Keys

Duplicate Key Breaks the Rule

A repeated parent key makes the row identity ambiguous.

highlighted = computed this step

Duplicate key breaks the rule

If two parent rows reuse the same id, the key no longer points to one clear row.

duplicate id is ambiguous\text{duplicate id is ambiguous}

Show the duplicate

The unique flag becomes no, and duplicate key count is 1.

duplicate keys=1\text{duplicate keys}=1

Key examples are tiny finite table checks; SQL dialects, indexing, query planning, deferrable constraints, enforcement timing, performance, and product behavior are out of scope.

Duplicate key breaks the rule: parent tablecustomer_idname1Ada1Ann2Ben Duplicate key breaks the rule: child tableorder_idcustomer_idtotal101150102230103170 key columnsparent keychild foreign keycustomer_idcustomer_id child-to-parent linkschildRowchildValueparentRowsstatus01[0, 1]ambiguous12[2]match21[0, 1]ambiguous duplicate parent keysvalueparentRows1[0, 1] dangling referenceschildRowvaluestatus01ambiguous21ambiguous key factsfactvalueparentRowCount3childRowCount3duplicateParentKeyCount1referenceCount3danglingReferenceCount2parentKeyUniquenovalidnodeleteStatusnone

Read the parent rows

The duplicate table lists the repeated value and the parent rows that share it.

ambiguous parent\text{ambiguous parent}

Summary

A duplicate parent key makes the relationship invalid before any child rows are trusted.

fail closed\text{fail closed}