Row Count in Trailer Record - sql

Question

I need to create a text file that contains a header row, multiple data rows (could be any number of rows once I put it to work), and a trailer row that includes the final row number, which is to be the number of data rows, plus 2 (the header and trailer).
I have been able to achieve the desired result with the following query, but I wonder if there is a more efficient way of handling the task?
My test query:
--SINGLE HEADER ROW
SELECT 'PH3', to_char('10000000')
FROM DUAL
UNION ALL
--MULTIPLE DATA ROWS
SELECT 'PD3', 'NO SSN'
FROM students s
WHERE s.schoolid = '999999'
UNION ALL
--SINGLE TRAILER ROW
SELECT 'PT3', to_char(count(*)+2)
FROM students s
WHERE s.schoolid = '999999'

Answer 1

I have been able to achieve the desired result with the following query, but I wonder if there is a more efficient way of handling the task?
As far as I know, it is not documented anywhere than UNION ALL will generate rows in any particular order. Even if it appears today to always work as you expected.
As of myself, I would consider than, without a specific ORDER BY clause, any set operation will produce an unordered result.
So for the sake of your peace of mind (or of some future maintainer), I would go for an explicit ordering. Probably not more efficient, but guarantying to output your rows in the expected order:
SELECT "col1", "col2" FROM (
SELECT 0 as "position", 'PH3' as "col1", to_char('10000000') as "col2"
FROM DUAL
UNION ALL
--MULTIPLE DATA ROWS
SELECT 1, 'PD3', 'NO SSN'
FROM students s
WHERE s.schoolid = '999999'
UNION ALL
--SINGLE TRAILER ROW
SELECT 2, 'PT3', to_char(count(*)+2)
FROM students s
WHERE s.schoolid = '999999'
) ORDER BY "position";
And, if you really want to get rid of the two SELECT having the same WHERE clause, maybe you could use a CTE?
WITH data AS (SELECT 'NO SSN'
FROM students s
WHERE s.schoolid = '999999')
SELECT "col1", "col2" FROM (
SELECT 0 as "position", 'PH3' as "col1", to_char('10000000') as "col2"
FROM DUAL
UNION ALL
--MULTIPLE DATA ROWS
SELECT 1, 'PD3', 'NO SSN'
FROM data
UNION ALL
--SINGLE TRAILER ROW
SELECT 2, 'PT3', to_char(count(*)+2)
FROM data
) ORDER BY "position";

Answer 2

You can use ROWNUM to obtain the final count of rows produced by the query, as in the following:
SELECT rectype,
case
when rectype <> 'PT3' THEN ssn
else to_char(rownum, 'TM9')
end as ssn
FROM
(--SINGLE HEADER ROW
SELECT 'PH3' as rectype, to_char('10000000') as ssn
FROM DUAL
UNION ALL
--MULTIPLE DATA ROWS
SELECT 'PD3' as rectype, 'NO SSN' as ssn
FROM students s
WHERE s.schoolid = '999999'
UNION ALL
--SINGLE TRAILER ROW
SELECT 'PT3' as rectype, NULL as ssn
FROM DUAL)
SQLFiddle here
Share and enjoy.