foreign-keys

问题 I want to delete a row/tuple from a parent table, but it is throwing an error message because it has a FOREIGN KEY reference in its child table. However, in my case I want to delete the record only from the parent table and maintain the data in the child table . Is it possible to achieve this? I know the usage of ON DELETE CASCADE, but I want to know if there is a solution for the secenario I described? 回答1: It is possible with some agreements in your data. To maintain child table data you'll

问题 I have two tables in my database: sliders and images . One slider can have many images, so the structure of tables is: --------- -------- | SLIDERS | | IMAGES | --------- -------- | id | | id | --------- -------- | title | | title | --------- -------- | sid | -------- SID is a foreign key associated to id in "SLIDERS" table. In entities I put bidirectional relationships OneToMany and ManyToOne , so fetched Slider result would contain Images Objects that belong to him, and Images would contain

问题 I have tables table_a and table_b and foreign key table_a_id references the primary key id in table_a . Now I have two JPA entities, A and B . A contains a reference to B . public class A { ... @JoinColumn(name="table_a_id") private B b; } This throws exceptions saying that column table_a_id cannot be found in table_a . I know JPA is looking for a foreign key table_a_id in table_a but how can I resolve this problem without moving the foreign key to table_a ? 回答1: You can have @OneToOne

问题 Please have a look at the below table structure. Client table has the foreign key for Provider Table, which is not NULL . Portfolio table has the foreign key for the Client table, which is also not NULL . I need to retrieve all the fields from the Portfolio table, Name of the Client and the Provider Name who is allocated to the Client which is referred by the Portfolio table.. How can I do this in SQL Code? 回答1: Try following query with INNER JOIN. SELECT Portfolio.*,Client.name as "Client

问题 I have three tables Teachers, Student, ViewType table Teachers{ Id uniqueidentifier, Name nvarchar } table Student{ Id uniqueidentifier, Name nvarchar } table ViewType{ Id uniqueidentifier, Type String } Note: Let's say for the example that ViewType is not a regular look up table. it contains data of how to present the Teacher or Student in the ui therefor shouldn't be in the Teacher or Studenttable model. Is there a way to create a foreign key to two tables where a key is enforced to from

问题 I have a scenario like this: CREATE TABLE `Users` ( `IdUser` bigint(20) NOT NULL PRIMARY KEY ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE `MainTable` ( `IdLite` bigint(20) NOT NULL PRIMARY KEY ) ENGINE=InnoDB DEFAULT CHARSET=utf8; CREATE TABLE `LinkedTable` ( `IdUser` bigint(20) NOT NULL, `IdLite` bigint(20) NOT NULL, PRIMARY KEY (`IdUser`, `IdLite`), FOREIGN KEY (`IdUser`) REFERENCES `Users` (`IdUser`), FOREIGN KEY (`IdLite`) REFERENCES `MainTable` (`IdLite`) ) ENGINE=InnoDB DEFAULT

问题 I have two tables,one is Information and another is WorkDetailsTable . The flow of activity is Information.java>>WorkDetailsTable.java by intent. When button in Information.java has clicked, it will save all the data and pass to WorkDetails.java . Information.java Button button=(Button)findViewById(R.id.button5); button.setOnClickListener(new View.OnClickListener() { public void onClick(View arg0) { Intent intent = new Intent(context, WorkDetailsTable.class); intent.putExtra("a",a); intent

问题 What do I have to do to have Entity Framework immediately sync the foreign key and the navigation property whenever either is changed, not just after calling SaveChanges ? public class Model { [Key] public int ModelId { get; set; } public string Name { get; set; } public virtual Model Other { get; set; } [ForeignKey("Other")] public int OtherId { get; set; } // Other regular mapped properties not shown } I'm using EF 6.1.3 Code First. Change tracking is not explicitly deactivated and should

问题 I am new in code igniter. Here is what I am trying to do. I have lists of products stored in database table name products. For each products i need to insert multiple images. I have created two tables, products and productimage. I have made the product_id of table productimage the foreign key, referencing the product_id of table products. Now i want to save the datas from form. Here is what i did previously Saving images in a single row by imploding But it became quite difficult for me to

问题 I have two table second and third second_id(PRIMARY KEY) second_name 1 ......... 2 ....... 3 ......... third_id(PRIMARY KEY) third_name second_id(FOREIGN KEY for second.second_id) 1 ..... 1 2 ..... 1 3 ..... 1 4 ..... 2 5 ..... 2 Now i want to delete a row from second where second_id=2 ( DELETE FROM second WHERE second_id=2 ) but it does not work. It says Successful 0 row(s) affected What is more is, it happened after i append a foreign key to third.second_id (i added foreign key after table