Query Query Query

In my last post, A Design Pattern for Oracle eBusiness Audit Trail Reports with XML Publisher, I described a database report module developed in Oracle's XML Publisher tool. Of the report structure I wrote:

It has a master entity with two independent detail entities, and therefore requires a minimum of two queries.

But why does such a structure require two queries? And can we determine the minimum number of queries for reports in general? To start with, let's define a report in this context as being a hierarchy of record groups, where:

  • a record group is a set of records having the same columns with (possibly) differing values
  • each group is linked to a single record in its (single) parent group by values in the parent record, except the top level (or root) group

For example, in the earlier post the root group is a set of bank accounts, with the two detail (or child) groups being the set of owners of the bank account and the set of audit records for the bank account parent record. Corresponding to this group structure, each bank account record is the root of the data hierarchies, comprising two sets of records below the bank account record, one for the owners and one for the audit records linked to the root record by the bank account id.

A (relational) query always returns a flat record set, and it's this fact that determines the minimum number of queries required for a given group structure. A master-detail group structure can be flattened in the query by simply copying master fields on to the child record sets. The set cardinality is then the cardinality of the child set. The report designer uses their chosen reporting tool to specify display of the queried data in either flat, or in master-detail format.

In fact this approach works for any number of child levels, with the query cardinality being the number of bottom level descendants (using null records for potential parents that are in fact childless). It's clear though that the approach will not work for any second child at the same level because there would be two cardinalities and no meaningful single record for both child groups could be constructed within a flat query.

This reasoning leads to the conclusion that the minimum number of queries required in general is equal to the number of groups minus the number of parent groups.

Query Query Query - example

In the earlier post I also stated:

This minimum number of queries is usually the best choice...

There are two main reasons for this:

  • each child query fires for every record returned by its parent, with associated performance impact
  • maintenance tends to be more difficult with extra queries; this is much worse when the individual groups, which should almost always be implemented by a maximum of one query each, are split, and then need to be joined back together procedurally

On thinking about this, it occurred to me that if the group structure were defined in a metadata table we might be able to return minimum query structures using an SQL query. Just one, obviously 🙂 . To save effort we could use Oracle's handy HR demo schema with the employee hierarchy representing groups.

The remainder of this article describes the query I came up with. As it's about hierarchies, recursion is the technique to use, and this is one of those cases where Oracle's old tree-walk syntax is too limited, so I am using the Oracle 11.2 recursive subquery factoring feature.

The query isn't going to be of practical value for report group structures since these are always quite small in size, but I expect there are different applications where this kind of Primogeniture Recursion would be useful.

Query Groups Query - Primogeniture Recursion

Query Structure Diagram
Query Query Query

SQL

WITH rsf (last_name, employee_id, lev, part_id, manager_id) AS (
SELECT last_name, employee_id, 0, employee_id, To_Number(NULL)
  FROM employees
 WHERE manager_id IS NULL
UNION ALL
SELECT e.last_name, e.employee_id, r.lev + 1, 
       CASE WHEN Row_Number() OVER (PARTITION BY r.employee_id ORDER BY e.last_name) = 1 THEN r.part_id ELSE e.employee_id END,
       e.manager_id
  FROM rsf r
  JOIN employees e
    ON e.manager_id = r.employee_id
)
SELECT part_id, LPad ('.', lev) || last_name last_name, employee_id, 
       Count(DISTINCT part_id) OVER () "#Partitions",
       Count(DISTINCT manager_id) OVER () "+ #Parents",
       Count(*) OVER () "= #Records"
  FROM rsf
 ORDER BY part_id, lev, last_name

