题解 | #在audit表上创建外键约束，其emp_no对应employees_test表的主键id#
在audit表上创建外键约束，其emp_no对应employees_test表的主键id
http://www.nowcoder.com/practice/aeaa116185f24f209ca4fa40e226de48
alter table audit add foreign key audit(emp_no) references employees_test(ID);
标准语法， 参考
To create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
To drop a FOREIGN KEY constraint, use the following SQL:
MySQL:
ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;
DROP FOREIGN KEY FK_PersonOrder;
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