如何修复 ORA-01427：单行子查询返回多行？

Question

CREATE TABLE 客户（id_customer NUMBER PRIMARY KEY，名称 VARCHAR2(10), 姓 VARCHAR2(20));

创建表 customer_wer ( id_customer 编号， 名称 VARCHAR2(10), 姓 VARCHAR2(20), data_from DATE， data_to 日期不为空， 约束 customer_wer_pk 主键（id_customer，data_from）， 约束 customer_wer_fk 外键 (id_customer) REFERENCES 客户 (id_customer));

SELECT COUNT(customer.id_customer) FROM customer JOIN customer_wer ON customer.id_customer = customer_wer.id_customer WHERE (SELECT customer_wer.id_customer FROM customer_wer JOIN customer ON customer_wer.id_customer = customer.id_customer GROUP BY customer.id_customer) >= 8;

'customer' 表中有 5 个客户，'customer_wer' 中有 48 个职位。 'customer' 表中的每个客户在 'customer_wer' 表中都有 8 个相关职位。我想知道有多少客户在“customer_wer”中有 8 个或更多相关职位。如何使用选择来做到这一点？

提前谢谢你。

Answer 1

例如，您可以使用MAX：

SELECT COUNT(customer.id_customer) 
  FROM customer JOIN customer_wer
    ON customer.id_customer = customer_wer.id_customer
 WHERE (SELECT MAX (customer_wer.id_customer) 
          FROM customer_wer JOIN customer
            ON customer_wer.id_customer = customer.id_customer
         GROUP BY customer.id_customer) >= 8;

或者 - 如果可能更好 - 为您的子选择添加一些条件，以便它返回一个唯一的结果。

另一种选择，如果要检查子查询返回的行是否为8，则使用IN运算符：

SELECT COUNT(customer.id_customer) 
  FROM customer JOIN customer_wer
    ON customer.id_customer = customer_wer.id_customer
 WHERE 8 IN (SELECT customer_wer.id_customer
               FROM customer_wer JOIN customer
                 ON customer_wer.id_customer = customer.id_customer
              GROUP BY customer.id_customer);

或使用EXISTS 并将条件>= 8 移动到子查询中：

SELECT COUNT(customer.id_customer) 
  FROM customer JOIN customer_wer
    ON customer.id_customer = customer_wer.id_customer
 WHERE EXISTS (SELECT customer_wer.id_customer
                 FROM customer_wer JOIN customer
                   ON customer_wer.id_customer = customer.id_customer
                WHERE customer_wer.id_customer >= 8
                GROUP BY customer.id_customer);

附言：（根据最新评论添加）

SELECT COUNT(c1.id_customer) 
  FROM customer c1
 WHERE 2 <= (SELECT COUNT (cw1.id_customer)
               FROM customer_wer cw1
              WHERE c1.id_customer = cw1.id_customer);