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Q:

Explain primary key constraint.

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) );

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Subject: Oracle

Q:

Explain Not Null constraint.

Answer

Oracle NOT NULL is used on a column to ensure that the value for that column can never be NULL.
Example:
Below, the constraint is that the id should never be NULL. If it is, oracle throws an error.
create table employee ( id number NOT NULL, Name varchar(200) );

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Subject: Oracle

Q:

Explain Check constraint.

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) );

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Subject: Oracle

Q:

What are the types of constraints avaialable in oracle?

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

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Subject: Oracle

Q:

How to recover a dropped table?

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.

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Subject: Oracle

Q:

How to Add a new column to an existing table?

Answer

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

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Subject: Oracle

Q:

What are the basic SQL*Plus commands?

Answer

1) START- Used to run a SQL script.
2) ACCEPT- Accepts input from user.
3) GET- Gets the sql file from user to place in buffer.
4) LIST- Displays the last command executed.
5) RUN- Used to list and run the command in buffer
6) SHOW- Shows the environment settings. Example: SHOW PAGESIZE

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Subject: Oracle

Q:

What is SQL*Plus? Explain its features

Answer

SQL*plus allows SQL and PL/SQL scripts to interactively run in command line.

It allows three kinds of commands to run; SQL, PL/SQL and SQL * Plus commands. It’s most commonly used by DBA’s to interact with the oracle database.

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Subject: Oracle