Foreign Key (cascade delete) Foreign Key (set null) SQL Server Loops/Conditionals String Functions Numeric/Math Functions Date/Time Functions Conversion Functions Configuration Functions Advanced Functions This SQL Server tutorial explains how to useForeign Keys with "set null on delete"in SQL Server ...
对于3,需要使用on delete cascade建立外键约束。实验: alter table emp_test add constraint fk_emp_dept_test foreign key(dept) references dept_test(deptno) on delete cascade; delete from dept_test where deptno = 1; 1 row deleted. SQL> select * from dept_test; DEPTNO DEPTNAME --- --- 2 ...
delete from dept where deptno = 1; 我们发现除了dept中的一条数据被删除了,emp中两条数据也被删除了,其中emp中的两条数据是参照了销售部的这条数据的,这就很容易理解on delete cascade了。 接下来我们再来看on delete set null,顾名思义了,这种方式建立的外键约束,当被参照的数据被删除是,参照该数据的那些...
外键定义时的on delete和on update表示在对父表进行delete和update操作时,对子表所在的操作。可定义的子表操作有: cascade:表示当父表发生delete或update操作时,对相应的子表中的数据也进行delete或者update操作 set null:表示当父表发生delete或update操作时,相对的字表中的数据被更新为NULL值,但是子表中相应的列必...
constraint emp1_foreign foreign key(deptno) references dept1(deptno) [on delete cascade] 删除子记录时级联删除主记录 若不写则子记录存在不可删除主记录。 十二、 连接 1、 左连接: 表示左表中指定的内容全部返回 select e.ename,d.dname from emp1 e,dept d ...
默认情况下,Django 的 ForeignKey 模拟了 SQL 约束 ON DELETE CASCADE——换句话说,任何外键指向要删除的对象的对象都会被一起删除。例如: >>> blogs = Blog.objects.all() # This will delete all Blogs and all of their Entry objects. >>> blogs.delete() (5, {'weblog.Blog': 1, 'weblog.Entry...
Applies to: SQL Server (Starting with SQL Server 2014 (12.x)) and Azure SQL Database Set AUTO_CREATE_STATISTICS to ON, and set INCREMENTAL to ON. This sets automatically created stats as incremental whenever incremental stats are supported. The default value is OFF. For more information, see...
默认情况下,Django的ForeignKey使用SQL约束ON DELETE CASCADE,任何具有指向要删除的对象的外键的对象将与它们一起被删除。 像这样:1 2 3 4 >>> blogs = Blog.objects.all() # This will delete all Blogs and all of their Entry objects. >>> blogs.delete() (5, {'weblog.Blog': 1, 'weblog....
Restrict: Nothing gonna be delete if there is a child row Cascade: the child row will be delete / update too Set Null: the child column will be set to null if you delete the parent No action: The child row will not be concern of the delete / update...
在执行SQL的UPDATE语句时,如果出现错误代码1242(Subquery returns more than 1 row),这意味着子查询返回了多于一行的结果,而UPDATE语句期望子查询只返回一行。 基础概念 UPDATE语句:用于修改表中的数据。 子查询:嵌套在另一个查询中的查询,用于提供数据或条件。 错误原因 错误1242通常发生在以下情况: 子查询返回多行...