On the use of GROUP BY ROLLUP

Transcription

1 On the use of GROUP BY ROLLUP The normal method of querying for data from the pivoted data table is via use of a regular GROUP BY clause: select 'D2 ' section_id '-' transaction_type as descr, group by transaction_type order by transaction_type; From here one would generate totals at the subheading and section level as follows: select * from (select 'T0DTotal', sum(v0) v0, as seq union all select 'T1DTotal', sum(v0) v0, subheading_id * as seq group by subheading_id union all select 'T2DTotal', sum(v0) v0, section_id * as seq group by section_id ) order by seq; Note the careful generation of SEQ numbers so that in a report all the rows can be ordered correctly.

2 However, the method above requires the execution of two independent SQL statements with the possibilities of errors entering into the process. What if the totals and detail rows could be generated in one SQL statement? Oracle has added Relational On-Line Analytical Processing (ROLAP) capability to their relational database product over the years. One of the simplest is the GROUP BY ROLLUP (and CUBE) extensions to the standard SELECT statement. ROLLUP allows for the generation of subtotals. The generation of subtotals via rollup can be invoked by simply changing the GROUP BY a1,a2,...,an clause of the SELECT statement to GROUP BY ROLLUP(a1,a2,...,an). Making this change gives us the following statement and result. select 'D3 ' section_id '-' transaction_type as descr, GROUP BY ROLLUP( transaction_type); Note that the subtotal is indicated in the output via a (null) value in the position of each of the ROLLUP attributes (note also the (null) in the SEQ column where there is a (null) in the SECTION_ID and TRANSACTION_TYPE columns). While one could use a CASE statement of a DECODE function to detect and process these (null) values i.e., by changing the (null) value to the word Total, it may be that a (null) value is an actual valid value of a column being summarised. To address this Oracle makes available the GROUPING function. The GROUPING function takes a single argument: the name of one of the attributes in the GROUP BY ROLLUP() clause. The following example shows the use of the GROUPING function. select 'D3 ' section_id '-' transaction_type as descr, GROUPING(subheading_id) as gshi, GROUPING(section_id) as gsi, GROUPING(transaction_type) as gtt,

3 group by rollup( transaction_type); And at the end of the query, the last rows are: How can we use the GROUPING function? There are two things we need to do with this single SQL statement to make it able to generate detail and totals in one. The first is to create the 'T0DTotal' line; the second is to generate a valid SEQ number for all subtotals that will ensure correct ordering in the report. First, to generate a Total at the lowest (TRANSACTION_TYPE) level we can execute this: select case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 else 'D3 ' section_id '-' transaction_type

4 group by rollup( transaction_type); To complete the generation of Totals at all grouping levels: select case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 when GROUPING(subheading_id) = 0 then 'T1 Total' when GROUPING(subheading_id) = 1 then 'T0 Total' else 'D3 ' transaction_type group by rollup( transaction_type); We can also use the GROUPING function to help us create sequence numbers that will guarantee that the Totals will be correctly displayed in the report. First, the sequence numbers: those of the total at the end of the TRANSACTION_TYPEs and SECTION_ID (because the value of the SECTION_ID and TRANSACTION_TYPE attributes is available at the point of grouping):

5 select case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 when GROUPING(subheading_id) = 0 then 'T1 Total' when GROUPING(subheading_id) = 1 then 'T0 Total' else 'D3 ' transaction_type case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 then section_id * when GROUPING(subheading_id) = 0 then subheading_id * else section_id * end as seq group by rollup( transaction_type); Note that the 900 added to the SECTION_ID and added to the SUBHEADING_ID ensures that correct ordering of the rows will always be guaranteed. Note, however, that the final T0 Total cannot be generated by reference to one of the GROUP BY attributes: The last subtotal SEQ, that for the grand total of all detail rows, can be generated by providing a number greater than the last one generated by the SELECT statement. select case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 when GROUPING(subheading_id) = 0 then 'T1 Total' when GROUPING(subheading_id) = 1

6 then 'T0 Total' else 'D3 ' transaction_type case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 then section_id * when GROUPING(subheading_id) = 0 then subheading_id * * 100 when GROUPING(subheading_id) = 1 then else section_id * end as seq group by rollup( transaction_type); But this is a hard-coded number dependent on the user knowing the range of the subheading_ids being processed. To make the SELECT independent of knowing the SUBHEADING_ID, we need to have recourse to the Oracle row comparison function called LAG (this is an analytic function standardised with SQL 99). From the Oracle documentation: The LAG and LEAD functions are useful for comparing values when the relative positions of rows can be known reliably. They work by specifying the count of rows which separate the target row from the current row. Because the functions provide access to more than one row of a table at the same time without a self-join, they can enhance processing speed. The LAG function provides access to a row at a given offset prior to the current position, and the LEAD function provides access to a row at a given offset after the current position. These functions have the following syntax:

7 {LAG LEAD} ( value_expr [, offset] [, default] ) OVER ( [query_partition_clause] order_by_clause ) In our situation we know that the row before the T0 (last) row has a valid subheading_id and that it is always ONE row before the last row of the whole ordered set (ordered by SUBHEADING_ID and SECTION_ID). To access this SUBHEADING_ID value the LAG clause would be (in isolation): LAG(1) OVER (ORDER BY section_id) When added to our SQL we get: select case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 when GROUPING(subheading_id) = 0 then 'T1 Total' when GROUPING(subheading_id) = 1 then 'T0 Total' else 'D3 ' transaction_type case when GROUPING(subheading_id) = 0 and GROUPING(section_id) = 0 then section_id * when GROUPING(subheading_id) = 0 then subheading_id * * 100 when GROUPING(subheading_id) = 1 then lag(1) over (order by section_id) * else section_id * end as seq group by rollup( transaction_type); With the result being:

8 Putting this all together by removing the grouping columns and padding the description column we get: