DataSys+ cheat sheet
DataSys+ · Keep the key ideas within reach.
Data, Databases, and Information Retrieval
Primary keys are unique and non-null; foreign keys enforce referential integrity.
WHERE filters rows before grouping; HAVING filters aggregated groups.
COUNT(*) counts rows; COUNT(column) skips NULL values.
LEFT JOIN preserves unmatched left rows; INNER JOIN removes them.
Precision = relevant retrieved / retrieved; recall = relevant retrieved / all relevant.
SQL models, types and constraints
SQL tables may contain duplicate rows unless keys or other constraints prevent them.
PRIMARY KEY is unique and nonnull; CHECK accepts TRUE or UNKNOWN.
Choose exact numerics for exact decimal requirements.
DDL transaction and identity effects depend on the specified DBMS.
SQL retrieval, joins and transaction semantics
WHERE retains TRUE predicates; NULL comparisons can be UNKNOWN.
Keep optional-side filtering consistent with the intended outer-join result.
Use a guaranteed nonnull marker for an unmatched-row test.
Check join cardinalities before trusting an aggregate total.
Database theory, administration and recovery
State candidate keys and functional dependencies before normalizing.
Transactions, logs, permissions and recovery require an explicit DBMS model.
Replication is not an independent recovery history.
Use actual query plans and waits to investigate performance.
Database deployment, performance and recovery lifecycle
Installation success does not establish production readiness.
Memory and connection settings require concurrency assumptions.
Use actual query plans, waits and representative workloads.
Verify consistent recovery chains, key access and service outcomes.
Identity, access and cryptographic protection
Identification, authentication, authorisation and accounting are four distinct steps.
MFA needs two different factor categories, not two instances of one.
RBAC suits stable job roles; ABAC suits context-dependent, fine-grained rules.
Kerberos uses a KDC with TGT and service tickets for SSO.
Symmetric ciphers handle bulk data; asymmetric ciphers solve key distribution and signatures.
Signing uses a specified signature scheme with the signer’s private key; verification and custody are essential to origin evidence.
ECB leaks patterns; use authenticated modes such as GCM or encrypt-then-MAC.
Security operations, monitoring and response
Asset inventory and configuration baselines underpin every operational control.
Changes need an appropriate authorization path, recovery planning and relevant evidence.
Emergency changes still require retrospective documentation within a defined window.
Monitor log sources for silence, not just for events.
Distinguish historical Revision 2 phases from current NIST incident-response guidance integrated with broader risk management.
Containment precedes eradication in almost every scenario.
Capture volatile evidence (memory, connections) before disk and before power-off.
Continuity, backup and technical recovery
Distinguish tolerated data loss from tolerated interruption.
A recovery point and restore time must both meet the stated targets.
A same-failure-domain snapshot alone does not cover loss of that domain.
Verify complete restore procedures and actual dependency behavior.