DBMS Topics
HubUnit 1 — Introduction to DBMS & Entity-Relationship ModelAdvantages and Disadvantages of DBMSAggregationCharacteristics of DBMSData IndependenceData ModelsDatabase Users and AdministratorsDBMS LanguagesER DiagramsEntity-Relationship ER ModelIntroduction to DBMSKeys in DBMSRelationships and CardinalitySchema and InstanceSpecialization and GeneralizationUnit 1 Summary — Introduction to DBMS & ER ModelThree-Level Architecture ANSI/SPARC ArchitectureWeak Entity SetUnit 2 — Relational Model & SQLConstraints in SQLCursorsDCL and TCL CommandsDDL CommandsDML CommandsDomain Relational Calculus DRCFunctions and Stored ProceduresJoins in SQLNested Queries SubqueriesRelational AlgebraRelational CalculusRelational ModelSQL IntroductionUnit 2 Summary — Relational Model & SQLTriggers in SQLTuple Relational Calculus TRCViews in SQLArmstrong's Axioms & Closure of AttributesBoyce-Codd Normal Form BCNFClosure of AttributesDependency PreservationFifth Normal Form 5NFFirst Normal Form 1NFFourth Normal Form 4NFFunctional DependencyLossless DecompositionMultivalued DependencyNormal Forms — OverviewNormalization — IntroductionSecond Normal Form 2NFUnit 3 Summary — NormalizationThird Normal Form 3NFUnit 4 — Transaction Management & Concurrency ControlACID PropertiesCheckpointsConcurrency Control & SerializabilityDeadlock in DBMSLock-Based Protocols & Two-Phase LockingLog-Based RecoveryRecovery SystemSerializabilityShadow PagingUnit 4 Summary — Transaction Management & Concurrency ControlTimestamp-Based ProtocolTransaction ManagementTransaction StatesTwo-Phase Locking 2PLUnit 5 — Storage & IndexingB+ TreeB-TreeCost EstimationDynamic HashingFile OrganizationHashing TechniquesIndexing — IntroductionMulti-Level IndexingQuery OptimizationQuery ProcessingSingle-Level IndexingStatic HashingUnit 5 Summary — Storage & IndexingUnit 6 — Advanced Topics in DBMSBig Data and DBMSCloud DatabaseData MiningData WarehouseDatabase SecurityDistributed DatabaseDistributed Query ProcessingDistributed DBMS Architecture, Fragmentation & ReplicationMongoDB — IntroductionNoSQL DatabaseReplication in Distributed DatabasesUnit 6 Summary — Advanced Topics in DBMSViva Voce — DBMSImportant Viva Questions — DBMSInterview Questions — DBMSViva Answers — DBMSDBMS Multiple Choice Questions MCQsDBMS Lab Programs
Weak Entity Set
Last Updated : 21 May, 2026
A weak entity is an entity that cannot be uniquely identified by its own attributes alone. It depends on another entity called the identifying entity or owner entity for
Unit 1305 words8 headingsTables includedExamples included
What is a Weak Entity?
A weak entity is an entity that cannot be uniquely identified by its own attributes alone. It depends on another entity (called the identifying entity or owner entity) for its identification.
Weak entities exist only in relation to their owner entity — they cannot exist independently.
Characteristics of a Weak Entity
- No primary key of its own — it has a partial key (discriminator) that is only unique within the context of the owner entity.
- Total participation — a weak entity always has total participation in the identifying relationship.
- Existence dependency — if the owner entity is deleted, the weak entity must also be deleted.
ER Diagram Notation
| Element | Weak Entity Notation | Regular Entity Notation |
|---|---|---|
| Entity | Double rectangle | Single rectangle |
| Identifying Relationship | Double diamond | Single diamond |
| Partial Key | Dashed underline | Solid underline (PK) |
Full Example — Employee and Dependents
Scenario: A company tracks employee dependents (family members). A dependent is identified by their name + the employee they depend on.
Employee Table (Strong Entity)
| EmpID | Name | Dept |
|---|---|---|
| E001 | Alice | CS |
| E002 | Bob | HR |
Primary Key: EmpID
Dependent Table (Weak Entity)
| EmpID | DepName | Relationship | DOB |
|---|---|---|---|
| E001 | Tom | Son | ... |
| E001 | Mary | Daughter | ... |
| E002 | John | Spouse | ... |
Primary Key: (EmpID, DepName). Partial Key: DepName. Foreign Key: EmpID → Employee.EmpID.
- "Tom" is only unique within "E001's dependents"
- "Tom" could also be a dependent of E002 — so DepName alone doesn't identify globally
- The full identity is (EmpID + DepName)
More Examples of Weak Entities
| Weak Entity | Identifying Entity | Discriminator |
|---|---|---|
| Dependent | Employee | DepName |
| Order Item | Order | ItemNumber |
| Room | Building | RoomNumber |
| Question | Exam | QuestionNumber |
| Chapter | Book | ChapterNumber |
| Section | Course | SectionNumber |
Building → Room
Room 101 in Building A is not the same as Room 101 in Building B.
RoomNumber is the discriminator.
Primary Key of Room: (BuildingID, RoomNumber)
Rules for Weak Entity
- A weak entity has total participation in the identifying relationship (shown as double line).
- The identifying relationship is a one-to-many relationship from identifying entity to weak entity.
- The primary key of the weak entity = Primary key of identifying entity + Partial key (discriminator).
- If the identifying (owner) entity is deleted, all associated weak entities are also deleted (CASCADE).
SQL Implementation
sql exampleWoHoTech
CREATE TABLE Employee (
emp_id INT PRIMARY KEY,
name VARCHAR(100) NOT NULL,
dept VARCHAR(50)
);
CREATE TABLE Dependent (
emp_id INT,
dep_name VARCHAR(100),
relationship VARCHAR(50),
dob DATE,
PRIMARY KEY (emp_id, dep_name), -- Composite PK
FOREIGN KEY (emp_id) REFERENCES Employee(emp_id)
ON DELETE CASCADE -- Weak entity deleted with owner
);Weak Entity vs. Strong Entity
| Strong Entity | Weak Entity |
|---|---|
| Can exist alone | Cannot exist without owner |
| Has complete primary key | Has partial key only |
| Single rectangle | Double rectangle + double diamond |
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Weak Entity Set.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this DBMS topic.
Search Terms
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