Using Relational Databases and SQL Steven Emory Department of Computer Science California State University, Los Angeles Lecture 2: Single-Table Selections

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Review: Part I Database Database Model Relational Model Tables Attributes Types Tuples Relationships Schema

Review: Part II Database Management System Query Language Structured Query Language

Candidate Keys A unique identifier (may contain multiple attributes). Example: Artists table  ArtistID, ArtistName,...

Primary Keys A candidate key that has been singled out to uniquely identify each record. Difference between candidate key and primary key? A table may have more than one candidate key, while only one primary key.

Foreign Keys Most table relationships almost always involve a primary key. A foreign key is just a key at the other end of the relationship. We usually say a foreign key ‘references’ a primary key (you’ll see this when we design our own database the last week of class).

SQL Data Types Strings (C-syntax) ‘Peterson’, ‘Joe\’s’, “Peterson”, “Joe’s”, “A\\B” Integers 1, 2, 3, etc. Decimals 1.45, 2.83, NULL NULL, null

Basic Queries Basic format is: SELECT field_name1, field_name2,... FROM table_name [WHERE conditions] To automatically select ALL fields: SELECT * FROM table_name [WHERE conditions] Code in [] is optional

Basic Examples List all attributes for all artists. SELECT * FROM Artists; List the first name of all members. SELECT FirstName FROM Members List all artists from the United States. SELECT ArtistName FROM Artists WHERE Country='USA'

Ordering By Attributes Syntax: SELECT A 1, A 2,... FROM tablename [WHERE conditions] ORDER BY F 1 [ASC | DESC], F 2 [ASC | DESC],... Example: List all artist names in decreasing order. SELECT ArtistName FROM Artists ORDER BY ArtistName DESC;

Ordering By Attributes Note that the ORDER BY clause can take multiple field names. Example: List all artists and their respective countries, sorted by country first, then by artist name. SELECT ArtistName, Country FROM Artists ORDER BY Country, ArtistName;

Ordering By Attributes Note that you can also mix ASC and DESC. Example: List all member’s first and last names along with their respective countries, sorted by country first in ascending order, followed by first name in descending order. SELECT FirstName, LastName, Country FROM Members ORDER BY Country ASC, FirstName DESC;

Operators Arithmetic +, -, *, /, % Conditional operators:, =, =, <>, BETWEEN Logical operators: AND, OR, NOT Other operators: IS NULL

Field Arithmetic You can use simple mathematical expressions in the field parameters of the SELECT clause. Examples: What is ((2 + 2)*4 – 7)/3? SELECT ((2 + 2)*4 – 7)/3; All sales people desire a 2 times pay raise. Display all sales people and their desired salaries. SELECT FirstName, LastName, 2*Base FROM SalesPeople;

The WHERE Clause Use the WHERE clause to filter results. Examples: SELECT * FROM Artists; SELECT * FROM Artists WHERE Country=‘Canada’; The statement in the WHERE clause gets executed once per row.

AND, OR, and NOT Examples: Display all member names from California or Texas. SELECT FirstName, LastName FROM Members WHERE Region=‘CA’ OR Region=‘TX’; Display all titles whose genre is not alternative. SELECT * FROM Titles WHERE NOT (Genre=‘alternative’); Display all member names from California or Texas who have a sales ID of 2. SELECT FirstName, LastName FROM Members WHERE (Region=‘CA’ OR Region=‘TX’) AND SalesID=2;

AND, OR, and NOT Be careful of parentheses: SELECT FirstName, LastName FROM Members WHERE (Region=‘CA’ OR Region=‘TX’) AND SalesID=2; SELECT FirstName, LastName FROM Members WHERE Region=‘CA’ OR Region=‘TX’ AND SalesID=2; OUCH! 2 nd query does not return the right result.

BETWEEN Like saying fieldname >= x AND fieldname <= y List the titles and tracks of all tracks with lengths between 240 and 300 seconds. SELECT TrackTitle, LengthSeconds FROM Tracks WHERE LengthSeconds BETWEEN 240 AND 300;

IS NULL Examples: Select all artist names who do not have a web page. SELECT ArtistName FROM Artists WHERE WebAddress IS NULL; Select all artist names who do have a web page. SELECT ArtistName FROM Artists WHERE NOT (WebAddress IS NULL);

LIKE Used for pattern matching %  any zero or more characters _  any single character Example: List all member names and their s whose s have a.org domain. SELECT FirstName, LastName, FROM Members WHERE LIKE '%.org';

NOT LIKE Negation of LIKE Ignores comparing NULL strings Example: List all member names and their s whose s do not come from a.org domain. SELECT FirstName, LastName, FROM Members WHERE NOT LIKE '%.org';

CASE Statements Use in the SELECT clause when you want to check and possibly modify/replace a column value before it is displayed Let’s go over the syntax first, and then go over some tips and examples

CASE: Switch Style Syntax: CASE case_value WHEN when_value THEN statement_list WHEN when_value THEN statement_list … ELSE statement_list END It’s like saying: if(case_value=when_value) then statement_list; else if(case_value=when_value) then statement_list; … else statement_list;

CASE: IF-ELSE Style Syntax: CASE WHEN condition THEN statement_list WHEN condition THEN statement_list … ELSE statement_list END It’s like saying: if(condition) then statement_list; else if(condition) then statement_list; … else statement_list;

CASE Examples List all track titles and their length in minutes. If the length of the track title is less than 3 minutes, display ‘Short Track’; otherwise, display ‘Long Track.’ SELECT TrackTitle, LengthSeconds/60, CASE WHEN LengthSeconds/60 < 3 THEN 'Short Track‘ ELSE 'Long Track’ END FROM Tracks;

CASE Examples List all track titles and their length in minutes. If the length of the track title is less than 3 minutes, display ‘Short Track’; less than 4 minutes, display ‘Medium Track’; otherwise, display ‘Long Track.’ SELECT TrackTitle, LengthSeconds/60, CASE WHEN LengthSeconds/60 < 3 THEN ‘Short Track’ WHEN LengthSeconds/60 < 4 THEN ‘Medium Track’ ELSE ‘Long Track’ END FROM Tracks;

Column Aliases Last CASE example had a really long column name. To get rid of that we can use column aliases SELECT expression AS alias FROM … Example: SELECT TrackTitle, LengthSeconds/60, CASE WHEN LengthSeconds/60 < 3 THEN ‘Short Track’ WHEN LengthSeconds/60 < 4 THEN ‘Medium Track’ ELSE ‘Long Track’ END AS ‘Track Description’ FROM Tracks;

DISTINCT Use when you want to remove duplicate results Example: Display a list of unique countries that all members come from. SELECT Country FROM Members; // WRONG!!! SELECT DISTINCT Country FROM Members; // CORRECT!!!