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Why do foreign key constraints fail?
The error comes when you are trying to add a row for which no matching row in in the other table. “Foreign key relationships involve a parent table that holds the central data values, and a child table with identical values pointing back to its parent. The FOREIGN KEY clause is specified in the child table.
What is a foreign key constraint error?
The error message itself showing there is a foreign key constraint error, which means you are deleting a parent table where the child table contains the Primary table identifier as a foreign key. To avoid this error, you need to delete child table records first and after that the parent table record.
How do you fix the alter table statement conflicted with the foreign key constraint?
- Make sure the table must empty or The column data should match.
- Make sure it is not null.
- If the table contains do not go to design and change, do it manually. alter table Table 1 add foreign key (Column Name) references Table 2 (Column Name) alter table Table 1 alter column Column Name attribute not null.
What problems do foreign keys introduce?
Foreign key problems. Many database users encounter foreign key errors, often due to referential integrity problems. A foreign key might point to data that no longer exists, or the foreign key’s data type doesn’t match the primary key data type, eroding referential integrity.
Can a foreign key be null?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.
How do I find foreign key constraints in SQL Server?
- Open the Table Designer for the table containing the foreign key you want to view, right-click in the Table Designer, and choose Relationships from the shortcut menu.
- In the Foreign Key Relationships dialog box, select the relationship with properties you want to view.
What does foreign key mismatch mean?
Mismatched data types
When you’re using a foreign key to reference a column in another table, the datatypes of both tables have to be the same. For example, if the referencing column where you’re declaring the foreign key uses the INT data type, then the referenced column must be INT as well.
How do you break a foreign key constraint?
- Expand your database view.
- Right Click on Table which has foreign key constraint. …
- Right click on the column which has the foreign key reference. …
- A list of relationships will appear (if you have one) in a pop up window.
- From there you can delete the foreign key constraint.
How can avoid foreign key constraint in SQL?
- In Object Explorer, expand the table with the constraint and then expand the Keys folder.
- Right-click the constraint and select Modify.
- In the grid under Table Designer, select Enforce Foreign Key Constraint and select No from the drop-down menu.
- Select Close.
How will you change the table to add FOREIGN KEY constraint in SQL?
The syntax for creating a foreign key using an ALTER TABLE statement in SQL Server (Transact-SQL) is: ALTER TABLE child_table ADD CONSTRAINT fk_name FOREIGN KEY (child_col1, child_col2, … child_col_n) REFERENCES parent_table (parent_col1, parent_col2, … parent_col_n);
What is Nocheck in SQL Server?
The default is WITH NOCHECK which means that existing data is not checked when re-enabling the constraint.
What is a FOREIGN KEY column?
A foreign key is a column (or combination of columns) in a table whose values must match values of a column in some other table. FOREIGN KEY constraints enforce referential integrity, which essentially says that if column value A refers to column value B, then column value B must exist.
What is not true about foreign key?
Q 26 – Which of the following is not true about a FOREIGN KEY constraint? A – It is a referential integrity constraint. B – It establishes a relationship between a primary key or a unique key in the same table or a different table. C – A foreign key value cannot be null.
Why are inconsistent values in foreign keys particularly troublesome?
Why are inconsistent values in foreign keys particularly troublesome? – Relationships will be missing or wrong when foreign key data are coded inconsistently or misspelled.
Why foreign keys are not redundant?
Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that data remains consistent and that the database contains no redundant data. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.
How do you break a foreign key constraint?
- Expand your database view.
- Right Click on Table which has foreign key constraint. …
- Right click on the column which has the foreign key reference. …
- A list of relationships will appear (if you have one) in a pop up window.
- From there you can delete the foreign key constraint.
Why foreign keys are not redundant?
Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that data remains consistent and that the database contains no redundant data. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.
What is not true about foreign key?
Q 26 – Which of the following is not true about a FOREIGN KEY constraint? A – It is a referential integrity constraint. B – It establishes a relationship between a primary key or a unique key in the same table or a different table. C – A foreign key value cannot be null.
Why do we use foreign key constraints?
Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table. For example, if the row for a salesperson is deleted from the Sales.
sql – mysql Foreign key constraint is incorrectly formed error – Stack Overflow
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mysql Foreign key constraint is incorrectly formed error
Although the other answers are quite helpful, just wanted to share my experience as well.
I faced the issue when I had deleted a table whose id was already being referenced as foreign key in other tables (with data) and tried to recreate/import the table with some additional columns.
The query for recreation (generated in phpMyAdmin) looked like the following:
CREATE TABLE `the_table` ( `id` int(11) NOT NULL, /* No PRIMARY KEY index */ `name` varchar(255) NOT NULL, `name_fa` varchar(255) NOT NULL, `name_pa` varchar(255) NOT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8; … /* SOME DATA DUMP OPERATION */ ALTER TABLE `the_table` ADD PRIMARY KEY (`id`), /* PRIMARY KEY INDEX */ ADD UNIQUE KEY `uk_acu_donor_name` (`name`);
As you may notice, the PRIMARY KEY index was set after the creation (and insertion of data) which was causing the problem.
Solution
The solution was to add the PRIMARY KEY index on table definition query for the id which was being referenced as foreign key, while also removing it from the ALTER TABLE part where indexes were being set:
MySQL FOREIGN KEY constraint is incorrectly formed
I have the following table definition:
CREATE TABLE `async_task` ( `idasync_task` bigint(20) unsigned NOT NULL AUTO_INCREMENT, `idasync_type` int(10) unsigned NOT NULL, `priority` tinyint(3) NOT NULL, `status` enum(‘todo’,’doing’,’failed’) NOT NULL DEFAULT ‘todo’, `iduser` int(11) NOT NULL, `date_added` datetime NOT NULL, PRIMARY KEY (`idasync_task`), KEY `priority_id` (`priority`,`idasync_task`), KEY `status_type` (`status`,`idasync_type`) ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
I’ve added a column to the my notification table which I want to point to async_task :
ALTER TABLE `notification` ADD COLUMN `async_task_id` BIGINT(20)
And when I add the following foreign key:
ALTER TABLE `notification` ADD CONSTRAINT `fk_notification_async_task` FOREIGN KEY (`async_task_id`) REFERENCES `async_task`(`idasync_task`);
I get:
ERROR 1005 (HY000): Can’t create table `my_database`.`#sql-182_2d` (errno: 150 “Foreign key constraint is incorrectly formed”)
I’ve looked elsewhere but only find the errors as being:
The table you’re referencing is not created (not the case) The table you’re referencing is not InnoDB (not the case, both notification and async_task are InnoDB) You’re not referencing the entire primary key (not the case, the only primary key is the ID column).
What else could it be?
Khắc phục lỗi “Foreign key constraint is incorrectly formed” QA Bug
Lỗi “Foreign key constraint is incorrectly formed” có rất nhiều nguyên nhân xảy ra như loại column không match với nhau, tên của khóa ngoại (Foreign key) không phải là duy nhất, bạn không thể tạo khóa ngoại với ON UPDATE SET NULL nếu cột không cho phép giá trị NULL …
Hướng dẫn xử lý Foreign key constraint is incorrectly formed
Nguyên nhân 1: Type column không khớp với nhau
Đây là nguyên nhân rõ ràng nhất. Nếu bạn cố gắng tạo khóa ngoại từ cột INT(10) sang cột SMALLINT(5) , bạn sẽ gặp lỗi ở trên.
Chỉ cần đảm bảo rằng các loại cột giống hệt nhau, bao gồm cả UNSIGNED flag.
Nguyên nhân 2: Tên khóa ngoại không phải duy nhất
Bạn có tin hay không, nhưng tên của các khóa ngoại phải là duy nhất trong cùng một cơ sở dữ liệu. Tùy thuộc vào phiên bản MySQL của bạn, bạn có thể gặp lỗi:
Can’t create table ‘mydatabase.#sql-798_ffe95d’ (errno: 121) Duplicate key on write or update
Nguyên nhân thứ 3: Không thể ON DELETE hay ON UPDATE
Bạn không thể tạo khóa ngoại với ON UPDATE SET NULL nếu cột không cho phép giá trị NULL .
Xem thêm.
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