Interview Q&A / SQL & DBMS
SQL & DBMS interview questions
Joins, keys, normalisation, indexes and transactions for placement and backend interviews.
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- Can a foreign key be NULL, and can it reference the same table?
Yes to both. A nullable foreign key means the relationship is optional for that row. A foreign key can also reference the same table's primary key, which is how hierarchies like employee and manager are stored.
IntermediateQuick answer1 min read - What do ON DELETE CASCADE, SET NULL and RESTRICT do in a foreign key?
They define what happens to child rows when the parent row is deleted. CASCADE deletes the children, SET NULL clears their foreign key, and RESTRICT or NO ACTION blocks the delete.
IntermediateQuick answer1 min read