Question

我需要创建一个文本文件，其中包含标题行，多个数据行（一旦我将其用于工作，可以是任意数量的行），以及包含最终行号的预告片行，这将是数据行数加上2（标题和预告片）。

我已经能够通过以下查询获得所需的结果，但我想知道是否有更有效的方法来处理任务？

我的测试查询：

--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

我已经能够通过以下查询获得所需的结果，但我想知道是否有更有效的方法来处理任务？

据我所知，除UNION ALL以外的任何地方都没有记录任何特定顺序的行。即使它今天出现也始终按预期工作。

至于我自己，我会考虑，如果没有特定的ORDER BY子句，任何设置操作都会产生无序结果。

因此，为了您的安心（或未来的维护者），我会寻求明确的订购。可能效率不高，但保证按预期顺序输出行：

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";

而且，如果你真的想摆脱具有相同SELECT条款的两个WHERE，也许你可以使用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

您可以使用ROWNUM获取查询生成的最终行数，如下所示：

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

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预告片记录中的行数

2 个答案: