Many to one relationship in rdbms ipc

One-to-many relationships

many to one relationship in rdbms ipc

Here's an implementation in Python to get the discussion started, although I'm sure I wasn't trying to say that it is "difficult" to express many to many relationships, but You're not suggesting that your RDBMS is your IPC mechanism, I hope?. The schema mapping module maps the relational database schema of the . a "many-to-one" relationship (or, equivalently, a "one-to-many" relationship). Database designs are closely related to database relationships, the association between two columns in one or more tables. Relationships are.

many to one relationship in rdbms ipc

Example of one-to-many relationship. One-to-Many relationships can also be viewed as Many-to-One relationships, depending on which way you look at it. Each customer can only be assigned one city. One city can be assigned to many customers. Many-to-Many In a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa. A many-to-many relationship could be thought of as two one-to-many relationships, linked by an intermediary table.

This table is used to link the other two tables together. It does this by having two fields that reference the primary key of each of the other two tables. The following is an example of a many-to-many relationship: This is the Relationships tab that is displayed when you create a relationship Microsoft Access.

Using Object Views in Applications Data in the rows of an object view may come from more than one table, but the object view still traverses the network in one operation. You can manipulate it like any other native structure. You can refer to object views in SQL statements in the same way you refer to an object table. You can also define object views on object views. You can access object view data on the client side using the same OCI calls you use for objects from object tables.

When you update or flush an object to the database in an object view, the database updates the object view.

Nesting Objects in Object Views An object type can have other object types nested in it as attributes. If the object type on which an object view is based has an attribute that itself is an object type, then you must provide column objects for this attribute as part of the process of creating the object view. If column objects of the attribute type already exist in a relational table, you can simply select them; otherwise, you must synthesize the object instances from underlying relational data just as you synthesize the principal object instances of the view.

You synthesize, or create, these objects by calling the respective constructor methods of the object type to create the object instances, and you can populate their attributes with data from relational columns specified in the constructor. For example, consider the department table dept in Example You might want to create an object view where the addresses are objects inside the department objects. That would allow you to define reusable methods for address objects, and use them for all kinds of addresses.

First, create the types for the address and department objects, then create the view containing the department number, name and address. The address objects are constructed from columns of the relational table. The relational table has no column that specifies whether or not the department address is null.

By using the DECODE function, or some other function, to return either a null or the constructed object, the null deptstreet column can be used to indicate that the whole address is null.

One-to-One, One-to-Many Table Relationships in SQL Server

You can select these collections from underlying collection columns or you can synthesize them using subqueries. This section contains the following topics: Next, define a department type with a department number, name, address, and a nested table of employees.

A query on this view could provide the list of departments, with each department row containing the department number, name, the address object and a collection of employees belonging to the department. Multilevel Collections in Object Views Multilevel collections and single-level collections are created and used in object views in the same way.

many to one relationship in rdbms ipc

The only difference is that, for a multilevel collection, you must create an additional level of collections. Example builds an object view containing a multilevel collection. The view is based on flat relational tables that contain no collections. As a preliminary to building the object view, the example creates the object and collection types it uses. In addition, the employee type has a nested table attribute of projects, and the department type has a nested table attribute of employees.

The latter nested table is a multilevel collection. Because the view data is not stored persistently, you must specify a set of distinct values to be used as object identifiers. Object identifiers allow you to reference the objects in object views and pin them in the object cache. If the view is based on an object table or an object view, then there is already an object identifier associated with each row and you can reuse them.

However, if the row object is synthesized from relational data, you must choose some other set of values. You can specify object identifiers based on the primary key. This turns the set of unique keys that identify the row object into an identifier for the object. These values must be unique within the rows selected out of the view, because duplicates would lead to problems during navigation through object references.

Because the underlying relational table has deptno as the primary key, each department row has a unique department number. In the view, the deptno column becomes the deptno attribute of the object type. Once you know that deptno is unique within the view objects, you can specify it as the object identifier.

In the relational case, the foreign key deptno in the emp employee table matches the deptno primary key value in the dept department table. This takes the view or table name that the reference points to, and a list of foreign key values, to create the object identifier portion of the reference that matches a specific object in the referenced view.

The following simple query retrieves all employees whose departments are located in the city of Redwood S: For example, you can create an employee view that has a reference to the employee's department, while the department view has a list of references to the employees who work in that department.

Different Types of Relations One to Many to One

Note that if the object view has a primary-key based object identifier, the reference to such a view is primary-key based. On the other hand, a reference to a view with system-generated object identifier is a system-generated object reference. This difference is only relevant when you create object instances in the OCI object cache and need to get the reference to the newly created objects. As with synthesized objects, you can also select persistently stored references as view columns and use them seamlessly in queries.

However, the object references to view objects cannot be stored persistently. According to this first query plan, the values in columns eid, ename, did, and dname are retrieved from tables Employee E and Department D for those rows in these tables where E. It is noted that the query plans discussed herein are presented using the well known SQL syntax.

Thus, the meaning and operation of these query plans will be apparent to persons skilled in the relevant art. However, if there are multiple Emp objects associated with a Dept object one-to-manythe first query plan of FIG.

In the case of one-to-many relationships, therefore, it is better in most cases to perform two queries on the RDBMSa first which retrieves the tuples corresponding to the Emp objects, and a second which retrieves the tuples corresponding to the Dept objects that relate to the Emp objects chosen.

This is indicated by the second query plan of FIG. Note that the first and second queries retrieve the primary key of Department D. These two streams are sorted by the join attribute D. This merge, which operates to match Employee tuple data with corresponding department tuple data i. Note that, with this second query plan shown in FIG. Let us now consider a case where a left join cannot be avoided.

An example is given in FIG. In this example, unlike in the earlier case of FIG. However, the answer to the object query in FIG. Therefore, a left outer join must be used here. Furthermore, even if left join support is available in the RDBMS, executing the left join directly will result in duplicate data retrieval for one-to-many and many-to-many relationships and hence is not desirable to use it in those cases.

So, the only case where a native left join may be used is the one-to-one case. In other cases, it is necessary to represent the object query using two relational, non-left join queries. Two possible query plans, each comprising two queries, are shown in FIG.

In both the first and second query plans of FIG. While each Department tuple is retrieved exactly once in the second query plan of FIG. Dept relationship is one-to-many. Therefore, the first query plan is suitable if the Emp. Dept is a one-to-one relationship and the second query plan is suitable if Emp. Dept is a one-to-many relationship.

Note that in the second plan, the final merge to find the related Employee and Department tuples is done in the gateway The query plans used in the previous examples FIGS. An example of a many-to-many relationship and more optimal query plans are given in FIG.

In the case of a many-to-many relationship, an intermediate table is involved in the join. In this example, two query plans are shown. The first plan shows the simplest way to do the left join and is likely to be inefficient for the many-to-many relationship case discussed here since it could get the same Employee and Project tuples multiple times.

The second query plan contains three queries and is very efficient in terms of data retrieval even though it results in two merges and also a sort of the intermediate tables's tuples in the gateway Keeping in mind the tradeoffs discussed above, the inventor has developed a set of rules for generating query plans for various types of relationships.

As should be clear from the discussion above, these query plans represent different ways that the second query generator generates the set of relational queries for a prefetch path component. In this embodiment of the invention, the query plans were developed according to the following principle: The inventor has defined three rules for one-to-one relationships given in FIG.

These rules have been optimized in such a way that the minimum number of tuples are retrieved from the RDBMS Also, the query plans chosen would have been different if the inventor had decided to minimize the number of relational queries rather than minimizing duplicate data retrieval. The nature of these rules, and the manner in which these rules are applied by the second query generatorshall now be described with reference to a flowchart contained in FIG.

many to one relationship in rdbms ipc

This flowchart represents the manner in which the second query generator translates a prefetch path component into one or more relational queries.

In other words, flowchart represents the manner in which the second query generator performs step of FIG. In stepthe second query generator determines whether the relationship defined by the prefetch path component is a 0,1 -to- 0,1 relationship.

The second query generator does this by querying the schema mapping module to obtain arity information. Given the prefetch path information, the schema mapping module has the ability to provide information regarding foreign keys associated with a relationship as well as information on whether the type of a prefetch component is a single pointer or a set of pointers.

The arity can be inferred by looking at the foreign key and type information for the prefetch component.

many to one relationship in rdbms ipc

If the second query generator determines that the prefetch path component defines a 0,1 -to- 0,1 relationship, then step is performed. In stepthe second query generator determines whether referential integrity is enforced. Referential integrity is defined as the mechanism by which a DBMS system maintains integrity of the relationships between foreign keys and primary keys, i.

