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Oracle provides additional capabilities for processing nested tables within SQL queries. A special THE keyword flattens the nested table, in effect, producing an unnested table with one row for each row of the nested table within each row of the main table. Here s a query that shows the schools from which one of the engineers has received degrees:
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SELECT NEST.SCHOOL FROM THE (SELECT DEGREES FROM ENGINEERS WHERE EMPL_NUM = 1234) NEST;
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The query within the inner parentheses is a query against the main (ENGINEERS) table. It selects the column containing the nested table, but it could select other columns as well. The THE operation, applied to the query results, flattens them out, creating a row for each nested row within each row of the main table. This flattened table is assigned an alias (NEST in this example), and it becomes the source of candidate query results rows from the FROM clause of the main, top-level query. With this table as a source, the main query in this example is quite simple; it selects one column that originated in the nested table. The ability to flatten nested tables in this way and process them as if they were actually joined versions of two separate tables is actually quite powerful. It allows many queries to be expressed in high-level SQL that would otherwise require you to resort to stored procedures. However, the logic behind such queries and the task of actually constructing them correctly can be extremely complicated, as even this simple example begins to show.
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Extensions to standard SQL syntax are used to insert new rows into a table containing collection-valued columns. Informix provides a trio of constructors the SET constructor, MULTISET constructor, and LIST constructor for this purpose. They transform a list of data items into the corresponding collections to be inserted. Here is a pair of INSERT statements that illustrates their use with the tables in Figure 24-6:
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INSERT INTO TECHNICIANS VALUES (1279, ROW('Sam', 'R', 'Jones'), ROW('164 Elm St.', 'Highland', 'IL', ROW(12345, 6789)), SET{'atlas', 'checkmate', 'bingo'}");
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INSERT INTO ENGINEERS VALUES (1281, ROW('Jeff', 'R', 'Ames'), ROW('1648 Green St.', 'Elgin', 'IL', ROW(12345, 6789)), MULTISET{ROW('BS', 'Michigan'), ROW('BS', 'Michigan'), ROW('PhD','Stanford')}");
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The first statement inserts a single row into the TECHNICIANS table with a three-item set in the PROJECTS column. The second inserts a single row into the ENGINEERS table with a three-item multiset in the DEGREES column. Because the members of this particular multiset are themselves row types, the row constructor must be used for each item. Oracle uses a different approach to constructing the collection-valued data items for insertion into the table. Recall from the discussion of Oracle abstract data types that each Oracle abstract data type automatically has an associated constructor method that is used to build a data item of the abstract type out of individual data items. This concept is extended to varying arrays and nested tables. A constructor method is automatically supplied for each varying array or nested table, and it is used in the INSERT statements:
INSERT INTO TECHNICIANS VALUES (NAME_TYPE('Sam', 'R', 'Jones'), ADDR_TYPE('164 Elm St.', 'Highland', 'IL', POST_TYPE(12345, 6789)), PROJECTS('atlas', 'checkmate', 'bingo')); INSERT INTO ENGINEERS VALUES (NAME_TYPE('Jeff', 'R', 'Ames'), ADDR_TYPE('1648 Green St.', 'Elgin','IL', POST_TYPE(12345, 6789)), DEGREES(DEGREE_TYPE('BS', 'Michigan'), DEGREE_TYPE('BS', 'Michigan'), DEGREE_TYPE('PhD', 'Stanford')));
Collections and Stored Procedures
Collections pose special problems for stored procedures that are retrieving and manipulating data in tables that contain them. Both Oracle and Informix provide special stored procedure language facilities for this purpose. In Informix, special SPL
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collection variables must be used. Here is an SPL stored procedure fragment that handles the PROJECTS collection column from the TECHNICIANS table:
define define define define proj_coll a_project proj_cnt empl_name collection; varchar(15); integer; name_type; /* /* /* /* holds project collection */ holds individual project */ number of projects */ buffer for tech name */
/* Check how many projects the technician is supporting */ select cardinality(projects) into proj_cnt from technicians where empl_num = 1234; /* If too many projects, then refuse to add a new one */ if (proj_cnt > 6) then . . . /* Retrieve row, including project set for the technician */ select name, projects into empl_name, proj_coll from technicians where empl_num = 1234; /* Add the 'gonzo' project to the list for this tech */ insert into table(proj_coll) values ('gonzo'); /* Search through project list one by one */ foreach proj_cursor for select * into a_project from table(proj_coll) if (a_project = 'atlas') then begin update table(proj_coll)(project) set project = 'bingo' where current of proj_cursor; exit foreach; end; end if; end foreach;
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