SQL Session 13: Subqueries

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Questions and Answers

What is crucial when combining tables to ensure accurate reporting?

Writing code that can adapt to changing data (correct)
Using a specific type of join
Using a self-join instead of other types of joins
Ensuring the code is compatible with current data

What is the primary purpose of a subquery in SQL?

To retrieve data without conditions
To combine multiple tables
To replace the main query
To return data for use in the main query as a condition (correct)

What is the name of the chapter that covers SQL fundamentals?

Chapter 20
Chapter 17
Chapter 19
Chapter 18 (correct)

What is the outcome of a subquery in SQL?

Data is returned to be used as a condition (B) Signup and view all the answers

Why is it essential to write code that can adapt to changing data?

To make the code more flexible and reliable (D) Signup and view all the answers

What is the term for a query that is embedded within the WHERE clause of another SQL query?

Subquery (C) Signup and view all the answers

What is the purpose of the having clause in a select statement?

To filter out rows after grouping (A) Signup and view all the answers

What happens when you join multiple tables together?

A new larger table is formed (D) Signup and view all the answers

What is the best approach when combining more than two tables?

Join only two tables at a time (A) Signup and view all the answers

What is the join condition used for?

To match rows of one table with rows of another table (D) Signup and view all the answers

What happens to unmatched rows in an inner join?

They are dropped from the result (D) Signup and view all the answers

What is the main difference between inner joins and outer joins?

Inner joins drop unmatched rows, outer joins include them (A) Signup and view all the answers

When designing a database, how are tables often designed to be joined?

In fixed ways (D) Signup and view all the answers

What can SQL use to get to the result more efficiently?

Shortcuts (C) Signup and view all the answers

What is the purpose of using a table of constants in a SQL query?

To make the code more flexible (C) Signup and view all the answers

Why is punctuation important in SQL queries?

It is essential for query syntax (D) Signup and view all the answers

What is the significance of case sensitivity in Oracle?

It influences query syntax (D) Signup and view all the answers

What is the purpose of using views in SQL?

To save the result of a select statement (C) Signup and view all the answers

What is three-valued logic in SQL?

A way to handle null values (A) Signup and view all the answers

What is the difference between a table and a view?

A table saves the result of a select statement, while a view saves the query (A) Signup and view all the answers

What is the benefit of saving the result of a select statement in a table?

Saves the data at a particular moment in time (C) Signup and view all the answers

What is the purpose of modifying data in a table?

To update the data to reflect changes (B) Signup and view all the answers

Study Notes

Combining Tables

There are several ways to combine tables, including inner join, three types of outer join, union, cross join, and self join.
The way tables are combined affects the results of later reporting, so it's essential to do it correctly.
Code should be written to handle any data that could be put into the tables, not just the current data.

Subqueries

A subquery or inner query or nested query is a query within another SQL query, embedded within the WHERE clause.
Subqueries return data used in the main query as a condition to further restrict the data to be retrieved.
The HAVING clause of the SELECT statement can be used to drop rows from a result table containing summarized data.

Inner Joins

An inner join is the most frequently used method to combine several tables together.
When joining tables, the tables are combined to form a new larger table, and then data is selected or summarized from this new table.
The inner join is a method to combine several tables together, other methods include outer joins and unions.
When combining more than two tables, the best approach is to join only two tables at a time, and then repeat the process.
The join condition is the rule that matches rows of one table with rows of another table.

SQL Fundamentals

Computers use the words AND, OR, and NOT in a precise way that is different from how they are used in English.
Punctuation is very important in SQL, and some databases are case-sensitive.
SQL uses three-valued logic, which means there can be nulls in the data.

Saving Results and Creating Tables

The result of any select statement can be saved in a table or view.
A table saves the actual data as it is at one particular moment in time, while a view saves the select statement and runs it again whenever the view is used.
A table contains fixed data unless changed, while a view contains data that is constantly updated.
One view can be built from another view.
Restrictions apply when modifying data in a table, and tables can be created and modified using SQL.

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Description

Learn about SQL subqueries and how to write flexible code that adapts to changing data. Understand the importance of correct table combinations for accurate reporting.