= Dataset Filtering 

Sequel offers unparalleled flexibility when it comes to filtering records. You can specify your conditions as a custom string, as a string with parameters, as a hash of values to compare against, or as ruby code that Sequel translates into SQL expressions.

== Filtering using a custom filter string

If you wish to write your SQL by hand, you can just supply it to the dataset's #filter method:

  items.filter('x < 10').sql
  #=> "SELECT * FROM items WHERE x < 10"

In order to prevent SQL injection, you can replace literal values with question marks and supply the values as additional arguments:

  items.filter('category = ?', 'ruby').sql
  #=> "SELECT * FROM items WHERE category = 'ruby'"

=== Specifying SQL functions

Sequel also allows you to specify functions by using the Symbol#sql_function method (and the Symbol#[] method on ruby 1.8):

  items.literal(:avg.sql_function(:price)) #=> "avg(price)"

== Filtering using a hash

If you just need to compare records against values, you can supply a hash:

  items.filter(:category => 'ruby').sql
  #=> "SELECT * FROM items WHERE (category = 'ruby')"

Sequel can check for null values:

  items.filter(:category => nil).sql
  #=> "SELECT * FROM items WHERE (category IS NULL)"

Or compare two columns:

  items.filter(:x => :some_table__y).sql
  #=> "SELECT * FROM items WHERE (x = some_table.y)"

And also compare against multiple values:

  items.filter(:category => ['ruby', 'perl']).sql
  #=> "SELECT * FROM items WHERE (category IN ('ruby', 'perl'))"

Ranges (both inclusive and exclusive) can also be used:

  items.filter(:price => 100..200).sql
  #=> "SELECT * FROM items WHERE (price >= 100 AND price <= 200)"

  items.filter(:price => 100...200).sql
  #=> "SELECT * FROM items WHERE (price >= 100 AND price < 200)"

== Filtering using expressions

Sequel allows you to use ruby expressions directly in the call to filter:

  items.filter(:price * 2 < 50).sql
  #=> "SELECT * FROM items WHERE ((price * 2) < 50) 

This works for the standard inequality and arithmetic operators (though you can't use the inequality operators directly on a symbol in ruby 1.9):

  items.filter(:price + 100 < 200).sql
  #=> "SELECT * FROM items WHERE ((price + 100) < 200) 

  items.filter(:price - 100 > 200).sql
  #=> "SELECT * FROM items WHERE ((price - 100) > 200) 

  items.filter(:price * 100 <= 200).sql
  #=> "SELECT * FROM items WHERE ((price * 100) <= 200) 

  items.filter(:price / 100 >= 200).sql
  #=> "SELECT * FROM items WHERE ((price / 100) >= 200) 

You use the overloaded bitwise and (&) and or (|) operators to combine expressions:

  items.filter((:price + 100 < 200) & (:price * 100 <= 200)).sql
  #=> "SELECT * FROM items WHERE (((price + 100) < 200) AND ((price * 100) <= 200)) 

  items.filter((:price - 100 > 200) | (:price / 100 >= 200)).sql
  #=> "SELECT * FROM items WHERE (((price - 100) > 200) OR ((price / 100) >= 200)) 

To filter by equality, you use the standard hash, which can be combined with other operators:

  items.filter({:category => 'ruby'} & (:price + 100 < 200)).sql
  #=> "SELECT * FROM items WHERE ((category = 'ruby') AND ((price + 100) < 200))"

This works with other hash values, such as arrays and ranges:

  items.filter({:category => ['ruby', 'other']} | (:price - 100 > 200)).sql
  #=> "SELECT * FROM items WHERE ((category IN ('ruby', 'other')) OR ((price - 100) <= 200))"

  items.filter({:price => (100..200)} & :active).sql
  #=> "SELECT * FROM items WHERE ((price >= 100 AND price <= 200) AND active)"

=== Negating conditions

You can use the negation operator (~) in most cases:

  items.filter(~{:category => 'ruby'}).sql
  #=> "SELECT * FROM items WHERE (category != 'ruby')"

  items.filter{~:active}.sql
  #=> "SELECT * FROM items WHERE NOT active"

  items.filter(~(:price / 100 >= 200)).sql
  #=> "SELECT * FROM items WHERE ((price / 100) < 200) 

=== Comparing against column references

You can also compare against other columns:

  items.filter{|o| o.credit > :debit}.sql
  #=> "SELECT * FROM items WHERE (credit > debit)

Or against SQL functions:

  items.filter{|o| :price - 100 < o.max(:price)}.sql
  #=> "SELECT * FROM items WHERE ((price - 100) < max(price))"

== String search functions

You can search SQL strings using the #like method:

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

You can specify a Regexp as a like argument, but this will probably only work
on PostgreSQL and MySQL:

  items.filter(:name.like(/Acme.*/)).sql
  #=> "SELECT * FROM items WHERE (name ~ 'Acme.*')"

Like can also take more than one argument:

  items.filter(:name.like('Acme%', /Beta.*/)).sql
  #=> "SELECT * FROM items WHERE ((name LIKE 'Acme%') OR (name ~ 'Beta.*'))"

== String concatenation

You can concatenate SQL strings using Array#sql_string_join: 

  items.filter([:name, :comment].sql_string_join.like('%acme%')).sql
  #=> "SELECT * FROM items WHERE ((name || comment) LIKE 'Acme%')"

Array#sql_string_join also takes a join argument:

  items.filter([:name, :comment].sql_string_join(' ').like('%acme%')).sql
  #=> "SELECT * FROM items WHERE ((name || ' ' || comment) LIKE 'Acme%')"

== Filtering using sub-queries

One of the best features of Sequel is the ability to use datasets as sub-queries. Sub-queries can be very useful for filtering records, and many times provide a simpler alternative to table joins. Sub-queries can be used in all forms of filters:

  refs = consumer_refs.filter(:logged_in).select(:consumer_id)
  consumers.filter(:id => refs).sql
  #=> "SELECT * FROM consumers WHERE (id IN (SELECT consumer_id FROM consumer_refs WHERE logged_in))"

Note that if you compare against a sub-query, you must select a single column in the sub-query.
