带有（来自）别名的 SQL 子查询

Posted 2023-03-31

【中文标题】带有（来自）别名的 SQL 子查询

【英文标题】：SQL subquery with (from) aliases

【发布时间】：2015-08-25 17:40:20

【问题描述】：

我需要关于 select 子句中的子查询的帮助。我有表：EMPLOYEES (EMPLOYEE_ID, EMPLOYEE_NAME, EMPLOYEE_GROUP) 和 ACTIVITIES (ACTIVITY_DATE, ACTIVITY_STATUS, EMPLOYEE_ID)。我需要为“W”组的员工选择一些活动。

很遗憾，我不能使用 CREATE TABLE/VIEW 功能，所以我尝试了这个，但它不起作用。

SELECT DISTINCT

TB1.EMPLOYEE_NAME,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'START' AND TB1.ACTIVITY_NUMBER = 1) AS FIRST_START,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'START' AND TB1.ACTIVITY_NUMBER = 2) AS SECOND_START,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'STOP' AND TB1.ACTIVITY_NUMBER = 1) AS FIRST_STOP,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'STOP' AND TB1.ACTIVITY_NUMBER = 2) AS SECOND_STOP

FROM( 

     SELECT DISTINCT

     A.EMPLOYEE_NAME,        
     B.ACTIVITY_DATE,
     B.ACTIVITY_STATUS,
     ROW_NUMBER() OVER (PARTITION BY A.EMPLOYEE_NAME,B.ACTIVITY_STATUS 
       ORDER BY B.ACTIVITY_DATE ASC) ACTIVITY_NUMBER

     FROM 
     (SELECT * FROM EMPLOYEES WHERE EMPLOYEE_GROUP ='W')  A

     LEFT JOIN ( SELECT * FROM ACTIVITIES  WHERE ACTIVITY_STATUS IN ('START','STOP','PAUSE') )B
      ON A.EMPLOYEE_ID = B.EMPLOYEE_ID

  ) TB1

你知道任何函数，我可以用它从带有别名的表中选择数据吗？

【问题讨论】：

你试过CTE（公用表表达式）吗？？

公用表表达式在 Oracle 文档中称为子查询因式分解。至少这是我最后一次阅读 Oracle 文档。

【参考方案1】：

我不确定您的 Oracle 版本是否支持它，但您要么必须将 TB1 定义为 CTE（公用表表达式）：

WITH TB1 AS ( 

     SELECT DISTINCT

     A.EMPLOYEE_NAME,        
     B.ACTIVITY_DATE,
     B.ACTIVITY_STATUS,
     ROW_NUMBER() OVER (PARTITION BY A.EMPLOYEE_NAME,B.ACTIVITY_STATUS 
       ORDER BY B.ACTIVITY_DATE ASC) ACTIVITY_NUMBER

     FROM 
     (SELECT * FROM EMPLOYEES WHERE EMPLOYEE_GROUP ='W')  A

     LEFT JOIN ( SELECT * FROM ACTIVITIES  WHERE ACTIVITY_STATUS IN ('START','STOP','PAUSE') )B
      ON A.EMPLOYEE_ID = B.EMPLOYEE_ID

  )
SELECT DISTINCT

TB1.EMPLOYEE_NAME,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'START' AND TB1.ACTIVITY_NUMBER = 1) AS FIRST_START,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'START' AND TB1.ACTIVITY_NUMBER = 2) AS SECOND_START,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'STOP' AND TB1.ACTIVITY_NUMBER = 1) AS FIRST_STOP,
(SELECT TB1.ACTIVITY_DATE FROM TB1 
WHERE TB1.ACTIVITY_STATUS = 'STOP' AND TB1.ACTIVITY_NUMBER = 2) AS SECOND_STOP

FROM TB1

或者，老式的方法是复制粘贴每个“TB1”实例的定义。 （是的，真的，这就是我们过去的做法。）