dataset_filtering.rdoc

Path: doc/dataset_filtering.rdoc
Last Update: Thu Jan 26 13:58:47 +0000 2012

Dataset Filtering

Sequel is very flexibile 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'"

You can also use placeholders with :placeholder and a hash of placeholder values:

  items.filter('category = :category', :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:

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

If you are specifying a filter/selection/order, you can use a virtual row block:

  items.select{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 an array

If you need to select multiple items from a dataset, you can supply an array:

        item_array = [1, 38, 47, 99]
        items.filter(:id => item_array).sql
        #=> "SELECT * FROM items WHERE (id IN (1, 38, 47, 99))"

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):

  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 inversion 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{credit > debit}.sql
  #=> "SELECT * FROM items WHERE (credit > debit)

Or against SQL functions:

  items.filter{price - 100 < 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 are checking for the inclusion of a single column in a subselect, the subselect should only select a single column.

Using OR instead of AND

By default, if you chain calls to filter, the conditions get ANDed together. If you want to use an OR for a condition, you can use the or method:

  items.filter(:name=>'Food').or(:vendor=>1).sql
  #=> "SELECT * FROM items WHERE ((name = 'Food') OR (vendor = 1))"

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