Questions and Answers
What is the purpose of the TRUNCATE statement in DDL?
When using the ALTER TABLE statement, what does the DROP command do?
What happens when a database or table is dropped using the DROP statement?
In the context of defining a column in a table, what does 'varchar(20)' signify?
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Which statement correctly describes the ADD command in the ALTER TABLE statement?
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What is the definition of a tuple in a relational model?
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Which of the following statements accurately describes attributes in a database?
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Which Codd's rule states that a database must manage information entirely through relational capabilities?
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According to Codd's rules, which is NOT a correct handling of NULL values?
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What does Rule 1, the Information Rule in Codd’s guidelines, emphasize?
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Which rule necessitates that views theoretically available for updating must be mutable by the system?
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Which Codd's rule requires that every unique piece of data must be accessible through a combination of Table Name, Primary Key, and Attribute?
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What role does Rule 5 of Codd’s guidelines pertain to in a relational database?
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What is the purpose of the ROLLBACK command in a database?
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What is the correct order of operations when using a WHERE clause with GROUP BY?
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How does the SAVEPOINT command function in a transaction?
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Which of the following statements about the ORDER BY clause is accurate?
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If you execute the command 'ROLLBACK TO A' after saving states at points A, B, and C, what will happen?
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In which clause can a subquery be utilized?
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In the given example, what does the command 'COMMIT;' do?
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What does a subquery return in SQL?
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What is the effect of using the ROLLBACK command without a SAVEPOINT specified?
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Why are nested queries considered powerful in SQL?
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Which of the following describes the role of DCL commands?
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What will the state of the class table look like after executing 'ROLLBACK TO B'?
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What should always enclose a subquery?
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Which SQL command would be used to create a temporary marker in a database transaction?
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When is the ORDER BY clause applied in the query execution process?
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What happens if the GROUP BY clause is omitted in a SQL statement with an aggregate function?
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What does the IN operator do in the context of a nested query?
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Which of the following queries correctly retrieves students with GPAs above the average?
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What is the purpose of the ANY operator in a nested query?
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How would you use the NOT IN operator effectively in a query?
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What will the following SQL query return? SELECT * FROM students WHERE GPA > 3.19;
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Which of the following describes the function of the subquery in SELECT * FROM students WHERE GPA > (SELECT AVG(GPA) FROM students);?
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What data type is returned by the subquery SELECT AVG(GPA) FROM students?
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Why is it important for the subquery in the context of GPA to return a single value?
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Study Notes
Database Concepts
- Tuple: Represents a single row in a table containing one record for that relation.
-
Attributes: Table headers that define permissible values known as the domain (e.g., the attribute
branch_nameincludes all valid branch names). - Relation: Refers to a table in the relational model of data storage.
- RDBMS (Relational Database Management System): A database system based on the relational model for storing and accessing data.
Codd's Rules for RDBMS
- E.F. Codd: Computer scientist who developed the relational model and proposed 12 rules to define the quality of a DBMS.
- Rule 0: A system qualifies as an RDBMS if it manages data solely through relational capabilities.
- Rule 1 (Information Rule): All information, including metadata, must be stored in tables, and rows and columns should remain unordered.
- Rule 2 (Guaranteed Access): Atomic data should be accessible through a combination of table name, primary key, and attribute.
- Rule 3 (Systematic Treatment of NULL): NULL should be consistently managed and must never occur in a primary key.
- Rule 4 (Active Online Catalog): The database catalog must be stored online and governed by the same rules as the database itself.
- Rule 5 (Powerful Language): A structured query language must provide access to database data, e.g., SQL.
- Rule 6 (View Updation): Updatable views must be supported by the system.
- Rule 7 (Relational Level Operations): Insert and Update operations must exist at each relation level.
SQL Commands
-
CREATE TABLE: Defines a new table with specified attributes. E.g.,
Students(ROLL_NO int(3), NAME varchar(20), SUBJECT varchar(20)); -
DROP: Deletes an entire table or database. Syntax:
DROP TABLE table_name; -
TRUNCATE: Removes all data from a table while preserving its structure for future use. Syntax:
TRUNCATE TABLE table_name; -
ALTER: Modifies an existing table to add, drop, or modify columns and constraints.
-
ADD: Adds new columns. Syntax:
ALTER TABLE table_name ADD (column_name datatype, ...); -
DROP: Deletes unwanted columns. Syntax:
ALTER TABLE table_name DROP COLUMN column_name; - MODIFY: Changes existing column attributes.
-
ADD: Adds new columns. Syntax:
Transaction Management
-
ROLLBACK: Restores a database to the last committed state. Useful to revert unwanted changes. Syntax:
ROLLBACK TO savepoint_name; -
SAVEPOINT: Temporarily saves a transaction state, allowing rollback to that point. Syntax:
SAVEPOINT savepoint_name;
Data Control Language (DCL)
- DCL: Manages user privileges for database operations.
- When used with
GROUP BY, theWHEREclause filters rows before groups are formed.
Query Features
-
ORDER BY Clause: Sorts query results in ascending or descending order, using specified columns for sorting. Syntax:
SELECT ... FROM table_name [WHERE condition] ORDER BY col1, col2, ...; -
Nested Queries: A query within a query, where the inner query's result informs the outer query.
- Allows tasks to be performed more efficiently by consolidating operations.
- Subqueries can filter data in various clauses (WHERE, FROM, SELECT, HAVING) and should always be enclosed in parentheses.
Example of Nested Query
- To find students with GPAs above the average:
- Use a subquery to calculate average GPA:
SELECT AVG(GPA) FROM students; - Combine with an outer query:
SELECT * FROM students WHERE GPA > (SELECT AVG(GPA) FROM students);
- Use a subquery to calculate average GPA:
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Description
This quiz covers fundamental concepts in databases, specifically focusing on tuples, attributes, and relations. Understand how single rows in a table represent records and the significance of attributes as headers. Dive into the relational data model and its structure to better grasp database organization.
