子查询作为 CASE WHEN 条件

Posted 2023-03-27

【中文标题】子查询作为 CASE WHEN 条件

【英文标题】：Subquery as CASE WHEN condition

【发布时间】：2021-09-07 07:22:36

【问题描述】：

以下查询，AS PQ_COUNT 发生语法错误

SELECT CASE WHEN
     RESULTS LIKE '%PQ - Duplicate%' AND 
     (SELECT COUNT(*) FROM MY_TABLE WHERE ID = '998877'AND FINAL_RESULTS='FL_57') AS PQ_COUNT >= 1 
    THEN 'PQ count = '|| PQ_COUNT 
ELSE RESULTS END AS RESULTS 

如果我将AS PQ_COUNT 移动到选择查询中，

(SELECT COUNT(*) AS PQ_COUNT FROM MY_TABLE WHERE ID = '998877'AND FINAL_RESULTS='FL_57') >= 1

THEN 块中 PQ_COUNT 的引用成为无效标识符 (ORA-00904)

将处理时可能会出现什么问题？

【参考方案1】：

一种选择是使用子查询（或 CTE，如我的示例）来计算满足条件的行数，然后 - 因为它只包含一行 - 将其交叉连接到 my_table。像这样的：

SQL> WITH
  2     my_table (id, final_results, results) AS
  3     -- sample data
  4        (SELECT '998877', 'FL_57', 'PQ - Duplicate' FROM DUAL),
  5     cnt AS
  6     -- calculate COUNT first ...
  7        (SELECT COUNT (*) pq_count               --> pq_count
  8           FROM MY_TABLE
  9          WHERE     ID = '998877'
 10                AND FINAL_RESULTS = 'FL_57')
 11  -- ... then re-use it in "main" query
 12  SELECT CASE
 13            WHEN     a.results LIKE '%PQ - Duplicate%'
 14                 AND b.pq_count >= 1            --> reused here
 15            THEN
 16               'PQ count = ' || b.PQ_COUNT      --> and here
 17            ELSE
 18               a.results
 19         END AS results
 20    FROM my_table a CROSS JOIN cnt b;

RESULTS
---------------------------------------------------
PQ count = 1

SQL>

【参考方案2】：

您不能在创建别名的子查询中引用别名；您需要嵌套子查询（或使用子查询因式分解子句；也称为 CTE 或 WITH 子句）并在外部引用它：

SELECT CASE
       WHEN results LIKE '%PQ - Duplicate%'
       AND  pq_count >= 1 
       THEN 'PQ count = '|| pq_count
       ELSE results
       END AS RESULTS
FROM   (
  SELECT results,
         ( SELECT COUNT(*)
           FROM   MY_TABLE
           WHERE  ID = '998877'
           AND    FINAL_RESULTS='FL_57'
         ) AS pq_count
  FROM   your_table
);