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SQL Basics - RRUG 1/6/2016
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kickinbahk
January 06, 2016
Technology
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SQL Basics - RRUG 1/6/2016
Overview of SQL and basic syntax.
kickinbahk
January 06, 2016
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Transcript
SQL BASICS
SQL - Structured Query Language
A Computer Language used for: •Storing, •Manipulating, •Querying
Data stored in relational databases
RELATIONAL DATABASES
A collection of data items organized as a set of
formally-described tables from which data can be accessed or reassembled in many different ways without having to reorganize the database tables.
None
Set of tables with data organized by predefined categories
Each row contains a unique instance of data for the
categories defined by the columns.
None
SQL FLAVORS
•Standard •PostgreSQL •DB2 •MicrosoftSQL •MySQL •Oracle •Informix
SQL is Standardized but different flavors are variations
Compare flavors: http://troels.arvin.dk/db/rdbms/ https://en.wikibooks.org/wiki/SQL_Dialects_Reference
SYNTAX
SELECT
Used to select data from the tables located in a
database.
Two keywords: we need to SELECT information FROM a table.
SELECT "column_name" FROM "table_name"; Syntax:
SELECT Store_Name FROM Store_Information; Syntax:
Store_Name Los Angeles San Diego Los Angeles Boston Result:
Select Multiple Columns
SELECT Store_Name, Sales FROM Store_Information; Syntax:
Store_Name Sales Los Angeles 1500 San Diego 250 Los Angeles
300 Boston 700 Result:
Select All Columns 2 ways…
SELECT Store_Name, Sales, Txn_Date FROM Store_Information; Syntax:
SELECT * FROM Store_Information; Syntax:
Select Distinct
SELECT - all information from a column (or columns) on
a table
This may cause redundancies.
To grab a distinct element, all we need to do
is to add DISTINCT after SELECT
SELECT DISTINCT Store_Name FROM Store_Information; Syntax:
Store_Name Los Angeles San Diego Boston Result:
Where
WHERE - allows filtering the result set based on certain
conditions.
SELECT "column_name" FROM "table_name" WHERE "condition";
SELECT Store_Name FROM Store_Information WHERE Sales > 1000; Syntax:
Store_Name Los Angeles Result:
Where with Or
SELECT * FROM Store_Information WHERE Sales > 1000 OR Txn_Date
= 'Jan-08-1999'; Syntax:
Store_Name Sales Txn_Date Los Angeles 1500 Jan-05-1999 Los Angeles 300
Jan-08-1999 Boston 700 Jan-08-1999 Result:
AND or OR
Allows for compound conditions
SELECT "column_name" FROM "table_name" WHERE "simple condition" [AND|OR] "simple condition";
We can use parenthesis ( ) to indicate the order
of the condition.
SELECT Store_Name FROM Store_Information WHERE Sales > 1000 OR (Sales
< 500 AND Sales > 275); Syntax:
Store_Name Los Angeles Result:
Inner Join
INNER JOIN - returns all rows from multiple tables where
the join condition is met.
SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders INNER JOIN Customers ON
Orders.CustomerID=Customers.CustomerID; Syntax:
Result:
Different Joins INNER JOIN: Returns all rows when there is
at least one match in BOTH tables LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
Different Joins RIGHT JOIN: Return all rows from the right
table, and the matched rows from the left table FULL JOIN: Return all rows when there is a match in ONE of the tables
WHY KNOW SQL IF I KNOW RAILS?
The Rails ORM is an attempt at an abstraction
The SQL queries that the Rails ORM generates for you
may need to be fine-tuned.
Resources Zed Shaw’s Learn SQL the Hard Way: http://sql.learncodethehardway.org/ book/introduction.html
Use the Index Luke: http://use-the-index-luke.com/sql/table- of-contents