Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. It only takes a minute to sign up. I was looking at the database sample that is provided by the DB2 server installaion. This database is the one created when running db2samle. I noticed that the table "ACT" contains a primary key with a references to it self, see sql below, and was wondering if there are any logic reason for this?
As noted from the DB2 Information Center :.
The delete rule of a referential constraint applies only when a row of the parent table is deleted. More precisely, the rule applies only when a row of the parent table is the object of a delete or propagated delete operation defined belowand that row has dependents in the dependent table of the referential constraint. Consider an example where P is the parent table, D is the dependent table, and p is a parent row that is the object of a delete or propagated delete operation.
The delete rule works as follows:.Genetics worksheet solutions answers
Any table that can be involved in a delete operation on P is said to be delete-connected to P. Thus, a table is delete-connected to table P if it is a dependent of P, or a dependent of a table to which delete operations from P cascade. So, while it appears like this may prevent records from being deleted from the table, it doesn't. It just prevents records from being deleted from the table if there are records in the child table.
It is fanciness that serves absolutely no purpose. I tested that myself against the sample database and it ran fine. You will find that it does in fact delete the record just added. I don't know how db2 works, but I think this is making it so rows can never be deleted once added.
A value cannot be updated or deleted when a row exists in a foreign key table that references the value in the referenced table. Similarly, a row cannot be deleted as long as there is a reference to it from a foreign key table. So restrict says the row can only be deleted if the destination key does not exist, and since it is pointing at itself it will always exist, therefor it makes it so you can add and edit a row, but you can't delete it.
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Asked 6 years, 10 months ago. Active 6 years, 10 months ago. Viewed 11k times. I can't see how that is possible, as the primary key column requires distinct not null values, meaning that the foreign key constraints can only point to the row itself.The script is intended to be run from the db2 command line.
The JUNK column is used to have a realistic row length. Without this column the table would become unrealistically small and many demonstrations would not work.SQL Tutorial - Create Table with Constraints
Random data is filled into the table, with exception to my entry, that is updated after the insert. Table statistics are gathered so that the optimizer knows a little bit about the table's content. If you are getting an error, try adding the schema name. Although that gives decent performance, it's better to use the index that supports the primary key:. Key expression: "1". Whereas reason code 5 means : " The key expression included a user-defined function. He is on a mission to introduce developers to the evolution of SQL in the 21st century.
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DB2 Alter Table Add Primary Key
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Stickers, coasters, books and coffee mugs. All you need for learning. Shop now. Deutsch English. The new records are randomly assigned to the subsidiaries 1 through The table and index are analyzed again to make the optimizer aware of the grown data volume. Dropping the primary key automatically drops the automatically created index supporting it. Adding the new primary key makes use of the new index automatically.
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Table of Contents.A table can be visualized as a two-dimensional arrangement of data that consists of rows and columns. The row is the horizontal part containing one or more columns. The column is the vertical part containing one or more rows of data of one data type.
All data for a column must be of the same type. A table in SQL is a keyed or non-keyed physical file. You provide a name for the table. The definition includes the names and attributes of its columns. The definition can include other attributes of the table, such as the primary key. Creating a table A table can be visualized as a two-dimensional arrangement of data that consists of rows and columns. Adding and removing constraints Constraints can be added to a new table or to an existing table.150cc go kart engine with reverse
Referential integrity and tables Referential integrity is the condition of a set of tables in a database in which all references from one table to another are valid.
Check pending Referential constraints and check constraints can be in a check pending state, where potential violations of the constraints exist.
Parent topic: Data definition language. Related concepts : Data types.In this article, I will explain the multiple ways to create table. Lot of people wants to know about sql create table with primary key with syntax and example. In this article I explain the sql create table with primary key with real world examples. I will explain multiple ways to create table in SQL also. Table is nothing but the collection of records.
User needs to follow specific syntax for creating table in sql. To create table in oracle the user should have the create table privileges. There are so many ways of creating table. User can create table using primary key or unique key also.
User can create table without using primary key and with using primary key. Here while creating table column name is any name and datatype is SQL supported datatype. User needs to give the size of the datatype according to datatype. This is simple example of creating table without any constraints.
DB2 Create Table
Constraints are very important for every table to identify the uniqueness of the records as well as identifying the relationship between different tables. T here is second way to create the table with primary key.
