In the DML of the network (CODASYL DBTG) data model, the RECONNECT command…

Concept — set membership retention in the network (CODASYL DBTG) model. Every set type in the network data model declares a membership class built from two independent parts: an insertion class (AUTOMATIC or MANUAL), which decides when a member record joins a set occurrence, and a retention class (FIXED, MANDATORY or OPTIONAL), which decides how firmly it stays there. The retention class alone governs which navigational DML statements — CONNECT, DISCONNECT, RECONNECT — the DBMS will accept for a member. RECONNECT is defined as the statement that moves an existing member from one occurrence of a set type to another occurrence of the same set type, so it is legal only where a member is permitted to change its owner.

Application. Test each retention class against the single privilege RECONNECT depends on — permission to change a member’s owner occurrence.

1. OPTIONAL retention leaves membership completely under program control: a member may even sit with no owner at all, so it can be detached from one owner and later attached to a different one. A change of owner is permitted, so RECONNECT is available.

2. MANDATORY retention requires that a member always belong to some owner occurrence, but it does not fix which one; RECONNECT performs exactly that one-step transfer from the old owner to the new one. A change of owner is permitted, so RECONNECT is available.

3. FIXED retention binds a member to the owner occurrence it was first linked to for the whole of the member’s lifetime; that owner can never change, so the statement has no legal target to move the member to. RECONNECT is therefore rejected — the only way to remove such a member from the set is to ERASE the record itself.

Cross-check. Lining up the two navigational statements against the three retention classes:

Exactly one row forbids RECONNECT, so the restriction does not extend to OPTIONAL or MANDATORY sets and the combined choice cannot hold either. Result: the RECONNECT command cannot be used with a FIXED set.