SQL Chapter 7: Coding Subqueries

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Questions and Answers

What does the WHERE clause with NOT IN accomplish in the first example query?

It retrieves all vendors with invoices.

It retrieves all vendors excluding those who have invoices. (correct)

It retrieves vendors with invoices sorted by vendor_id.

It retrieves all vendor information irrespective of invoices.

What does the LEFT JOIN in the second example query achieve?

It retrieves only vendors with invoices.

It retrieves all invoice details without vendor information.

It retrieves all vendors and lists their invoices if they exist. (correct)

It retrieves vendors without invoices and excludes them from the result.

In the subquery calculating 'balance_due', what is being compared to the average balance?

The total payments made by a vendor.

The invoice total minus payment and credit totals. (correct)

The average payment to different vendors.

The total invoice amount.

What does the ALL keyword signify in a comparison operator?

It requires the expression to be greater than all values in the list. Signup and view all the answers

How many rows were returned by the subquery calculating the average balance_due?

9 rows Signup and view all the answers

What is an example of where a subquery can be used in a SELECT statement?

In the SELECT clause Signup and view all the answers

Which clause is NOT a typical place for a subquery?

JOIN clause Signup and view all the answers

Which statement correctly describes a noncorrelated subquery?

It can be executed independently of the outer query. Signup and view all the answers

What is a significant advantage of using subqueries?

They can simplify complex queries. Signup and view all the answers

Which of these results would be returned by the example subquery in the WHERE clause from the content?

Invoices where total is higher than the average. Signup and view all the answers

What distinguishes a correlated subquery from a noncorrelated subquery?

It references values from the outer query. Signup and view all the answers

What does the subquery in the FROM clause act as in a SQL statement?

A table specification Signup and view all the answers

Which of the following is a key reason for utilizing joins over subqueries?

Easier to understand when relationships are clear. Signup and view all the answers

Which of the following MySQL integer types uses the least amount of storage?

TINYINT Signup and view all the answers

What is a distinguishing feature of ENUM and SET data types in MySQL?

ENUM supports a fixed set of options while SET supports multiple selections from that set. Signup and view all the answers

How does MySQL handle implicit data conversion when necessary?

MySQL automatically converts values to the appropriate type if they differ. Signup and view all the answers

Which data type in MySQL is considered a Large Object (LOB)?

BLOB Signup and view all the answers

What type of data can the DATE and TIME data types in MySQL store?

Both date and time values. Signup and view all the answers

What is the maximum value that can be stored in an INT UNSIGNED data type?

4294967295 Signup and view all the answers

How does the ZEROFILL attribute affect the display of an INT data type?

It pads the value with leading zeros to a specified width. Signup and view all the answers

Which of the following fixed-point types uses the most memory?

DECIMAL(18,9) Signup and view all the answers

What happens when a date literal is incorrectly formatted, such as '2014-02-31'?

It results in an ERROR. Signup and view all the answers

Which floating-point type has a fixed size and uses 4 bytes for storage?

FLOAT Signup and view all the answers

What is the storage size for the DATE data type?

3 bytes Signup and view all the answers

Which of the following describes the nature of a floating-point number?

Approximate numeric type that can represent fractions. Signup and view all the answers

When the value '20140815' is stored in a DATE column, what value is retained?

2014-08-15 Signup and view all the answers

What value is stored in a TIME column when a literal value of '19:61:11' is used?

ERROR Signup and view all the answers

How many bytes are used to store an ENUM type in MySQL?

1-2 Signup and view all the answers

What happens when you attempt to add 1 to a date column using implicit conversion?

The date increases by one day Signup and view all the answers

What value is stored in a SET column when the value 'Olives, Pepperoni' is inserted?

Olives, Pepperoni Signup and view all the answers

What is the maximum byte size for a LONGBLOB in MySQL?

L + 4 Signup and view all the answers

Which of the following values would be incorrectly interpreted in an ENUM column defined as ENUM ('Yes', 'No', 'Maybe')?

'Possibly' Signup and view all the answers

What is stored in a DATETIME or TIMESTAMP column when the literal value '2014-08-15' is used?

2014-08-15 00:00:00 Signup and view all the answers

When a string is implicitly converted to a number, which of the following statements is correct?

Mathematical operations can be performed on the converted number Signup and view all the answers

Study Notes

Chapter 7: How to Code Subqueries

Subqueries are queries embedded within other queries.
Subqueries can be used in the WHERE, HAVING, FROM, and SELECT clauses.
Four common ways to use subqueries in a SELECT statement are as a search condition, a table specification, or a column specification in separate clauses.

Objectives

Apply subqueries in SELECT statements.
Describe how subqueries are used in WHERE, HAVING, FROM, and SELECT clauses.
Define the difference between correlated and non-correlated subqueries.

Four Ways to Introduce a Subquery

WHERE clause: A subquery can be used as a condition.
HAVING clause: A subquery can be used as a condition within the HAVING clause.
FROM clause: In certain situations, a subquery can define a table used in a FROM clause.
SELECT clause: A useful way to specify subqueries to define a column in a SELECT result.

A Subquery in a `WHERE` Clause

A subquery can be used in WHERE clauses. In the provided example, a subquery finds the average invoice total.
Invoices with a invoice total greater than the average are listed.

Advantages of Joins

Joins combine data from multiple tables.
Joins are intuitive when based on existing relationships between tables.

Advantages of Subqueries

Subqueries can deliver aggregate values to main queries.
Subqueries handle ad hoc relationships more intuitively.
Complex queries are more readily implemented using subqueries.

Syntax of a `WHERE` Clause - `IN` Phrase

The IN keyword can be used to check if a value exists within a given set.
The NOT operator can prevent the inclusion of values within a set.
A sample query displays vendors without invoices.

The `ALL` Keyword

ALL(value) means a condition must be true for all values in a subquery's result set.

The `ANY` Keyword

ANY(value) means a condition must be true for at least one value.

A Correlated Subquery

A correlated subquery is a query that is reliant on a column or data from another query on the same set of data for calculation inside the query's logic.
Subqueries included inside the WHERE clause filter data dependent on another query.

A Subquery in the `SELECT` Clause

A subquery in the SELECT clause adds a new column.
A sample query finds the latest invoice date for each vendor.

The Same Query Restated Using a Join

A query using a JOIN restructures the same data from a query using a subquery.

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Description

This quiz focuses on the application of subqueries within SQL statements, specifically within Chapter 7. You'll explore how to use subqueries in various clauses such as WHERE, HAVING, FROM, and SELECT. Additionally, the quiz will help you distinguish between correlated and non-correlated subqueries.