SQL Subquery Basics Quiz

An INNER JOIN in SQL is used to combine rows from two or more tables based on a related column between them. It returns only those rows where the specified join condition holds true, effectively filtering out any non-matching entries and ensuring that the result set contains only relevant data from both tables.

A LEFT JOIN retrieves all records from the left table, ensuring that every row is included in the result set. When there are matching rows in the right table, those are included; if there are no matches, NULL values are returned for the right table's columns. This join is useful for preserving all data from the left table.

A correlated subquery is one that depends on the outer query for its values. Unlike a regular subquery that can be executed independently, a correlated subquery is evaluated repeatedly, once for each row processed by the outer query, allowing it to reference and use columns from the outer query in its condition.

A scalar subquery is designed to return a single value, specifically one row and one column. This type of subquery is often used in SQL statements where a single value is needed for comparison or assignment, making it distinct from other subqueries that may return multiple rows or columns.

Subqueries can be utilized in multiple SQL clauses, primarily in the WHERE, FROM, and SELECT clauses. In the WHERE clause, they filter results based on conditions; in the FROM clause, they can define a dataset; and in the SELECT clause, they can derive values for the output. This versatility allows for complex queries and data manipulation.

A subquery in the WHERE clause that returns multiple rows is commonly paired with the IN operator. This allows for filtering results based on whether a value matches any value within the set returned by the subquery, effectively enabling more complex conditions in SQL queries.

A LEFT JOIN retrieves all rows from the left table and only the matching rows from the right table. If there are no matches in the left table, the right table's rows are not included. Thus, the statement is false, as it incorrectly suggests that all rows from the right table are included regardless of matches.

A RIGHT JOIN returns all rows from the right table, ensuring that every entry is included in the result set. For those rows that have corresponding matches in the left table, the related data is also included. If there is no match, the result will show NULL values for columns from the left table.

In SQL, comparison operators such as = and > are used to directly compare a value with the result of a scalar subquery. These operators evaluate the outcome of the subquery and allow for conditional filtering in the WHERE clause, enabling precise data retrieval based on specific criteria.

A subquery in the FROM clause is referred to as a view or derived table because it allows you to create a temporary result set that can be treated like a table. This enables complex queries to be simplified by breaking them into manageable parts, enhancing readability and maintainability of SQL statements.

A correlated subquery depends on the outer query for its values, meaning it must be executed repeatedly—once for each row processed by the outer query. This relationship allows the subquery to return results that are specific to the current row being evaluated, thus affirming the statement as true.

An INNER JOIN retrieves only the rows that have matching values in both tables, excluding any non-matching entries. In contrast, a LEFT OUTER JOIN returns all rows from the left table, including those without matches in the right table, filling in with NULLs where necessary. This allows for a more comprehensive view of the left table's data.