All foreign keys will therefore point to valid primary keys if referential integrity is automatically maintained by the system. The second query generator performs step by appropriately querying the schema mapping module If the second query generator determines that referential integrity is enforced, then step is performed.

In stepthe second query generator generates relational queries corresponding to the prefetch path component according to a Rule 1a. The form of the resulting relational queries for a given object query, a prefetch path, a relational schema, and an object schema is shown in FIG. As indicated by FIG. The first query operates to retrieve from the RDBMS data to construct objects of the top-level class which are not related to i. The second query operates to retrieve from the RDBMS data to construct objects of the top-level class which are related to i.

The second query also operates to retrieve from the RDBMS data to construct these objects along the prefetch path. The manner in which the second query generator generates relational queries in accordance with Rule 1a shall now be described with reference to a flowchart in FIG.

In stepthe second query generator queries the schema mapping module to identify the table in the RDBMS that maps to the queried class also called the top-level class. For reference purposes, this is called the top-level table. Also in stepthe second query generator queries the schema mapping module to identify the columns of the top-level table which map to the base attributes in the queried class.

Further in stepthe second query generator generates the "select" statement of the first query such that this "select" statement lists the columns of the top-level table which map to the base attributes in the queried class. In stepthe second query generator generates the "from" statement of the first query so that this "from" statement references the top-level table.

In stepthe second query generator generates the "where" statement of the first query so that this "where" statement operates to retrieve tuples of the top-level table where 1 the condition list as specified in the translated object query are satisfied; and 2 the foreign key in the top-level table associated with the table indicated by the prefetch path component is null. Class Y maps to table Yr.

Thus, table Yr is indicated by the prefetch path component X. The top-level table, Xr, includes a foreign key Yfk which is associated with table Yr. Thus, this second condition specifies that tuples having Yfk equal to null are retrieved from table Xr subject to the first condition.

For reference purposes, the class that lies along the prefetch path i. The table that maps to the prefetch class i.

many to one relationship in rdbms ipc

In stepthe second query generator generates the "select" statement for the second query such that this "select" statement lists the columns of the top-level table which map to the base attributes in the queried class, and also lists the columns of the prefetch table which map to the base attributes in the prefetch class. The second query generator may query the schema mapping module to obtain schema mapping information to perform step In stepthe second query generator generates the "from" statement of the second query so that this "from" statement references the top-level table and the prefetch table.

In stepthe second query generator generates the "where" statement of the second query so that this "where" statement operates to retrieve tuples of the top-level table and the prefetch table where 1 the condition list as specified in the translated object query are satisfied; and 2 the pertinent foreign key in the top-level table is equal to the primary key in the prefetch table.

The "pertinent foreign key" is the foreign key that is associated with the prefetch table. The primary key in the prefetch table Yr is assumed to be y1. Thus, this second condition specifies that tuples having Yfk equal to y1 are retrieved from tables Xr and Yr subject to the first condition. Referring again to FIG. In stepthe second query generator generates relational queries corresponding to the prefetch path component according to a Rule 1b. An example of Rule 1b is discussed above as the first query plan in FIG.

The manner in which the second query generator generates relational queries in accordance with Rule 1b shall now be described with reference to a flowchart in FIG.

Oracle IO on linux: database writer IO and wait events | Frits Hoogland Weblog

In stepthe second query generator generates the "select" statement of the first query such that this "select" statement lists the columns of the top-level table which map to the base attributes in the queried class. The performance of step is similar to the performance of stepdescribed above. In stepthe second query generator generates the "where" statement of the first query so that this "where" statement operates to retrieve tuples of the top-level table where 1 the condition list as specified in the translated object query are satisfied; and 2 the foreign key in the top-level table associated with the prefetch table is not equal to the primary key in the prefetch table.

The prefetch table is Yr, and Yfk is the foreign key in the top-level table Xr which is associated with this prefetch table Yr. It is assumed that the primary key in the prefetch table Yr is y1.

Thus, this second condition specifies that tuples having Yfk not equal to any y1 are retrieved from table Xr subject to the first condition. In stepthe second query generator determines whether the relationship defined by the prefetch path component is a 1,1 -to- 1,1 relationship. The operation of step is similar to the operation of step If the second query generator determines that the prefetch path component defines a 1,1 -to- 1,1 relationship, then step is performed.

