Selects
This page describes how to write SELECT
statements with drift's dart_api.
To make examples easier to grasp, they're referencing two common tables forming
the basis of a todo-list app:
class TodoItems extends Table {
IntColumn get id => integer().autoIncrement()();
TextColumn get title => text().withLength(min: 6, max: 32)();
TextColumn get content => text().named('body')();
IntColumn get category => integer().nullable().references(Categories, #id)();
}
@DataClassName('Category')
class Categories extends Table {
IntColumn get id => integer().autoIncrement()();
TextColumn get name => text()();
}
For each table you've specified in the @DriftDatabase
annotation on your database class,
a corresponding getter for a table will be generated. That getter can be used to
run statements:
@DriftDatabase(tables: [TodoItems, Categories])
class MyDatabase extends _$MyDatabase {
// the schemaVersion getter and the constructor from the previous page
// have been omitted.
// loads all todo entries
Future<List<TodoItem>> get allTodoItems => select(todoItems).get();
// watches all todo entries in a given category. The stream will automatically
// emit new items whenever the underlying data changes.
Stream<List<TodoItem>> watchEntriesInCategory(Category c) {
return (select(todos)..where((t) => t.category.equals(c.id))).watch();
}
}
Drift makes writing queries easy and safe. This page describes how to write basic select queries, but also explains how to use joins and subqueries for advanced queries.
Simple selects¶
You can create select
statements by starting them with select(tableName)
, where the
table name
is a field generated for you by drift. Each table used in a database will have a matching field
to run queries against. Any query can be run once with get()
or be turned into an auto-updating
stream using watch()
.
Where¶
You can apply filters to a query by calling where()
. The where method takes a function that
should map the given table to an Expression
of boolean. A common way to create such expression
is by using equals
on expressions. Integer columns can also be compared with isBiggerThan
and isSmallerThan
. You can compose expressions using a & b, a | b
and a.not()
. For more
details on expressions, see this guide.
Limit¶
You can limit the amount of results returned by calling limit
on queries. The method accepts
the amount of rows to return and an optional offset.
Future<List<TodoItem>> limitTodos(int limit, {int? offset}) {
return (select(todoItems)..limit(limit, offset: offset)).get();
}
Ordering¶
You can use the orderBy
method on the select statement. It expects a list of functions that extract the individual
ordering terms from the table. You can use any expression as an ordering term - for more details, see
this guide.
Future<List<TodoItem>> sortEntriesAlphabetically() {
return (select(todoItems)
..orderBy([(t) => OrderingTerm(expression: t.title)]))
.get();
}
You can also reverse the order by setting the mode
property of the OrderingTerm
to
OrderingMode.desc
.
Single values¶
If you know a query is never going to return more than one row, wrapping the result in a List
can be tedious. Drift lets you work around that with getSingle
and watchSingle
:
Stream<TodoItem> entryById(int id) {
return (select(todoItems)..where((t) => t.id.equals(id))).watchSingle();
}
If an entry with the provided id exists, it will be sent to the stream. Otherwise,
null
will be added to stream. If a query used with watchSingle
ever returns
more than one entry (which is impossible in this case), an error will be added
instead.
Mapping¶
Before calling watch
or get
(or the single variants), you can use map
to transform
the result.
Stream<List<String>> contentWithLongTitles() {
final query = select(todoItems)
..where((t) => t.title.length.isBiggerOrEqualValue(16));
return query.map((row) => row.content).watch();
}
Deferring get vs watch¶
If you want to make your query consumable as either a Future
or a Stream
,
you can refine your return type using one of the Selectable
abstract base classes;
// Exposes `get` and `watch`
MultiSelectable<TodoItem> pageOfTodos(int page, {int pageSize = 10}) {
return select(todoItems)..limit(pageSize, offset: page);
}
// Exposes `getSingle` and `watchSingle`
SingleSelectable<TodoItem> selectableEntryById(int id) {
return select(todoItems)..where((t) => t.id.equals(id));
}
// Exposes `getSingleOrNull` and `watchSingleOrNull`
SingleOrNullSelectable<TodoItem> entryFromExternalLink(int id) {
return select(todoItems)..where((t) => t.id.equals(id));
}
These base classes don't have query-building or map
methods, signaling to the consumer
that they are complete results.
