Data Management (Foundations) D426

Correct Answer

A. They should be simple, easy to type, and quick to process in queries

Explanation

Primary key values should be simple, easy to type, and efficient for processing in queries. This ensures that indexing and lookups are fast, contributing to better performance in the database. Complex primary key values or long strings can slow down query performance and make it harder for users to work with the data.

Why other options are wrong

B. They should be able to store large amounts of data

This is incorrect. Primary keys should not store large amounts of data. They are typically small, simple values like integers or short strings that are easy to index and quick to search.

C. They need to be long strings for uniqueness

This is incorrect. While uniqueness is important, primary keys should not necessarily be long strings. Simple numeric or short alphanumeric keys are sufficient for ensuring uniqueness and are more efficient for indexing and querying.

D. They should contain complex data structures

This is incorrect. Complex data structures are not recommended for primary keys. Simple and efficient values ensure better performance in querying and indexing.

Correct Answer

A. UPDATE PRODUCTS SET PRICE = 5.45 WHERE (PROD_ID = '24');

Explanation

The correct SQL syntax to update an existing table involves the UPDATE command, followed by the table name (PRODUCTS), the SET clause to specify the column and the new value (PRICE = 5.45), and the WHERE clause to define the condition for which rows should be updated (PROD_ID = '24').

Why other options are wrong

B. SELECT PRODUCTS WHERE (PROD_ID = '24') SET PRICE = 5.45;

This is incorrect because the SELECT statement is used for querying data, not updating it. The UPDATE command is required to modify data in a table.

C. SELECT PRODUCTS UPDATE PRICE = 5.45 WHERE (PROD_ID = '24');

Again, this is incorrect because SELECT should not be used in an update query. The UPDATE statement must be used for modifying data.

D. UPDATE PROD_ID = '24' SET PRICE = 5.45;

This is incorrect because the UPDATE statement must specify the table name (e.g., UPDATE PRODUCTS), and conditions like WHERE should be used to specify which rows to update.

Correct Answer:

Not Null

Explanation:

Entity integrity is a fundamental rule in relational databases stating that every table must have a primary key and that the primary key must contain unique, non-null values. A primary key uniquely identifies each record, and allowing null values would break this rule because null indicates "unknown" and cannot serve as a reliable identifier. Therefore, a primary key must always be defined as NOT NULL to maintain entity integrity.

Why Other Options Are Wrong:

Null

A primary key cannot be null because null means no value or unknown, making it impossible to identify a record uniquely. If allowed, multiple rows could have null in the primary key, violating entity integrity.

Both Null & Not Null

This is incorrect because a primary key must consistently follow the NOT NULL rule. Having some rows null and others not null would defeat the purpose of uniquely identifying records and would violate relational database design principles.

Any value

Allowing any value implies including nulls or duplicates, both of which are forbidden for primary keys. A primary key must always hold unique and not null values to ensure proper identification of rows.

Correct Answer

B. many

Explanation

In a one-to-many relationship, the foreign key is placed in the table on the "many" side of the relationship. This is because the foreign key is used to reference the primary key in the table on the "one" side, establishing the relationship between the two tables. For example, in a relationship where one department can have many employees, the foreign key (department ID) will be placed in the employee table.

Why other options are wrong

A. weak

A weak entity is one that cannot be uniquely identified without the help of another entity. Weak entities are not relevant to the placement of foreign keys in a one-to-many relationship.

C. many-to-many

In a many-to-many relationship, a junction table is typically used to represent the relationship, which contains foreign keys referencing both related tables. The foreign key does not reside on just one side in a many-to-many relationship but is instead distributed across the junction table.

D. one

In a one-to-many relationship, the foreign key is placed in the table on the "many" side, not the "one" side. The "one" side will have the primary key, which is referenced by the foreign key on the "many" side.

E. cardinal

Cardinal refers to the cardinality of the relationship (such as one-to-many or many-to-many), not to the location of the foreign key in a one-to-many relationship.

Correct Answer

 A. Left Join

Explanation

A Left Join (also called Left Outer Join) retrieves all records from the left table and only the matching records from the right table. If no match is found in the right table, NULL values will be returned for columns from the right table.

