Interview Questions and Answers for 'Constraint' | Search Interview Question

Interview Questions and Answers for 'Constraint' - 10 question(s) found - Order By Newest

Q1. Which constraint cannot be specified as an explicit constraint and should be specified with the column only ?

Database

Ans. NOT NULL

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Q2. When do we generally get this database error - integrity constraint violated - parent key not found ?

Ans. This error most likely will be thrown during an insert statement, while inserting a value within a foreign key column which doesnt exist within the Parent column. For example - Trying to add a dept number reference within a Employee Table when the Dept doesnt exist in the Dept Table.

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Q3. When do we generally get the database error - Unique Constraint Violated ?

Ans. This error can result either on an insert, update or delete when any change in data results in duplicate record or subset of it having unique constraint on record or its subset. For example - If we have a unique constraint on a combination of columns and then trying to insert duplicate column values.

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Q4. Write an SQL Statement to add a foreign key constraint in oracle ?

Database

Ans. ALTER TABLE <Table_Name> ADD CONSTRAINT <Constraint_Name> FOREIGN KEY (<Column_Name>) REFERENCES <Foreign_Table>(<Foreign_Column>);

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Q5. What do you make out of this error

org.hibernate.exception.ConstraintViolationException: could not insert: [<Entity>]
Caused by: java.sql.SQLIntegrityConstraintViolationException: ORA-00001: unique constraint (<Constraint Name>) violated

and how would you go about debugging it ?

Database

Ans. Application is unable to insert a record as it violates a unique constraint.

The exception states the constraint and Table can be located by the Entity mapping. So I will go to the DB and will first check to which all columns the unique constraint applies. And then I will go and check the code and logs to see how come the duplicate column values were attempted to be inserted when they were not supposed to be.

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Q6. What is ConstraintViolationException in Hibernate ?

Hibernate

Ans. The exception is thrown when a database constraint is violated.

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Q7. What are the different database constraint types ?

Database

Ans. Not Null
Unique
Primary Key
Foreign Key
Check Constraint

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Q8. What is a check database constraint ?

Database

Ans. Constraint specifies the values allowed in one or more columns of every row of a table.

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Q9. What is a unique constraint ?

Database

Ans. Unique constraint ensures that a columns or combination or columns are always unique in a table and hence doesn't allow null or duplicates to be entered for the column or combination of columns.

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Q10. What is the relation between Primary Key, Unique and Not Null constraint ?

Database

Ans. Primary Key constraint means that the column(s) should be unique and doesn't allow null. So Primary key constraint implies unique and not null constraint too.

Unique constraint implies not null constraint too as allowing null would violate uniqueness on those columns.

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