In stepthe second query generator generates relational queries corresponding to the prefetch path component according to a Rule 1c. An example of Rule 1c is presented in FIG. This query operates to retrieve from the RDBMS data to construct objects of the top-level class which point to objects along the prefetch path, and to construct these objects along the prefetch path.

The manner in which the second query generator generates relational queries in accordance with Rule 1c shall now be described with reference to a flowchart in FIG. In stepthe second query generator generates the "select" statement of the query such that this "select" statement lists the columns of the top-level table which map to the base attributes in the queried class, and the columns of the prefetch table which map to the base attributes of the prefetch class.

In stepthe second query generator generates the "from" statement of the query so that this "from" statement references the top-level table and the prefetch table. In stepthe second query generator generates the "where" statement of the query so that this "where" statement operates to retrieve tuples of the top-level table and the prefetch table where 1 the condition list as specified in the translated object query are satisfied; and 2 the pertinent foreign key in the top-level table is equal to the primary key in the prefetch table.

In stepthe second query generator determines whether the relationship defined by the prefetch path component is a one-to-many relationship. The operation of step is similar to the operation of steps and If the second query generator determines that the prefetch path component defines a one-to-many relationship, then step is performed. In stepthe second query generator generates relational queries corresponding to the prefetch path component according to a Rule 2.

An example of Rule 2 is presented in FIG. The first query operates to retrieve from the RDBMS data to construct objects of the top-level class that satisfy the conditions present in the original object query. The tuples that are retrieved by the first query are sorted according to the foreign key in the top-level table that is related to i.

The second query operates to retrieve from the RDBMS data to construct objects that are along the prefetch path and that are related to i. The tuples that are retrieved by the second query are sorted according to the primary key of the prefetch table. The gateway merges the tuples retrieved by the first and second queries. The manner in which the second query generator generates relational queries in accordance with Rule 2 shall now be described with reference to a flowchart in FIG.

In stepthe second query generator generates the "select" statement of the first query such that this "select" statement lists the columns of the top-level table which map to the base attributes in the queried class, and also lists the foreign key in the top-level table associated with the prefetch table.

The second query generator queries the schema mapping module to obtain schema mapping information to perform step In stepthe second query generator generates the "where" statement of the first query so that this "where" statement operates to retrieve tuples of the top-level table where the conditions related to the queried class and specified in the translated object query are satisfied. In stepthe second query generator generates an "order by" statement to order the tuples retrieved from the top-level table by the foreign key described above with respect to step In stepthe second query generator generates the "select" statement for the second query such that this "select" statement lists the columns of the prefetch table which map to the base attributes in the prefetch class.

In stepthe second query generator generates the "where" statement of the second query so that this "where" statement operates to retrieve tuples of the prefetch table where 1 the condition list as specified in the translated object query are satisfied; and 2 the pertinent foreign key in the top-level table is equal to the primary key in the prefetch table.

Thus, this second condition specifies that tuples having Yfk equal to y1 are retrieved from table Yr subject to the first condition. In stepthe second query generator generates an "order by" statement to order the tuples retrieved from the prefetch table by the primary key of the prefetch table. Note that the flowchart is for the case when the foreign key is present in the top-level table and points to the prefetch table.

In the case where the foreign key is present in the direction opposite to the prefetch path i. The foreign key will be fetched in the query generated for the prefetch table and not in the query for the top-level table as described above in step Also, step will need to order the top-level table's tuples by their primary key.

In stepthe additional join i. Finally, step should be changed to order the tuples of the prefetch table by the foreign key. The merging will be done using the primary key in the top-level table with the foreign key in the prefetch table. Note that the above applies for both 1-to-1 and 1-to-many cases but not for the many-to-many case.

In this case, step is performed. In stepthe second query generator generates relational queries corresponding to the prefetch path component according to a Rule 3. An example of Rule 3 is presented in FIG. The tuples that are retrieved by the first query are sorted according to the primary key of the top-level table. The intermediate table stores the foreign key from the top-level table that references the prefetch table.

These foreign keys are called the top-level foreign keys for reference purposes. The intermediate table also stores the foreign key from the prefetch table that references the top-level table. These foreign keys are called the prefetch foreign keys for reference purposes.