Web Development Database Basics 2

Question 1

Which SQL clause filters rows AFTER aggregation has been applied?

Accepted Answer

HAVING

Answer

HAVING filters grouped/aggregated results, whereas WHERE filters individual rows before grouping.

Question 2

What does the term 'cardinality' refer to in a database relationship?

Accepted Answer

The number of related records between two entities

Answer

Cardinality describes how many instances of one entity relate to instances of another (one-to-one, one-to-many, many-to-many).

Question 3

Which isolation level prevents dirty reads but still allows non-repeatable reads?

Accepted Answer

Read Committed

Answer

Read Committed prevents dirty reads by only reading committed data but allows non-repeatable reads if another transaction commits between reads.

Question 4

What is a composite key?

Accepted Answer

A key made up of two or more columns

Answer

A composite key uses two or more columns together to uniquely identify a row in a table.

Question 5

Which type of JOIN returns only rows where there is a match in BOTH tables?

Accepted Answer

INNER JOIN

Answer

INNER JOIN returns only the rows that have matching values in both joined tables.

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