User directly needs to add the constraint while creating table with primary key. The syntax is bit different for creating table with primary key. In Mysql the Constraint keyword is not used. Instead of that user needs to add primary key keyword directly and in bracket give the column name. So using two different syntax user can create the table in different enviroments and databases. Sometimes user needs to add the constraint after creating the table.
Sometimes user forget to add the constraints to the table. There is some specific way to add the constraint to the table. Hope this article is helpful to users for create table using primary key. This is the most common question for all the programmer and database developers. Your email address will not be published.
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Skip to content. Leave a Reply Cancel reply Your email address will not be published. Show purposes Show vendors.Primary key, as you all know is the attribute that can be used to uniquely identify a row in a table.
That is primary key is also a unique key. On the other hand, Foreign keys depend on primary key values. That means any value in a foreign key column also exist in the primary key column which foreign key depends on.Cmh
There are two ways to define primary keys, while creating the table and after creating the table by altering the table. The unique key definition on the column userName has nothing to do with the primary key definition.
It is just an extra information to show how to force column values to be unique. Notice that if you use second way, you have to explicitly create a unique index on the primary key column before defining it as a primary key column. Int the first way, the unique index is created implicitly. When it comes to foreign keys, you have to be sure that the primary key is already exist. The Users table is already created in the previous SQL code. Again you can declare foreign keys in two ways.
While creating the table and after creating the table by altering the table. Cascade means that if the primary key is deleted, the foreign key which references to primary key is also deleted. No Action means there will be no action on the Foreign key record, which will make data structure a bit awkward.
Restrict means that you can not delete the primary key record, if there is an associated foreign key record. Physically impossible.
Db2 Foreign Key
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Notify me of new posts via email. How to rule the DB2!Summary : in this tutorial, you will learn how to use the Db2 foreign key constraint to enforce the referential integrity between the data across tables. In this diagram, each contact may have zero or many phones such as home phone, work phone, and emergency phone.
However, each phone must belong to one and only one contact. The relationship between the contacts and phones is one-to-many. For each row in the phones table, you can always find a corresponding row in the contacts table. But the current setup does not ensure this relationship. Furthermore, if you delete a contact, all the phones of the deleted contact will remain in the phones table. The rows in the phones table that does not have corresponding rows in the contacts table are called orphaned rows.
In order to enforce the relationship between contacts and phones tables, you need to use a foreign key constraint. A foreign key is a column or group of columns in a table that uniquely identifies a row in another table.
The foreign key constraints define foreign keys. Because for each phone in the phones table, you can find a corresponding contact in the contacts table.Toyota hilux obd2 protocol
The contacts table is called the parent table to which the foreign key references. The phones table is called the child table or dependent table to which the foreign key constraint is applied.
In the database world, referential integrity is a mechanism to ensure that the relationship of data between tables remains consistent. And to enforce the referential integrity, you use foreign key constraints. Therefore, foreign key constraints are also known as referential integrity constraints or referential constraints. If you omit it, Db2 will generate a name for the foreign key constraint.
Third, specify the name of the parent table and a list of comma-separated columns to which the foreign key columns reference. Db2 determines whether or not to delete the rows in the child table based on the following options:. First, insert a new contact into the contacts table:. Then, delete the contact id 1 from the contacts table.
The following statement creates a new table called members :. Suppose each member can have one or many favorite books and each book may belong to favorite lists of many users.
The relationship between members and books are many-to-many. We often use these kinds of tables to manage the many-to-many relationship. In this tutorial, you have learned about Db2 foreign key and how to use the foreign key constraint to enforce referential integrity.
The db2tutorial. What is a foreign key? Was this tutorial helpful?The constraint name cannot be the same as any other constraint specified within an ALTER TABLE statement, and must be unique within the table this includes the names of any referential integrity constraints that are defined.
Existing data is checked against the new condition before the statement succeeds. DB2 Version 9. About this task. To define another column, or set of columns, as the primary key, the existing primary key definition must first be dropped, and then recreated. When a foreign key is added to a table, packages and cached dynamic SQL containing the following statements might be marked as invalid: Statements that insert or update the table containing the foreign key Statements that update or delete the parent table.
Restriction: After you define informational constraints on a table, you can only alter the column names for that table after you remove the informational constraints. Query results might be unpredictable if the data does not actually conform to the constraint. Parent topic: Constraints.
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