Mysql 如何正确创建外键？

我有下面的SQL

mysql-sql> create table Employees (Emp_ID int not null primary key, Emp_Name varchar(20),Hours_LoggedIn int);
Query OK, 0 rows affected (0.09 sec)
mysql-sql> insert into Employees (Emp_ID,Emp_Name,Hours_LoggedIn) values (001,'Dinesh',9);
Query OK, 1 row affected (0.03 sec)
mysql-sql> insert into Employees (Emp_ID,Emp_Name,Hours_LoggedIn) values (002,'Shruthi',8);
Query OK, 1 row affected (0.02 sec)
mysql-sql> insert into Employees (Emp_ID,Emp_Name,Hours_LoggedIn) values (003,'Sukanya',7);
Query OK, 1 row affected (0.00 sec)
mysql-sql> insert into Employees (Emp_ID,Emp_Name,Hours_LoggedIn) values (004,'Varun',6);
Query OK, 1 row affected (0.00 sec)
mysql-sql> insert into Employees (Emp_ID,Emp_Name,Hours_LoggedIn) values (005,'Karthi',5);
Query OK, 1 row affected (0.01 sec)


mysql-sql>  create table Project (Project_ID int not null primary key,Project_Name varchar(20),Project_Manager varchar(20),CostPerHour int,Minimum_LoggedInHours int,foreign key(Minimum_LoggedInHours) references Employees(Hours_LoggedIn));

我得到下面的错误

ERROR: 1215 (HY000): Cannot add foreign key constraint

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我需要用Hours_LoggedIn作为FK。有人能帮我找到正确的代码吗？

MySQL需要索引外键引用的任何列。请尝试以下操作：

ALTER TABLE Employees ADD INDEX hours_logged_in_idx(Hours_LoggedIn);

在创建项目表之前。请注意，由于小时数不一定是唯一的，因此这可能会导致意外行为