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Questions and Answers
What is the primary difference between a UNION and a JOIN?
What is the primary difference between a UNION and a JOIN?
What does the 'UNION ALL' clause do in a SQL query?
What does the 'UNION ALL' clause do in a SQL query?
Which of the following statements about data types in a UNION operation is true?
Which of the following statements about data types in a UNION operation is true?
What happens when combining rows from two queries with a UNION if the number of columns differs?
What happens when combining rows from two queries with a UNION if the number of columns differs?
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In a SQL UNION operation, what is required regarding the order of columns?
In a SQL UNION operation, what is required regarding the order of columns?
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What will be the result of executing a UNION operation without the ALL keyword?
What will be the result of executing a UNION operation without the ALL keyword?
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In the context of a UNION operation, what is meant by the term 'data type precedence'?
In the context of a UNION operation, what is meant by the term 'data type precedence'?
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What must be true about the XML data type when using UNION in SQL?
What must be true about the XML data type when using UNION in SQL?
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What does the INTO clause accomplish in the example provided?
What does the INTO clause accomplish in the example provided?
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What is the effect of using UNION ALL compared to UNION in SQL?
What is the effect of using UNION ALL compared to UNION in SQL?
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Which of the following SQL statements correctly uses ORDER BY with UNION?
Which of the following SQL statements correctly uses ORDER BY with UNION?
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What result set is produced when using UNION without ALL among three identical tables?
What result set is produced when using UNION without ALL among three identical tables?
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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?
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?
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What is the purpose of dropping a table before creating a new one in SQL?
What is the purpose of dropping a table before creating a new one in SQL?
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In the provided examples, what criteria are used to select data from the ProductModel and Gloves tables?
In the provided examples, what criteria are used to select data from the ProductModel and Gloves tables?
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Which SQL statement will execute without error based on the given examples?
Which SQL statement will execute without error based on the given examples?
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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!