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Cheat Sheet

Open a database

require 'rubygems'
require 'sequel'

DB = Sequel.sqlite('my_blog.db')
DB = Sequel.connect('postgres://user:password@localhost/my_db')
DB = Sequel.mysql('my_db', :user => 'user', :password => 'password', :host => 'localhost')
DB = Sequel.ado('mydb')

Open an SQLite memory database

Without a filename argument, the sqlite adapter will setup a new sqlite database in RAM.

DB = Sequel.sqlite

Logging SQL statements

require 'logger'
DB = Sequel.sqlite '', :loggers => [Logger.new($stdout)]
# or
DB.loggers << Logger.new(...)

Using raw SQL

DB << "CREATE TABLE users (name VARCHAR(255) NOT NULL, age INT(3) NOT NULL)"
DB.fetch("SELECT name FROM users") do |row|
  p r[:name]
end
dataset = DB["SELECT age FROM users"]
dataset.print
dataset.map(:age)

Create a dataset

dataset = DB[:items]
dataset = DB.dataset.from(:items)

Most dataset methods are chainable

dataset = DB[:managers].where(:salary => 5000..10000).order(:name, :department)
# or
dataset = DB.query do
  from :managers
  where :salary => 5000..10000
  order :name, :department
end

Insert rows

dataset.insert(:name => 'Sharon', :grade => 50)
dataset << {:name => 'Sharon', :grade => 50} # same effect as above

Retrieve rows

dataset.each {|r| p r}
dataset.all #=> [{...}, {...}, ...]
dataset.first
dataset.order(:name).last # works only for ordered datasets

Update/Delete rows

dataset.filter(:active => false).delete
dataset.filter('price < ?', 100).update(:active => true)

Datasets are Enumerable

dataset.map {|r| r[:name]}
dataset.map(:name) # same effect as above

dataset.inject {|sum, r| sum + r[:value]}

Filtering (see also doc/dataset_filtering.rdoc)

dataset.filter(:name => 'abc')
dataset.filter('name = ?', 'abc')
dataset.filter(:value > 100)
dataset.exclude(:value <= 100)

dataset.filter(:value => 50..100)
dataset.where((:value >= 50) & (:value <= 100))

dataset.where('value IN ?', [50,75,100])

# Get the first record that matches a condition
dataset[:name => 'abc'] # Same as:
dataset.filter(:name => 'abc').first

# Filter using a subquery
dataset.filter('price > ?', dataset.select('AVG(price) + 100'))

Advanced filtering using ruby expressions without blocks

Available as of Sequel 2.0:

DB[:items].filter(:price < 100).sql 
#=> "SELECT * FROM items WHERE (price < 100)" 

DB[:items].filter(:name.like('AL%')).sql 
#=> "SELECT * FROM items WHERE (name LIKE 'AL%')"

There's support for nested expressions with AND, OR and NOT:

DB[:items].filter((:x > 5) & (:y > 10)).sql 
#=> "SELECT * FROM items WHERE ((x > 5) AND (y > 10))" 

DB[:items].filter({:x => 1, :y => 2}.sql_or & ~{:z => 3}).sql 
#=> "SELECT * FROM items WHERE (((x = 1) OR (y = 2)) AND (z != 3))"

You can use arithmetic operators and specify SQL functions:

DB[:items].filter((:x + :y) > :z).sql 
#=> "SELECT * FROM items WHERE ((x + y) > z)" 

DB[:items].filter(:price < :AVG[:price] + 100).sql 
#=> "SELECT * FROM items WHERE (price < (AVG(price) + 100))"

Ordering

dataset.order(:kind)
dataset.reverse_order(:kind)
dataset.order(:kind.desc, :name)

Row ranges

dataset.limit(30) # LIMIT 30
dataset.limit(30, 10) # LIMIT 30 OFFSET 10

Pagination

paginated = dataset.paginate(1, 10) # first page, 10 rows per page
paginated.page_count #=> number of pages in dataset
paginated.current_page #=> 1
paginated.next_page #=> next page number or nil
paginated.prev_page #=> previous page number or nil
paginated.first_page? #=> true if page number = 1
paginated.last_page? #=> true if page number = page_count

Joins

DB[:items].left_outer_join(:categories, :id => :category_id).sql #=>
  "SELECT * FROM items LEFT OUTER JOIN categories ON categories.id = items.category_id"

Summarizing

dataset.count #=> record count
dataset.max(:price)
dataset.min(:price)
dataset.avg(:price)
dataset.sum(:stock)

dataset.group(:category).select(:category, :AVG[:price])

SQL Functions / Literals

dataset.update(:updated_at => :NOW[])
dataset.update(:updated_at => 'NOW()'.lit)

dataset.update(:updated_at => "DateValue('1/1/2001')".lit)
dataset.update(:updated_at => :DateValue['1/1/2001'])

Schema Manipulation

DB.create_table :items do
  primary_key :id
  text :name, :unique => true, :null => false
  boolean :active, :default => true
  foreign_key :category_id, :categories

  index :grade
end

DB.drop_table :items

DB.create_table :test do
  varchar :zipcode, :size => 10
  enum :system, :elements => ['mac', 'linux', 'windows']
end

Aliasing

DB[:items].select(:name.as(:item_name))
DB[:items].select(:name => :item_name)
DB[:items].select(:name___item_name)
DB[:items___items_table].select(:items_table__name___item_name)
# => "SELECT items_table.name AS item_name FROM items AS items_table"

Transactions

DB.transaction do
  dataset << {:first_name => 'Inigo', :last_name => 'Montoya'}
  dataset << {:first_name => 'Farm', :last_name => 'Boy'}
end # Either both are inserted or neither are inserted

Database#transaction is re-entrant:

DB.transaction do # BEGIN issued only here
  DB.transaction
    dataset << {:first_name => 'Inigo', :last_name => 'Montoya'}
  end
end # COMMIT issued only here

Transactions are aborted if an error is raised:

DB.transaction do
  raise "some error occurred"
end # ROLLBACK issued and the error is re-raised

Transactions can also be aborted by raising Sequel::Error::Rollback:

DB.transaction do
  raise(Sequel::Error::Rollback) if something_bad_happened
end # ROLLBACK issued and no error raised

Miscellaneous:

dataset.sql #=> "SELECT * FROM items"
dataset.delete_sql #=> "DELETE FROM items"
dataset.where(:name => 'sequel').exists #=> "EXISTS ( SELECT 1 FROM items WHERE name = 'sequel' )"
dataset.print #=> pretty table print to $stdout
dataset.columns #=> array of columns in the result set, does a SELECT
DB.schema_for_table(:items) => [[:id, {:type=>:integer, ...}], [:name, {:type=>:string, ...}], ...]
                               # Works on PostgreSQL, MySQL, SQLite, and possibly elsewhere
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