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Add better description of qualified identifiers/aliased expressions t…


I'm moving the description of qualified tables to the general
section on qualification. The problem with using the :schema__table
example earlier is that we haven't explained what it means, so it
looks like magic.

I noticed the join table example was also using the __ syntax
before the explanation, so I removed that as well.
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commit 0b269a85f5ae0fb6bbd6306eed630d4e9791141d 1 parent 8e62130
@jeremyevans authored
Showing with 18 additions and 8 deletions.
  1. +18 −8 README.rdoc
26 README.rdoc
@@ -154,8 +154,6 @@ Datasets are the primary way records are retrieved and manipulated. They are ge
posts = DB.from(:posts)
posts = DB[:posts] # same
- # Can also qualify with a database name
- posts = DB[:other_database__posts]
Datasets will only fetch records when you tell them to. They can be manipulated to filter records, change ordering, join tables, etc..
@@ -397,10 +395,10 @@ and not raise an exception outside the block, you can raise the
Sequel makes it easy to join tables:
order_items = DB[:items].join(:order_items, :item_id => :id).
- where(:order_items__order_id => 1234)
+ where(:order_id => 1234)
# SELECT * FROM items INNER JOIN order_items
# ON order_items.item_id =
- # WHERE order_items.order_id = 1234
+ # WHERE order_id = 1234
The important thing to note here is that item_id is automatically qualified with
the table being joined, and id is automatically qualified with the last table
@@ -439,9 +437,10 @@ Ruby strings are generally treated as SQL strings:
items.where(:x => 'x')
# SELECT * FROM items WHERE (x = 'x')
-=== Qualifying column names
+=== Qualifying identifiers (column/table names)
-Column references can be qualified by using the double underscore special notation <tt>:table__column</tt>:
+An identifier in SQL is a name that represents a column, table, or schema.
+Identifiers can be qualified by using the double underscore special notation <tt>:table__column</tt>:
# items.price
@@ -451,9 +450,15 @@ Another way to qualify columns is to use the <tt>Sequel.qualify</tt> method:
items.literal(Sequel.qualify(:items, :price))
# items.price
-=== Column aliases
+While it is more common to qualify column identifiers with table identifiers, you can also qualify table identifiers with schema identifiers
+to select from a qualified table:
+ posts = DB[:some_schema__posts]
+ # SELECT * FROM some_schema.posts
+=== Identifier aliases
-You can also alias columns by using the triple undersecore special notation <tt>:column___alias</tt> or <tt>:table__column___alias</tt>:
+You can also alias identifiers by using the triple undersecore special notation <tt>:column___alias</tt> or <tt>:table__column___alias</tt>:
# price AS p
@@ -465,6 +470,11 @@ Another way to alias columns is to use the <tt></tt> method:
items.literal(, :p))
# price AS p
+You can use the <tt></tt> method to alias arbitrary expressions, not just identifiers:
+ items.literal([:posts].select{max(id)}, :p))
+ # (SELECT max(id) FROM posts) AS p
== Sequel Models
A model class wraps a dataset, and an instance of that class wraps a single record in the dataset.
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