EMP as a
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