Defining an IMS segment 3 levels down

Answer

The pure relational approach would be to map 3 tables:

Table 1: Map A by itself with all fields in the segment.
Table 2: Map A+B with only the key from A and all fields from B.
Table 3: Map A+B+C with only key fields from A and B, and all fields from C.

If mapping a single table with all fields from all segments, the result of a query against that table is essentially an inner join of the 3 tables described above without actually having to perform a join.

The bottom line is that your specific needs should drive how you map the data. If your needs are more generic in nature, then the pure relational approach is probably the best way to go. Note that this general approach can be more expensive if joins are performed.

Here are steps for CDA and mapping 3 segements.

When mapping an IMS table in the CDA using the mapping wizard, if the leaf segment is set to a third level segment (Figure 1 below), the 'MAP IMS SEGMENT' window will appear once for each segment in the hierarchy. For each segment, a separate copybook can be selected and the required
columns can be selected to map into the table. Figure 2 below shows the first appearance of the window and we have selected only the root key for mapping.

Here is the generated DDL for a 3 level mapping, with the first 3 columns in highlighted in blue showing they come from 3 different segments.

Generated DDL for 3 level mapping

Figure 2 - Mapping the key-only for the root segment