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Indexes
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=======
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* An index is a data structure (usually a B-tree) for fast column lookups.
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* You don't need to (and probably shouldn't) put an index on every column, but if for example a column is used in the WHERE clause of an SQL statement, you should consider adding an index to that column.
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* If a table is not indexed, queries may go slow because the database may have to do a full table scan (i.e. linear search).
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CREATE TABLE RandomData(
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ID INTEGER PRIMARY KEY,
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Num FLOAT NOT NULL
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);
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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INSERT INTO RandomData(Num) VALUES (RANDOM());
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SELECT * FROM RandomData;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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INSERT INTO RandomData(Num) SELECT a.Num/b.Num AS Num FROM RandomData AS a JOIN RandomData AS b ON a.ID != b.ID;
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SELECT COUNT(*) FROM RandomData;
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SELECT * FROM RandomData WHERE ID=1234567; /* SQLite automatically puts an index on PRIMARY KEY and UNIQUE columns */
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SELECT * FROM RandomData WHERE Num>=.998 AND Num<=.999; /* Not indexed; it takes a second. */
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CREATE INDEX Idx_RandomData_Num ON RandomData(Num); /* Naming convention for an index: Idx_TableName_ColumnName; */
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SELECT * FROM RandomData WHERE Num>=.998 AND Num<=.999;
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DROP INDEX Idx_RandomData_Num;
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SELECT * FROM RandomData LIMIT 10;
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Transactions
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============
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A transaction is a sequence of statements that are treated as one unit; either all of them occur or none of them.
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Syntax and features vary somewhat based on the DBMS.
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Classic example: moving money from one bank account to another.
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Autocommit mode:
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Every statement is treated as a transaction of its own.
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Most DBMSs have autocommit mode on by default.
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To do a multi-statement transaction: BEGIN...COMMIT
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Systems support various means to explicitly ROLLBACK a transaction using if statements, etc.
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SQLite Example:
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---------------
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CREATE TABLE Customers(
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CustomerID INTEGER PRIMARY KEY,
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Name TEXT NOT NULL
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);
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CREATE TABLE Accounts(
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AccountID INTEGER PRIMARY KEY,
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CustomerID INTEGER NOT NULL,
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Balance INTEGER NOT NULL,
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FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID),
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CHECK (Balance >= 0) /* Enforces a constraint that no account may be overdrawn. */
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);
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INSERT INTO Customers (Name) VALUES("DrKow");
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INSERT INTO Accounts (CustomerID, Balance) VALUES(1,1000);
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INSERT INTO Accounts (CustomerID, Balance) VALUES(1,2000);
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/* This statement is a single-line transaction; since it failed for one row, it will not modify any rows. */
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UPDATE Accounts SET Balance = Balance - 1500;
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BEGIN;
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UPDATE OR ROLLBACK Accounts SET Balance = Balance + 50 WHERE AccountID=1;
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UPDATE OR ROLLBACK Accounts SET Balance = Balance / 0 WHERE AccountID=1; /* This makes it crash, but the transaction is safely rolled back. */
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UPDATE OR ROLLBACK Accounts SET Balance = Balance - 50 WHERE AccountID=2;
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COMMIT;
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/* The following command will also be rolled back, because the non-negative balance constraint would be violated. */
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BEGIN;
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UPDATE OR ROLLBACK Accounts SET Balance = Balance + 5000 WHERE AccountID=1;
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UPDATE OR ROLLBACK Accounts SET Balance = Balance - 5000 WHERE AccountID=2;
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COMMIT;
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/* Note that the "OR ROLLBACK" part is required in SQLite; if you run the command above without it, 5000 will be added to one account without deducting it from the other. */
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SELECT Name,AccountID,Balance FROM Customers JOIN Accounts ON Customers.CustomerID=Accounts.CustomerID;
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@ -0,0 +1,35 @@
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'Created with https://www.planttext.com/
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@startuml
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skin rose
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title Bird database
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class Species {
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SpeciesID
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Name
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}
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class Regions {
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RegionID
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Name
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}
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class Sightings {
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SightingID
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Latitude
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Longitude
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Date
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}
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class Spotters {
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SpotterID
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Name
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}
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Regions -right- Species: "0..* 0..*"
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Species -right- Sightings: "1 0..*"
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Sightings -right- Spotters: "0..* 0..1"
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@enduml
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After Width: | Height: | Size: 16 KiB |
@ -0,0 +1,79 @@
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta http-equiv="X-UA-Compatible" content="IE=edge">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<title>Homework 1: Bird database</title>
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<link rel="stylesheet" href="../style.css">
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<style>
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img {
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border: 1px solid black;
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}
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</style>
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</head>
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<body>
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<div id="page">
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<h1>Homework 1: Bird database</h1>
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<p>This homework gives pracice with the most important aspects of SQL we covered.</p>
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<h2>Assignment</h2>
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<p>
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Write commands to implement the database represented in the class diagram below in SQLite. You can use <a href="https://sql.js.org/examples/GUI/">this SQL interpreter</a> or a local installation of SQLite, at your discretion. You will be submitting <strong>all of the SQL commands</strong> to implement the database, as described below.
