IMS Essentials

Overview of IMS Databases

An IMS database is a large, centralized collection of information comprising one or more physical files that can be accessed by the SAS/ACCESS interface to IMS. An IMS database is a hierarchical database. Information is structured in records that are subdivided into a hierarchy of related segments.

A record is a root segment and all of its dependent segments. Segments are further subdivided into fields. The data in any record relates to one entity. Ideally, information in the database records is subdivided into segments and fields on some logical basis, either by the inherent structure of the data or by consideration of the uses to which the data will be put.

The term hierarchical implies that there are levels of data. You can think of a hierarchical database as one that starts at the top with general information about the item, individual, or case. As you progress from level to level down through the hierarchy, more and more information related to the general information at the top level is given. Each level in the hierarchy has one or more segments.

In some ways the structures of IMS databases and tabular files (such as SAS data sets) are comparable, but in other ways they differ. For example, database fields and data set variables are similar, and database records are like data set observations because both contain data about one entity. At the same time, however, database records differ from data set observations because subsets of records can be accessed while you cannot access a subset of a data set observation. The observation is stored and accessed as a unit.

A tabular file has nothing comparable to a segment. The concept of data segments is one of the things that makes a hierarchical database different from a SAS data set and other tabular files.

Consider banking data as an example. Customer information maintained by a bank might include the following:

name	checking account debits
Social Security number	checking account credits
address	savings account number
home phone	savings account balance
work phone	savings account date of last statement
checking account number
checking account balance	savings account balance at last statement
checking account date of last statement	savings account debits
checking account balance at last statement	savings account credits

If this information is stored in a tabular file, each item of information is a variable, and all of the variables for any given person comprise one observation. You can visualize the layout of banking data in a tabular file as shown in the following figure.

Tabular File Structure

[Tabular File Structure]

The rows in the table represent observations (customers), and the columns represent variables. The structure of the file is such that a maximum number of variables for debits and credits must be defined when the file is created. In the previous figure there are variables for up to only three debits and three credits per customer, which presents a problem if a customer has more than three debit or credit transactions.

The same data can also be stored in an IMS database but would be structured very differently. For example, the following figure shows one way the banking information could be structured in IMS. The sample database, called ACCTDBD, is used in this document and described in About the Example Data in the Document in Chapter 1, "Introducing the SAS/ACCESS Interface to IMS."

Hierarchical File Structure

[Hierarchical File Structure]

Each block in the figure represents a segment type, which is a grouping of related fields of data. There are three levels in the ACCTDBD database hierarchy and seven segment types. For each database record, the top or first level has only one segment, called the root segment. The root segment in the ACCTDBD database is called CUSTOMER; it contains fields with these data: Social Security number, customer name, address, city, state, country, ZIP code, home phone, and work phone. The segments under the root segment are dependent segments called CHCKACCT, CHCKDEBT, CHCKCRDT, SAVEACCT, SAVEDEBT, and SAVECRDT. Each of the dependent segments contains fields of data, as shown in the following table.

Dependent Segments and Corresponding Fields
DependentSegment	Fields
CHCKACCT	checking account number, current balance, last statement date, last statement balance
CHCKDEBT	checking account debit date and time, amount, description
CHCKCRDT	checking account credit date and time, amount, description
SAVEACCT	savings account number, current balance, last statement date, last statement balance
SAVEDEBT	savings account debit date and time, amount, description
SAVECRDT	savings account credit date and time, amount, description

Segment Occurrences

The hierarchical database structure is useful for storing multiple occurrences of any given element of information, especially if there are varying numbers of occurrences of the data for each record.

Consider the dependent segment SAVEACCT, which contains the following fields:

savings account number
savings account balance
date of last statement
savings account balance at last statement

Different customers can have different numbers of savings accounts; some might have none, others might have two or three. If the data is not segmented, there must be space in each customer's record for the maximum number of savings accounts per customer. With the segmented structure, however, it is possible to have one SAVEACCT segment occurrence for each savings account a customer has. Any segment type can have an unlimited number of segment occurrences. Although the segment types are predefined, the number of segment occurrences is not predefined. Note that each occurrence of a root segment represents a separate record.

Here is an example of how a segment type can have an unlimited number of segment occurrences. A certain customer has two savings accounts. In one month, the customer has two deposits for account number 111 and one deposit and two withdrawals for account number 222. The following figure shows the customer's record.

