Explain Not Null constraint.

Correct Answer: Oracle check constraint is used to ensure that before inserting the data in the database, it is validated and checked for the condition Example: Below, the constraint is that the id has to be between 0 and 1000 create table employee ( id number check (id between 0 and 1000), Name varchar(200) );

Correct Answer: Oracle constraints are used to maintain consistent of data and ensure the data is properly maintained A constraint is more or less a restriction we try to apply on a table Types of constraints: - Check constraints - NOT NULL constraint - PRIMARY KEY constraint - REFERENCES constraint - UNIQUE constraint

Correct Answer: Dropped tables can be recovered using DROP TABLE flashback It works the way recycle bin works Example: FLASHBACK TABLE EMPLOYEE TO BEFORE DROP; The most recently dropped table with that original name is retrieved from the recycle bin, with its original name

Correct Answer: New column can be added using the ALTER TABLE command as shown below: Example: ALTER TABLE employee ADD DateOfBirth date

Correct Answer: When any file is attempted to edit without any arguments (example ed or edit), the last SQL command is saved in AFIEDTBUF, It acts like a default buffer file for edit commands

Correct Answer: Primary key constraint ensures that the column(s) always has a unique value to identify the record Example: Below, the primary key is created for column id with name prim_id create table employee ( id number NOT NULL, Name varchar(200) Constraint prim_id primary key(id) );

Correct Answer: A foreign key is a reference to another table It is used to establish relationships between tables For example, relationship between employee and professor table One employee can have multiple professors The Primary key of employee becomes foreign key of professor Example: create table employee ( id number NOT NULL, professor_id NOT NULL, Name varchar(200) Constraint prim_id Foreign key(id) references professor(professor_id) );

Correct Answer: A unique constraint on a column uniquely identifies the record by a combination of one or more fields Few unique constraint fields can have a NULL value as long as the combination of values is unique Example: create table employee ( id number NOT NULL, dob DATE, professor_id NOT NULL, Name varchar(200) Constraint id_unique UNIQUE(id,dob) );

Correct Answer: The number of rows returned by a select query can be restricted by the LIMIT clause Example: SELECT * from employee LIMIT 20

Correct Answer: Joins are used to combine data of one or more tables Joins should be used when there is abundant data Joins can be LEFT, RIGHT, OUTER, INNER or even SELF JOIN The purpose is to bind data from multiple tables without any receptivity