CSE 414: Section 2

A SeQueL to SQL

October 6th, 2022

Joke of the day!

Announcements

• HW1 is due tomorrow @ 11 PM
• You can use up to 2 late days per assignment (totally 6 late days)
• HW2 will be out soon
• Querying over a large, real-world dataset!
• Name the files EXACTLY as specified
• Come to OH or post on Ed if you're stuck

Importing Files (HW2)

First, create the table.�Then, import the data.

.mode csv� .import population.csv Population� .import gdp.csv GDP

.import /path/to/file NameOfTable

Make sure you import the tables in the order you create them so there are no foreign key constraint issues. For example, if GDP had a foreign key constraint to Population, it would be illegal to import GDP before Population.

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SQL 3-Valued Logic

SQL has 3-valued logic

• FALSE = 0

[ex] price < 25 is FALSE when price = 99

• UNKNOWN = 0.5

[ex] price < 25 is UNKNOWN when price = NULL

• TRUE = 1

[ex] price < 25 is TRUE when price = 19

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SQL 3-Valued Logic (con’t)

Formal definitions:

C1 AND C2 means min(C1,C2)� C1 OR C2 means max(C1,C2)� NOT C means means 1-C

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The rule for SELECT ... FROM ... WHERE C is the following:� if C = TRUE then include the row in the output� if C = FALSE or C = unknown then do not include it

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Aliasing

• Good style for renaming attribute operations to more intuitive labels
• Especially for more complex queries, you can save time by aliasing
• Essential for self joins (ex: FROM [table] AS T1, [table] AS T2)
• You can alias without “AS” in the FROM clause (i.e. “AS” keyword can be omitted)

SELECT [attribute] AS [attribute_name]

FROM [table] AS [table_name]

… [table_name].[attribute_name] …

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Misc. Filters

LIMIT number - limits the amount of tuples returned

[ex] SELECT * FROM table LIMIT 1;

• Super useful when you want to get the largest value (LIMIT + ORDER BY)

DISTINCT - only returns unique values (eliminates duplicates)

[ex] SELECT DISTINCT column_name FROM table;

ORDER BY - Orders result tuples by specified attributes (default ascending)

[ex] SELECT column_name FROM table ORDER BY column_name [DESC];

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Join Semantics

• Think as “nested loops”.

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• NOT the most efficient implementation on a large database! (we will talk about other ways to join later in the course)
• Hash Join
• Sort-Merge Join
• Often requires a foreign key to execute

For more information and different types of joins see:

https://blogs.msdn.microsoft.com/craigfr/2006/08/16/summary-of-join-properties/

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Nested Loop Semantics

SELECT x_1.a_1, …, x_n.a_n�FROM x_1, …, x_n�WHERE <cond>

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for each tuple in x_1:

…

for each tuple in x_n:

if <cond>(x_1, …, x_n):

output(x_1.a_1, …, x_n.a_n)

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Join Types

There will be times we use inner join, full join, and left outer join.

There is never a scenario in this class we need to use a right outer join and sqlite3 does not support this operation. It also doesn’t support full outer join, which you most likely won’t need for this class.

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Worksheet

Time!

Where we started

FWS

(From, Where, Select)

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And now...

FWGHOS^TM

(From, Where, Group By, Having, Order By, Select)

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Group By

• Powerful tool to handle “categories”
• Groups rows with the same value of an attribute into a “bucket” (think dividing into categories)
• Careful when selecting
• Only select attributes in GROUP BY or aggregates
• SQLite will guess (arbitrarily pick a value)¯\_(ツ)_/¯
• SQL Server will throw an error ง •̀_•́)ง

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Aggregates

• Computes summary values for a set of tuples.

COUNT(attribute) - counts the number of tuples

SUM(attribute) - sums the value of the attribute among all tuples in set

MIN/MAX(attribute) - min/max value of the attribute among all tuples in the set

AVG(attribute) - avg value of the attribute among all tuples in the set

...

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Group By - Examples

Do these queries work?

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Enrolled(stu_id, course_num)

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SELECT stu_id, course_num

FROM Enrolled

GROUP BY stu_id

SELECT stu_id, count(course_num)

FROM Enrolled

GROUP BY stu_id

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Group By - Examples

Do these queries work?

​

Enrolled(stu_id, course_num)

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​

​

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SELECT stu_id, course_num

FROM Enrolled

GROUP BY stu_id

SELECT stu_id, count(course_num)

FROM Enrolled

GROUP BY stu_id

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Group By - Examples

Do these queries work?

​

Enrolled(stu_id, course_num)

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​

​

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SELECT stu_id, course_num

FROM Enrolled

GROUP BY stu_id

SELECT stu_id, count(course_num)

FROM Enrolled

GROUP BY stu_id

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Grouping and Ordering

GROUP BY [attribute], …, [attribute_n]

HAVING [predicate] - operates on groups, chooses to keep or remove the entire group

ORDER BY [attribute] [ASC/DESC]

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