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Redefinitions

A redefinition is a column definition or a set of column definitions that provides an alternate definition for another column. Columns in a redefinition do not add length to a row but redefine columns previously listed in the table. Columns of a redefinition may have a different data type.

Redefinitions are commonly used to describe a table with multiple record types. Leading columns are common to all record types, but trailing columns depend upon the record type that is specified by one or more of the leading common columns. Reference to columns dependent upon the record type must be delayed until predicate(s) that reference the record type column(s) have been evaluated, or attempts to reference data contained in rows of the wrong record type could occur.

To prevent these invalid references, the following rules are required:

• Columns contained in a redefinition are visible only to the CREATE DATACOM VIEW statement. CA Datacom/DB enforces this rule.
• Any predicates required to limit the rows of the view to a certain "record type" or otherwise prevent access to invalidly typed data must appear as the first predicates in the WHERE clause and be RQA-able, where "RQA-able" means that the predicates should be simple and presentable in a RQA (Request Qualification Area) used by Compound Boolean Selection (CBS) logic. For example, LIKE predicates are generally not RQA-able, but predicates that use the basic comparison operators are. For more information on Compound Boolean Selection in CA Datacom/DB, see the CA Datacom/DB Programming Guide. Failure to follow this rule could result in an attempt to process non-data-type-conformant data, resulting in "invalid data" errors and various other problems. CA Datacom/DB does not enforce this rule. The user must enforce this rule, that is to say, because CA Datacom/DB has no way of knowing how users distinguish which rows belong to what record type or whether there are separate record types at all, users must supply predicates in the CREATE DATACOM VIEW as needed to prevent CA Datacom/DB from retrieving rows of, for example, "record type B" when reading from the "record type A" view.

  Note: If the data does not contain alternate record types, no predicates to distinguish between record types are needed.

Following are additional rules:

• The FROM clause of the DATACOM VIEW may only reference a single database table. A DATACOM VIEW can therefore not be nested. However, queries and standard views may join DATACOM VIEWS, and those views may be nested.
• See Additional Items to Consider for additional rules that apply to any arrays referenced by the view.