SelectResults: Using Queries

Overview

SelectResults are returned from .select and .selectBy methods on SQLObject classes, and from SQLMultipleJoin, and SQLRelatedJoin accessors on SQLObject instances.

Select results are generators, which are lazily evaluated. The SQL is only executed when you iterate over the select results, fetching rows one at a time. This way you can iterate over large results without keeping the entire result set in memory. You can also do things like .reversed() without fetching and reversing the entire result -- instead, SQLObject can change the SQL that is sent so you get equivalent results.

Note

To retrieve the results all at once use the python idiom of calling list() on the generator to force execution and convert the results to a stored list.

You can also slice select results. This modifies the SQL query, so peeps[:10] will result in LIMIT 10 being added to the end of the SQL query. If the slice cannot be performed in the SQL (e.g., peeps[:-10]), then the select is executed, and the slice is performed on the list of results. This will generally only happen when you use negative indexes.

In certain cases, you may get a select result with an object in it more than once, e.g., in some joins. If you don't want this, you can add the keyword argument MyClass.select(..., distinct=True), which results in a SELECT DISTINCT call.

You can get the length of the result without fetching all the results by calling count on the result object, like MyClass.select().count(). A COUNT(*) query is used -- the actual objects are not fetched from the database. Together with slicing, this makes batched queries easy to write:

start = 20
size = 10
query = Table.select()
results = query[start:start+size]
total = query.count()
print "Showing page %i of %i" % (start/size + 1, total/size + 1)

Note

There are several factors when considering the efficiency of this kind of batching, and it depends very much how the batching is being used. Consider a web application where you are showing an average of 100 results, 10 at a time, and the results are ordered by the date they were added to the database. While slicing will keep the database from returning all the results (and so save some communication time), the database will still have to scan through the entire result set to sort the items (so it knows which the first ten are), and depending on your query may need to scan through the entire table (depending on your use of indexes). Indexes are probably the most important way to improve importance in a case like this, and you may find caching to be more effective than slicing.

In this case, caching would mean retrieving the complete results. You can use list(MyClass.select(...)) to do this. You can save these results for some limited period of time, as the user looks through the results page by page. This means the first page in a search result will be slightly more expensive, but all later pages will be very cheap.

Retrieval Methods

Iteration

As mentioned in the overview, the typical way to access the results is by treating it as a generator and iterating over it (in a loop, by converting to a list, etc).

getOne(default=optional)

In cases where your restrictions cause there to always be a single record in the result set, this method will return it or raise an exception: SQLObjectIntegrityError if more than one result is found, or SQLObjectNotFound if there are actually no results, unless you pass in a default like .getOne(None).

Cloning Methods

These methods return a modified copy of the SelectResult instance they are called on, so successive calls can chained, eg results = MyClass.selectBy(city='Boston').filter(MyClass.q.commute_distance>10).orderBy('vehicle_mileage') or used independently later on.

orderBy(column)

Takes a string column name (optionally prefixed with '-' for DESCending) or a SQLBuilder expression.

limit(num)

Only return first num many results. Equivalent to results[:num] slicing.

lazyColumns(v)

Only fetch the IDs for the results, the rest of the columns will be retrieved when attributes of the returned instances are accessed.

reversed()

Reverse-order. Alternative to calling orderBy with SQLBuilder.DESC or '-'.

distinct()

In SQL, SELECT DISTINCT, removing duplicate rows.

filter(expression)

Add additional expressions to restrict result set. Takes either a string static SQL expression valid in a WHERE clause, or a SQLBuilder expression. ANDed with any previous expressions.

Aggregate Methods

These return column values (strings, numbers, etc) not new SQLResults instances, by making the appropriate SQL query (the actual result rows are not retrieved). Any that take a column can also take a SQLBuilder column instance, e.g. MyClass.q.size.

count()

Returns the length of the result set, by a SQL SELECT COUNT(...) query.

sum(column)

The sum of values for column in the result set.

min(column)

The minimum value for column in the result set.

max(column)

The maximum value for column in the result set.

avg(column)

The average value for the column in the result set.