Joins¶
Drift supports sql joins to write queries that operate on more than one table. To use that feature, start
a select regular select statement with select(table)
and then add a list of joins using .join()
. For
inner and left outer joins, a ON
expression needs to be specified.
// We define a data class to contain both a todo entry and the associated
// category.
class EntryWithCategory {
EntryWithCategory(this.entry, this.category);
// The classes are generated by drift for each of the tables involved in the
// join.
final TodoItem entry;
final Category? category;
}
// in the database class, we can then load the category for each entry
Stream<List<EntryWithCategory>> entriesWithCategory() {
final query = select(todoItems).join([
leftOuterJoin(categories, categories.id.equalsExp(todoItems.category)),
]);
return query.watch().map((rows) {
return rows.map((row) {
return EntryWithCategory(
row.readTable(todoItems),
row.readTableOrNull(categories),
);
}).toList();
});
}
Of course, you can also join multiple tables:
/// Searches for todo entries in the same category as the ones having
/// `titleQuery` in their titles.
Future<List<TodoItem>> otherTodosInSameCategory(String titleQuery) async {
// Since we're adding the same table twice (once to filter for the title,
// and once to find other todos in same category), we need a way to
// distinguish the two tables. So, we're giving one of them a special name:
final otherTodos = alias(todoItems, 'inCategory');
final query = select(otherTodos).join([
// In joins, `useColumns: false` tells drift to not add columns of the
// joined table to the result set. This is useful here, since we only join
// the tables so that we can refer to them in the where clause.
innerJoin(categories, categories.id.equalsExp(otherTodos.category),
useColumns: false),
innerJoin(todoItems, todoItems.category.equalsExp(categories.id),
useColumns: false),
])
..where(todoItems.title.contains(titleQuery));
return query.map((row) => row.readTable(otherTodos)).get();
}
Parsing results¶
Calling get()
or watch
on a select statement with join returns a Future
or Stream
of
List<TypedResult>
, respectively. Each TypedResult
represents a row from which data can be
read. It contains a rawData
getter to obtain the raw columns. But more importantly, the
readTable
method can be used to read a data class from a table.
In the example query above, we've read the todo entry and the category from each row like this:
return query.watch().map((rows) {
return rows.map((row) {
return EntryWithCategory(
row.readTable(todoItems),
row.readTableOrNull(categories),
);
}).toList();
});
Note: readTable
will throw an ArgumentError
when a table is not present in the row. For instance,
todo entries might not be in any category. To account for that, we use row.readTableOrNull
to load
categories.
Custom columns¶
Select statements aren't limited to columns from tables. You can also include more complex expressions in the query. For each row in the result, those expressions will be evaluated by the database engine.
Future<List<(TodoItem, bool)>> loadEntries() {
// assume that an entry is important if it has the string "important" somewhere in its content
final isImportant = todoItems.content.like('%important%');
return select(todoItems).addColumns([isImportant]).map((row) {
final entry = row.readTable(todoItems);
final entryIsImportant = row.read(isImportant)!;
return (entry, entryIsImportant);
}).get();
}
Note that the like
check is not performed in Dart - it's sent to the underlying database engine which
can efficiently compute it for all rows.
Aliases¶
Sometimes, a query references a table more than once. Consider the following example to store saved routes for a navigation system:
class GeoPoints extends Table {
IntColumn get id => integer().autoIncrement()();
TextColumn get name => text()();
TextColumn get latitude => text()();
TextColumn get longitude => text()();
}
class Routes extends Table {
IntColumn get id => integer().autoIncrement()();
TextColumn get name => text()();
// contains the id for the start and destination geopoint.
IntColumn get start => integer()();
IntColumn get destination => integer()();
}
Now, let's say we wanted to also load the start and destination GeoPoint
object for each route. We'd have to use
a join on the geo-points
table twice: For the start and destination point. To express that in a query, aliases
can be used:
class RouteWithPoints {
final Route route;
final GeoPoint start;
final GeoPoint destination;
RouteWithPoints({this.route, this.start, this.destination});
}
// inside the database class:
Future<List<RouteWithPoints>> loadRoutes() async {
// create aliases for the geoPoints table so that we can reference it twice
final start = alias(geoPoints, 's');
final destination = alias(geoPoints, 'd');
final rows = await select(routes).join([
innerJoin(start, start.id.equalsExp(routes.start)),
innerJoin(destination, destination.id.equalsExp(routes.destination)),
]).get();
return rows.map((resultRow) {
return RouteWithPoints(
route: resultRow.readTable(routes),
start: resultRow.readTable(start),
destination: resultRow.readTable(destination),
);
}).toList();
}
SELECT
routes.id, routes.name, routes.start, routes.destination,
s.id, s.name, s.latitude, s.longitude,
d.id, d.name, d.latitude, d.longitude
FROM routes
INNER JOIN geo_points s ON s.id = routes.start
INNER JOIN geo_points d ON d.id = routes.destination
ORDER BY
and WHERE
on joins¶
Similar to queries on a single table, orderBy
and where
can be used on joins too.
The initial example from above is expanded to only include todo entries with a specified
filter and to order results based on the category's id:
Stream<List<EntryWithCategory>> entriesWithCategory(String entryFilter) {
final query = select(todos).join([
leftOuterJoin(categories, categories.id.equalsExp(todos.category)),
]);
query.where(todos.content.like(entryFilter));
query.orderBy([OrderingTerm.asc(categories.id)]);
// ...
}
As a join can have more than one table, all tables in where
and orderBy
have to
be specified directly (unlike the callback on single-table queries that gets called
with the right table by default).
Group by¶
Sometimes, you need to run queries that aggregate data, meaning that data you're interested in comes from multiple rows. Common questions include
- how many todo entries are in each category?
- how many entries did a user complete each month?
- what's the average length of a todo entry?
What these queries have in common is that data from multiple rows needs to be combined into a single row. In sql, this can be achieved with "aggregate functions", for which drift has builtin support.
Additional info: A good tutorial for group by in sql is available here.
To write a query that answers the first question for us, we can use the count
function.
We're going to select all categories and join each todo entry for each category. What's special is that we set
useColumns: false
on the join. We do that because we're not interested in the columns of the todo item.
We only care about how many there are. By default, drift would attempt to read each todo item when it appears
in a join.
Future<void> countTodosInCategories() async {
final amountOfTodos = todoItems.id.count();
final query = select(categories).join([
innerJoin(
todoItems,
todoItems.category.equalsExp(categories.id),
useColumns: false,
)
]);
query
..addColumns([amountOfTodos])
..groupBy([categories.id]);
final result = await query.get();
for (final row in result) {
print('there are ${row.read(amountOfTodos)} entries in'
'${row.readTable(categories)}');
}
}
To find the average length of a todo entry, we use avg
. In this case, we don't even have to use
a join
since all the data comes from a single table (todos).
That's a problem though - in the join, we used useColumns: false
because we weren't interested
in the columns of each todo item. Here we don't care about an individual item either, but there's
no join where we could set that flag.
Drift provides a special method for this case - instead of using select
, we use selectOnly
.
The "only" means that drift will only report columns we added via "addColumns". In a regular select,
all columns from the table would be selected, which is what you'd usually need.
Stream<double> averageItemLength() {
final avgLength = todoItems.content.length.avg();
final query = selectOnly(todoItems)..addColumns([avgLength]);
return query.map((row) => row.read(avgLength)!).watchSingle();
}
Using selects as inserts¶
In SQL, an INSERT INTO SELECT
statement can be used to efficiently insert the rows from a SELECT
statement into a table.
It is possible to construct these statements in drift with the insertFromSelect
method.
This example shows how that method is used to construct a statement that creates a new category
for each todo entry that didn't have one assigned before:
Future<void> createCategoryForUnassignedTodoEntries() async {
final newDescription = Variable<String>('category for: ') + todoItems.title;
final query = selectOnly(todoItems)
..where(todoItems.category.isNull())
..addColumns([newDescription]);
await into(categories).insertFromSelect(query, columns: {
categories.name: newDescription,
});
}
The first parameter for insertFromSelect
is the select statement statement to use as a source.
Then, the columns
map maps columns from the table in which rows are inserted to columns from the
select statement.
In the example, the newDescription
expression as added as a column to the query.
Then, the map entry categories.description: newDescription
is used so that the description
column
for new category rows gets set to that expression.
Subqueries¶
Starting from drift 2.11, you can use Subquery
to use an existing select statement as part of more
complex join.
This snippet uses Subquery
to count how many of the top-10 todo items (by length of their title) are
in each category.
It does this by first creating a select statement for the top-10 items (but not executing it), and then
joining this select statement onto a larger one grouping by category:
Future<List<(Category, int)>> amountOfLengthyTodoItemsPerCategory() async {
final longestTodos = Subquery(
select(todoItems)
..orderBy([(row) => OrderingTerm.desc(row.title.length)])
..limit(10),
's',
);
// In the main query, we want to count how many entries in longestTodos were
// found for each category. But we can't access todos.title directly since
// we're not selecting from `todos`. Instead, we'll use Subquery.ref to read
// from a column in a subquery.
final itemCount = longestTodos.ref(todoItems.title).count();
final query = select(categories).join(
[
innerJoin(
longestTodos,
// Again using .ref() here to access the category in the outer select
// statement.
longestTodos.ref(todoItems.category).equalsExp(categories.id),
useColumns: false,
)
],
)
..addColumns([itemCount])
..groupBy([categories.id]);
final rows = await query.get();
return [
for (final row in rows) (row.readTable(categories), row.read(itemCount)!),
];
}
Any statement can be used as a subquery. But be aware that, unlike subquery expressions, full subqueries can't use tables from the outer select statement.
JSON support¶
sqlite3 has great support for JSON operators that are also available
in drift (under the additional 'package:drift/extensions/json1.dart'
import).
JSON support is helpful when storing a dynamic structure that is best represented with JSON, or when
you have an existing structure (perhaps because you're migrating from a document-based storage)
that you need to support.
As an example, consider a contact book application that started with a JSON structure to store contacts:
import 'dart:convert';
import 'package:drift/drift.dart';
import 'package:drift/extensions/json1.dart';
import 'package:json_annotation/json_annotation.dart';
@JsonSerializable()
class ContactData {
final String name;
final List<String> phoneNumbers;
ContactData(this.name, this.phoneNumbers);
factory ContactData.fromJson(Map<String, Object?> json) =>
_$ContactDataFromJson(json);
Map<String, Object?> toJson() => _$ContactDataToJson(this);
}
To easily store this contact representation in a drift database, one could use a JSON column:
class _ContactsConverter extends TypeConverter<ContactData, String> {
@override
ContactData fromSql(String fromDb) {
return ContactData.fromJson(json.decode(fromDb) as Map<String, Object?>);
}
@override
String toSql(ContactData value) {
return json.encode(value.toJson());
}
}
class Contacts extends Table {
IntColumn get id => integer().autoIncrement()();
TextColumn get data => text().map(_ContactsConverter())();
TextColumn get name => text().generatedAs(data.jsonExtract(r'$.name'))();
}
Note the name
column as well: It uses generatedAs
with the jsonExtract
function to
extract the name
field from the JSON value on the fly.
The full syntax for JSON path arguments is explained on the sqlite3 website.
To make the example more complex, let's look at another table storing a log of phone calls:
class Calls extends Table {
IntColumn get id => integer().autoIncrement()();
BoolColumn get incoming => boolean()();
TextColumn get phoneNumber => text()();
DateTimeColumn get callTime => dateTime()();
}
Let's say we wanted to find the contact for each call, if there is any with a matching phone number.
For this to be expressible in SQL, each contacts
row would somehow have to be expanded into a row
for each stored phone number.
Luckily, the json_each
function in sqlite3 can do exactly that, and drift exposes it:
Future<List<(Call, Contact)>> callsWithContact() async {
final phoneNumbersForContact =
contacts.data.jsonEach(this, r'$.phoneNumbers');
final phoneNumberQuery = selectOnly(phoneNumbersForContact)
..addColumns([phoneNumbersForContact.value]);
final query = select(calls).join(
[innerJoin(contacts, calls.phoneNumber.isInQuery(phoneNumberQuery))]);
return query
.map((row) => (row.readTable(calls), row.readTable(contacts)))
.get();
}
Selects without tables¶
Some queries don't need a FROM
clause at all and instead just select some expressions directly.
An example for this may be a select that just uses subquery expressions, like here to query whether
any rows exist in a table:
Future<bool> hasTodoItem() async {
final todoItemExists = existsQuery(select(todoItems));
final row = await selectExpressions([todoItemExists]).getSingle();
return row.read(todoItemExists)!;
}
The selectExpressions
API is similar to selectOnly
, except that it doesn't require any table
at all.
Instead, the expressions in the list passed to selectExpressions
are evaluated in a standalone
select statement and can be parsed from the TypedResult
class returned when evaluating the
query.