Sql &引用；ORA-00998:必须使用列别名命名此表达式；我犯了这个错误，谁能告诉我是什么'；问题和解决办法是什么

这是我的示例代码-

CREATE TABLE orders
(

ord_no int,
purch_amt float,
ord_date varchar(50),
customer_id int,
salesman_id int,
PRIMARY KEY(ord_no)
);

INSERT INTO orders (ord_no, purch_amt, ord_date, customer_id, salesman_id)
VALUES (70001, 150.5, '2012-10-05', 3005, 5002);

CREATE VIEW totalforday 
 AS SELECT ord_date , COUNT(DISTINCT customer_id),
 AVG(purch_amt), SUM(purch_amt)
 FROM orders
 GROUP BY ord_date;

---

想象一下为新视图编写一个查询

SELECT ord_date,
       COUNT(DISTINCT customer_id),
       AVG(purch_amt),
       SUM(purch_amt)
 FROM totalforday;

您的列名-您是在要求一个名为SUM（purch_amt）的列，还是在尝试对一个名为purch_amt的列进行求和

您需要隐式或显式地为视图提供有效的列名。添加别名允许数据库隐式地确定列的名称

像这样

CREATE or replace VIEW totalforday 
 AS SELECT ord_date , COUNT(DISTINCT customer_id) how_many_customers,
 AVG(purch_amt) avg_amt, SUM(purch_amt) total_amt
 FROM orders
 GROUP BY ord_date;

说：

如果省略[view]别名，则数据库将从查询中的列或列别名中派生它们。因此，如果查询包含表达式而不是列名，则必须使用别名

因此，如果视图定义不包含视图本身的列/别名列表，则必须在查询中提供它们，如@Jeff所示：

CREATE OR REPLACE VIEW totalforday
AS
SELECT ord_date,
  COUNT(DISTINCT customer_id) AS count_customers,
  AVG(purch_amt) AS avg_purch_amt,
  SUM(purch_amt) AS sum_purch_amt
FROM orders
GROUP BY ord_date;

describe totalforday

Name            Null? Type   
--------------- ----- ------ 
ORD_DATE              DATE   
COUNT_CUSTOMERS       NUMBER 
AVG_PURCH_AMT         NUMBER 
SUM_PURCH_AMT         NUMBER 

另一种方法是显式指定视图的列名：

CREATE OR REPLACE VIEW totalforday
  (ord_date, count_customers, avg_purch_amt, sum_purch_amt)
AS
SELECT ord_date,
  COUNT(DISTINCT customer_id),
  AVG(purch_amt),
  SUM(purch_amt)
FROM orders
GROUP BY ord_date;

describe totalforday

Name            Null? Type   
--------------- ----- ------ 
ORD_DATE              DATE   
COUNT_CUSTOMERS       NUMBER 
AVG_PURCH_AMT         NUMBER 
SUM_PURCH_AMT         NUMBER 

查询仍然可以包含列和列表达式的别名，如果您希望独立运行该查询，这会很有帮助，但视图会忽略这些别名-仍然使用显式名称/别名：

CREATE OR REPLACE VIEW totalforday
  (ord_date, count_customers, avg_purch_amt, sum_purch_amt)
AS
SELECT ord_date AS ignored_1,
  COUNT(DISTINCT customer_id) AS ignored_2,
  AVG(purch_amt) AS ignored_3,
  SUM(purch_amt) AS ignored_4
FROM orders
GROUP BY ord_date;

describe totalforday

Name            Null? Type   
--------------- ----- ------ 
ORD_DATE              DATE   
COUNT_CUSTOMERS       NUMBER 
AVG_PURCH_AMT         NUMBER 
SUM_PURCH_AMT         NUMBER 

---

来自MySQL的Oracle错误。。。将标记替换为。您必须在“创建”视图中命名“选择”列。例如，

COUNT（DISTINCT customer\u id）cust\u cnt

。这是Oracle更清晰的错误消息之一*8-）它在工作，但我怀疑，每当我必须创建视图时，是否有必要进行此计数（DISTINCT customer\u id）多少客户，平均（purch\u amt）平均金额，总和（purch\u amt）总计这是一个完全不同的问题，为了帮助您做到这一点，我们需要更多地了解您正在构建的视图需求背后的业务需求