Question

我尝试在FOREIGN KEY上包括ON DELETE SET NULL约束，但出现错误1215。

    CREATE TABLE branch_supplier (
    -> branch_id INT,
    -> supplier_name VARCHAR(40),
    -> supply_type VARCHAR(40),
    -> PRIMARY KEY(branch_id, supplier_name),
    -> FOREIGN KEY(branch_id) REFERENCES branch(branch_id) ON DELETE SET NULL
    -> );
ERROR 1215 (HY000): Cannot add foreign key constraint

否则，如果我尝试包括ON DELETE CASCADE约束，则不会出现该错误。有人知道发生这种情况的原因吗？

    CREATE TABLE branch_supplier ( 
    -> branch_id INT,
    -> supplier_name VARCHAR(40), 
    -> supply_type VARCHAR(40), 
    -> PRIMARY KEY(branch_id, supplier_name), 
    -> FOREIGN KEY(branch_id) REFERENCES branch(branch_id) ON DELETE CASCADE 
    -> );
Query OK, 0 rows affected (0.26 sec)

Answer 1

这是因为您将branch_id字段作为主键，并且不允许使用Null值。

...
PRIMARY KEY(branch_id, supplier_name),
..

当我尝试添加ON DELETE SET NULL时出现错误，但是当我在SQL（mysql服务器）中添加ON DELETE CASCADE时不出现错误

1 个答案: