如何摆脱错误代码1242

Question

我在My SQL Workbench中有代码行，但是我收到错误：错误代码：1242。子查询返回超过1行。

 SELECT Fname, Lname, Dnum, Pno 
 FROM EMPLOYEE e, PROJECT p, WORKS_ON w 
 WHERE p.Dnum = (SELECT Dno FROM EMPLOYEE WHERE e.Ssn = w.Essn AND w.Pno = p.Pnumber)

我正在尝试显示正在处理未由其部门赞助的项目的员工表。

Answer 1

试试这个

do(onNext:)

使用SELECT Fname, Lname, Dnum, Pno FROM EMPLOYEE e, PROJECT p, WORKS_ON w WHERE p.Dnum in (SELECT Dno FROM EMPLOYEE WHERE e.Ssn = w.Essn AND w.Pno = p.Pnumber); 子句instad in

Answer 2

优化查询：

SELECT Fname, Lname, Dnum, Pno 
FROM EMPLOYEE e
INNER JOIN  WORKS_ON  w ON e.Ssn = w.Essn
INNER JOIN  PROJECT  p  ON  w.Pno = p.Pnumber AND p.Dnum = e.Dno