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</p>
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<h2>Database design</h2>
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<p>
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<img src="class diagram.png" width="550"><br>
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Implement the database as pictured above. Note that the many-to-many relationship between Species and Regions will require a junction table. Write the following in SQL (invent data as necessary).
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<ul>
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<li>Create all tables
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<ul>
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<li>Use a primary key for each table (a junction table should get a two-column primary key if each pairing should happen at most once)</li>
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<li>Declare foreign key constraints where appropriate</li>
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<li>Declare an index for the SpotterID foreign key column of Sightings</li>
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</ul>
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</li>
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<li>Fill tables with initial data
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<ul>
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<li>Put in at least 6 actual bird species</li>
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<li>Put in the regions: Midwest, Northeast, South, West</li>
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<li>Put in 3 spotters (one of them should be JD - the Math/CS department head who is a birder)</li>
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<li>Put in at least 4 sightings; at least two of them should be by JD</li>
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<li>Associate each bird with the regions it actually lives in</li>
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</ul>
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</li>
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<li>
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Include the following query commands:
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<ul>
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<li>Select the lat, lon, date, and species name for each sighting made by JD.</li>
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<li>Pick some specific bird species and write a query that selects all region names in which it lives.</li>
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</ul>
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</li>
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<li>
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Include the following update command:
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<ul>
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<li>Update some bird's name to its scientific name.</li>
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</ul>
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</li>
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<li>
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Include the following delete command:
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<ul>
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<li>Delete a given spotter by ID and NULL out his/her ID in all of the associated sightings. Do this as a transaction and specify OR ROLLBACK for the commands therein.</li>
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</ul>
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</li>
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</ul>
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</p>
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<p>Note that in real life, it would likely be useful to have a geospatial index on latitude and longitude. SQLite doesn't support this, but there are <a href="https://www.gaia-gis.it/fossil/libspatialite/index">add-ons</a> and <a href="https://www.linkedin.com/pulse/what-geospatial-index-gaurav-pandey">workarounds</a> (though that is not required for this assignment).</p>
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<hr>
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<p>Submit a text document with all of your SQL commands in Educat, by November 6, 2024.</p>
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</div>
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</body>
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</html>
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@ -0,0 +1,75 @@
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.tab {
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padding-left: 15px;
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}
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body {
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background-color: #ffffb0;
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margin: 0;
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padding: 0;
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}
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#page {
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margin-left: 30px;
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margin-right: 30px;
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margin-top: 10px;
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margin-bottom: 10px;
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background-color: #fffff0;
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padding: 7px;
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border-style: double;
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border-color: #000000;
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border-width: 3px;
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}
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.underline {
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text-decoration: underline;
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}
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img {
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max-width: 100%;
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height: auto;
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border: 1px solid black;
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}
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/* This is so code samples look reasonable */
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pre {
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-moz-tab-size: 4;
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-o-tab-size: 4;
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tab-size: 4;
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overflow-x: auto;
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border-width: 1px;
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border-color: black;
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border-style: solid;
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background-color: white;
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padding: 4px;
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}
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blockquote {
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border-width: 1px;
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border-color: black;
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border-style: solid;
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background-color: white;
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padding: 4px;
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}
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.output {
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border-width: 3px;
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border-style: double;
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border-color: white;
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background-color: #000000;
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color: #ffffff;
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padding: 3px;
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}
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img {
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max-width: 100%;
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}
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table {
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border-collapse: collapse;
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}
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td, th {
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border: 1px solid black;
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padding: 4px;
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background-color: white;
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}
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Loading…
Reference in new issue