SQL Union and Join Operations

Questions and Answers

What is the primary difference between a UNION and a JOIN?

A JOIN includes duplicate rows while a UNION excludes them.

A UNION creates individual rows from columns while a JOIN does not.

A UNION combines result sets while a JOIN combines column data. (correct)

A UNION compares rows from two tables while a JOIN concatenates them.

What does the 'UNION ALL' clause do in a SQL query?

It combines result sets without permitting duplicates.

It ensures that the result set is sorted automatically.

It includes all rows in the results, including duplicates. (correct)

It excludes duplicate rows in the result set.

Which of the following statements about data types in a UNION operation is true?

The data types must be identical in all queries.

Data types must be compatible through implicit conversion. (correct)

Only the first query's data type determines the result set data type.

All columns must be of integer type to use a UNION.

What happens when combining rows from two queries with a UNION if the number of columns differs?

The operation will fail and result in an error. Signup and view all the answers

In a SQL UNION operation, what is required regarding the order of columns?

Columns must be in the same order across all queries. Signup and view all the answers

What will be the result of executing a UNION operation without the ALL keyword?

Only unique rows will be included in the result set. Signup and view all the answers

In the context of a UNION operation, what is meant by the term 'data type precedence'?

How SQL determines the resulting data type when types differ. Signup and view all the answers

What must be true about the XML data type when using UNION in SQL?

All XML columns must be either typed to an XML schema or untyped. Signup and view all the answers

What does the INTO clause accomplish in the example provided?

It creates a new table to hold the result set. Signup and view all the answers

What is the effect of using UNION ALL compared to UNION in SQL?

UNION only allows unique records, while UNION ALL allows duplicates. Signup and view all the answers

Which of the following SQL statements correctly uses ORDER BY with UNION?

SELECT Name FROM Table1 UNION SELECT Name FROM Table2 ORDER BY Name; Signup and view all the answers

What result set is produced when using UNION without ALL among three identical tables?

Only unique rows across the three tables. Signup and view all the answers

If the query produces 15 rows using UNION ALL, how many rows would it produce using only UNION if there are 3 tables with identical data?

5 rows, since all are duplicates. Signup and view all the answers

What is the purpose of dropping a table before creating a new one in SQL?

To avoid errors related to table existence. Signup and view all the answers

In the provided examples, what criteria are used to select data from the ProductModel and Gloves tables?

ProductModelID is excluded for certain values. Signup and view all the answers

Which SQL statement will execute without error based on the given examples?

SELECT ProductModelID FROM Production.ProductModel WHERE ProductModelID = 1 UNION SELECT ProductModelID FROM Gloves ORDER BY ProductModelID; Signup and view all the answers

Study Notes

UNION and UNION ALL

UNION combines the results of two queries into a single result set.
UNION ALL includes duplicate rows while UNION excludes them.

Key Differences Between UNION and JOIN

A UNION concatenates result sets without creating individual rows from columns gathered from two tables.
A JOIN compares columns from two tables to create result rows composed of data from both.

Rules for Using UNION

Number and order of columns must be identical in all queries.
Data types of the corresponding columns must be compatible.
If data types differ, resulting data type follows data type precedence rules.

Data Type Compatibility

Columns in a UNION do not need identical definitions but must support implicit conversion.
Columns typed as XML must follow the same XML schema collection.

Syntax Overview

Basic syntax includes { UNION [ ALL ] { | ( ) } [...n ] } to specify queries.
Queries within parentheses are required for proper specification.

Example A: Simple UNION

Creates a Gloves table and combines data from ProductModel and Gloves tables.
Uses UNION to select ProductModelID and Name from both, sorted by Name.

Example B: SELECT INTO with UNION

The INTO clause in a SELECT statement specifies a new table (ProductResults) for storing the combined result of two queries.
Selects data from Production.ProductModel that is not in Gloves alongside Gloves data, creating a consolidated table.

Example C: ORDER BY with UNION

Order matters when using ORDER BY with UNION.
Shows correct and incorrect sequencing of ORDER BY in conjunction with UNION operations.

Example D: UNION of Three SELECT Statements

Demonstrates effects of UNION with ALL and without ALL.
Using UNION ALL results in 15 rows (including duplicates) versus 5 rows when using UNION to eliminate duplicates.

Practical Applications

UNION and UNION ALL are critical for data consolidation in SQL.
Understanding the difference and application scenarios enhances query efficiency and data accuracy.

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Description

This quiz will test your understanding of SQL UNION and JOIN operations. Learn the differences between these two concepts, how to concatenate query results, and control duplicates in your result sets. Get ready to boost your database knowledge!