Why other options are wrong

B. Right Join

A Right Join retrieves all records from the right table and only the matching records from the left table. This is the opposite of a Left Join.

C. Inner Join

An Inner Join only retrieves rows that have matching values in both tables. It does not return unmatched rows from either table.

D. Full Join

A Full Join retrieves all records when there is a match in either the left or the right table. If there is no match, NULL values are returned for the non-matching side.

Correct Answer:

composite

Explanation:

When no single column can uniquely identify a record, multiple columns can be combined to form a composite key. This ensures that the combination of values across these columns uniquely identifies each row. Composite keys are widely used in relational database design, especially in junction tables that model many-to-many relationships. They maintain data integrity while avoiding duplication.

Why Other Options Are Wrong:

joined

Although it sounds related, "joined" is not the correct term in database terminology for a key made up of multiple columns. Joins are operations used to combine rows from different tables, not to define keys.

aggregate

Aggregate refers to functions like SUM, AVG, COUNT, etc., which perform calculations on sets of values. It has no relation to primary or candidate keys in database design, so this is an incorrect option.

union

Union is an SQL set operator used to combine results from two queries. It does not refer to key structures and cannot uniquely identify rows within a single table. Therefore, it is not the right choice.

Correct Answer

B. INSERT INTO table_name (column1, column2) VALUES (value1, value2);

Explanation

The correct syntax for adding a new record to a table in SQL is INSERT INTO table_name (column1, column2) VALUES (value1, value2);. This statement specifies the table, the columns to insert data into, and the corresponding values for those columns. The INTO keyword is required to define the table in the statement.

Why other options are wrong

A. INSERT table_name (column1, column2) VALUES (value1, value2);

This is incorrect because the INTO keyword is missing. The correct syntax requires INSERT INTO to specify the table where the data is to be inserted.

C. INSERT INTO table_name VALUES (value1, value2);

This is incorrect because this statement assumes the values provided will match the exact order of all columns in the table. If any columns are missing or out of order, it will lead to an error. It's safer to explicitly define which columns you're inserting values into.

D. INSERT INTO table_name (column1, column2) SET (value1, value2);

This is incorrect because SET is used in the UPDATE statement, not in an INSERT statement. The correct keyword to use in INSERT is VALUES, not SET.

Correct Answer

 A. To connect two tables with a many-to-many relationship

Explanation

A junction table is used to resolve a many-to-many relationship between two tables. It contains foreign keys that reference the primary keys of the two tables being connected. This allows multiple records from both tables to be associated with each other, providing a way to store many-to-many relationships efficiently.

Why other options are wrong

B. To store primary keys

While a junction table contains foreign keys, it is not primarily used to store primary keys. It stores relationships between tables, with the foreign keys referencing the primary keys of the related tables. The primary key itself is typically stored in the main tables, not the junction table.

C. To perform complex queries

Junction tables are not designed specifically for performing complex queries. They are meant to store the relationship between tables in a many-to-many relationship. Complex queries can involve multiple tables, including junction tables, but the table itself is not responsible for the complexity of the queries.

D. To enforce referential integrity

While junction tables play a role in maintaining relationships between tables, they do not inherently enforce referential integrity. Referential integrity is enforced by the database system through constraints like FOREIGN KEY, not by the junction table itself.

Correct Answer:

UNIQUE()

Explanation:

UNIQUE() is not a standard SQL aggregate function. Aggregate functions operate across multiple rows to return a single summary value, such as COUNT(), MAX(), MIN(), SUM(), and AVG(). UNIQUE() may exist in certain non-SQL contexts or as part of constraints, but it is not recognized as a built-in SQL aggregation function.

Why Other Options Are Wrong:

COUNT()

COUNT() is a standard aggregate function that returns the number of rows matching a condition. It is widely used in SQL to summarize data.

MAX()

MAX() is an aggregate function that returns the maximum value from a column across rows. It is commonly used in queries for analysis.

SUM()

SUM() is another aggregate function that calculates the total of all numeric values in a column across rows. It is a fundamental SQL function for aggregation.