Sample Record

[Sample Record]

The figure shows seven segment occurrences within three (shaded) segment types.

Segment Relationships

The information in a hierarchical database is subdivided or segmented according to a logical scheme. Moving from top to bottom through the database, there is a relationship between the segments. A segment that is hierarchically dependent on a segment one level up in the hierarchy is said to be the child. The segment on which it is dependent is the parent. CUSTOMER is a parent segment with two children, CHCKACCT and SAVEACCT. CHCKACCT, in turn, is the parent of CHCKDEBT and CHCKCRDT, and SAVEACCT is the parent of SAVEDEBT and SAVECRDT. Segments that share a parent are called siblings; for example, CHCKACCT and SAVEACCT are siblings. Multiple segment occurrences of one segment type with the same parent occurrence are called twins. For example, SAVEACCT 111 and SAVEACCT 222 are twins.

All dependent segments are children but are not necessarily parents. The root segment (CUSTOMER), on the other hand, is a parent if any dependent segments exist, but it is never a child. (It is possible to have a database with no dependent segments, that is, with only one level, the root segment.) In a hierarchical structure, there can be only one parent segment for a child segment.

Segments can also be grouped by paths. Two segments belong to the same path if one is a dependent of the other. You can access multiple segments in a path at the same time. These relationships are shown in the following figure.

Segment Relationships

[Segment Relationships]

Parents:: A, B, E1, E2
Children:: B, E1, E2, C1, C2, D, F1, F2, G, H
Twins:: C1 and C2, E1 and E2, F1 and F2
Siblings:: B, E1, and E2; C1, C2, and D; G and H
Paths:: A, B, and C1; A, B, and C2; A, B, and D; A, E1, and F1; A, E1, and F2; A, E2, and G; A, E2, and H

Path Navigation

You can navigate one path of an IMS database at a time with the interface view engine in Version 7 and later of SAS. That is, you can select items in one path of the database when creating a view descriptor. Consider A Database Path, which shows one path of data shaded. The SAS/ACCESS interface processes each record occurrence from top to bottom and from left to right following these rules:

The first occurrence of a root segment is processed first.
Then, the first child of a root segment in the defined path is processed before twins of the root segment.
Twins are processed after a child (if any) down that path. Twins are processed in order of occurrence. Any child of a twin is processed according to this rule.
After the child and twins are processed for that one path, the next eligible root segment in the path is processed.

Note: No siblings are processed. [cautionend]

The following figure illustrates a path of data in a particular program view. The numbering indicates the order of processing.

A Database Path

[A Database Path]

Fields

There are three types of fields in the segments of an IMS database:

A sequence field (or key field) is a field that identifies and provides access to segments in a database. A sequence field is defined to IMS in the database description (DBD), which specifies characteristics of a database. In some cases, a sequence field sequences twin segment occurrences in ascending order, according to their sequence field values. For example, if the sequence field of the CHCKACCT segment is ACNUMBER, twin CHCKACCT occurrences in a given customer's record are ordered from the lowest to highest account number. Root segments usually have a sequence field, but dependent segments do not necessarily have them.

In a root segment, the sequence field also uniquely identifies the record. In dependent segments, the sequence field can provide unique identification, but this is not required. Root segments might or might not be sequenced by the sequence field, depending on the IMS access method used to store the database.
A search field is defined to IMS in the DBD and is used to search through the database for particular values. For example, CUSTZIP is defined as a search field in the CUSTOMER segment, permitting the SAS/ACCESS interface to IMS to search the database for records containing a specific ZIP code.
An undefined field is not defined to IMS. All fields other than sequence fields and search fields do not have to be defined in the DBD. IMS does not know the format of an undefined field and cannot search for segments based on values in an undefined field. The format of an undefined field is determined by the program that loads the database initially.

The CUSTOMER segment of the ACCTDBD database contains examples of two kinds of fields. There are fields for Social Security number, name, city, state, country, ZIP code, address, home phone, and work phone. The Social Security number field is defined as the sequence field, meaning that it uniquely identifies the record. The name field of CUSTOMER does not uniquely identify a record because customer names might be duplicated. However, because names can be used to search through the database, the name field is defined as a search field, as are the address, city, state, country, ZIP code, home phone, and work phone fields.

The sequence field of a root segment enables direct access to the root segment. The sequence field of a dependent segment does not enable direct access to the record, but IMS finds segments faster when searching on sequence fields rather than search fields.

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