Query Output

   PART_ID LAST_NAME            EMPLOYEE_ID #Partitions + #Parents = #Records
---------- -------------------- ----------- ----------- ---------- ----------
       100 King                         100          89         18        107
           .Cambrault                   148          89         18        107
            .Bates                      172          89         18        107
       101 .Kochhar                     101          89         18        107
            .Baer                       204          89         18        107
       102 .De Haan                     102          89         18        107
            .Hunold                     103          89         18        107
             .Austin                    105          89         18        107
       104   .Ernst                     104          89         18        107
       106   .Pataballa                 106          89         18        107
       107   .Lorentz                   107          89         18        107
       108  .Greenberg                  108          89         18        107
             .Chen                      110          89         18        107
       109   .Faviet                    109          89         18        107
       111   .Sciarra                   111          89         18        107
       112   .Urman                     112          89         18        107
       113   .Popp                      113          89         18        107
       114 .Raphaely                    114          89         18        107
            .Baida                      116          89         18        107
       115  .Khoo                       115          89         18        107
       117  .Tobias                     117          89         18        107
       118  .Himuro                     118          89         18        107
       119  .Colmenares                 119          89         18        107
       120 .Weiss                       120          89         18        107
            .Fleaur                     181          89         18        107
       121 .Fripp                       121          89         18        107
            .Atkinson                   130          89         18        107
       122 .Kaufling                    122          89         18        107
            .Chung                      188          89         18        107
       123 .Vollman                     123          89         18        107
            .Bell                       192          89         18        107
       124 .Mourgos                     124          89         18        107
            .Davies                     142          89         18        107
       125  .Nayer                      125          89         18        107
       126  .Mikkilineni                126          89         18        107
       127  .Landry                     127          89         18        107
       128  .Markle                     128          89         18        107
       129  .Bissot                     129          89         18        107
       131  .Marlow                     131          89         18        107
       132  .Olson                      132          89         18        107
       133  .Mallin                     133          89         18        107
       134  .Rogers                     134          89         18        107
       135  .Gee                        135          89         18        107
       136  .Philtanker                 136          89         18        107
       137  .Ladwig                     137          89         18        107
       138  .Stiles                     138          89         18        107
       139  .Seo                        139          89         18        107
       140  .Patel                      140          89         18        107
       141  .Rajs                       141          89         18        107
       143  .Matos                      143          89         18        107
       144  .Vargas                     144          89         18        107
       145 .Russell                     145          89         18        107
            .Bernstein                  151          89         18        107
       146 .Partners                    146          89         18        107
            .Doran                      160          89         18        107
       147 .Errazuriz                   147          89         18        107
            .Ande                       166          89         18        107
       149 .Zlotkey                     149          89         18        107
            .Abel                       174          89         18        107
       150  .Tucker                     150          89         18        107
       152  .Hall                       152          89         18        107
       153  .Olsen                      153          89         18        107
       154  .Cambrault                  154          89         18        107
       155  .Tuvault                    155          89         18        107
       156  .King                       156          89         18        107
       157  .Sully                      157          89         18        107
       158  .McEwen                     158          89         18        107
       159  .Smith                      159          89         18        107
       161  .Sewall                     161          89         18        107
       162  .Vishney                    162          89         18        107
       163  .Greene                     163          89         18        107
       164  .Marvins                    164          89         18        107
       165  .Lee                        165          89         18        107
       167  .Banda                      167          89         18        107
       168  .Ozer                       168          89         18        107
       169  .Bloom                      169          89         18        107
       170  .Fox                        170          89         18        107
       171  .Smith                      171          89         18        107
       173  .Kumar                      173          89         18        107
       175  .Hutton                     175          89         18        107
       176  .Taylor                     176          89         18        107
       177  .Livingston                 177          89         18        107
       178  .Grant                      178          89         18        107
       179  .Johnson                    179          89         18        107
       180  .Taylor                     180          89         18        107
       182  .Sullivan                   182          89         18        107
       183  .Geoni                      183          89         18        107
       184  .Sarchand                   184          89         18        107
       185  .Bull                       185          89         18        107
       186  .Dellinger                  186          89         18        107
       187  .Cabrio                     187          89         18        107
       189  .Dilly                      189          89         18        107
       190  .Gates                      190          89         18        107
       191  .Perkins                    191          89         18        107
       193  .Everett                    193          89         18        107
       194  .McCain                     194          89         18        107
       195  .Jones                      195          89         18        107
       196  .Walsh                      196          89         18        107
       197  .Feeney                     197          89         18        107
       198  .OConnell                   198          89         18        107
       199  .Grant                      199          89         18        107
       200  .Whalen                     200          89         18        107
       201 .Hartstein                   201          89         18        107
            .Fay                        202          89         18        107
       203  .Mavris                     203          89         18        107
       205  .Higgins                    205          89         18        107
             .Gietz                     206          89         18        107

107 rows selected.

 






One thought on “Query Query Query

  1. Oracle mentions an interesting alternative, called the "union join", in the context of JSON support: http://docs.oracle.com/database/121/ADXDB/json.htm#ADXDB6287

    Function JSON_TABLE allows for multiple NESTED COLUMNS clauses, including siblings. "The virtual tables defined by parent and child COLUMNS clauses are joined using an outer join, with the parent being the outer table. The virtual columns defined by sibling COLUMNS clauses are joined using a union join."

    The UNION JOIN is equivalent to a FULL JOIN...ON 1=0;

    The same technique could be used for any "family tree" of real tables.

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