### Database Schema:
CREATE TABLE gender_age_test (
	device_id	INTEGER	NOT NULL	PRIMARY KEY
);

CREATE TABLE sample_submission (
	device_id	INTEGER	NOT NULL	PRIMARY KEY,
	`F23-`	REAL	NULL	DEFAULT NULL,
	`F24-26`	REAL	NULL	DEFAULT NULL,
	`F27-28`	REAL	NULL	DEFAULT NULL,
	`F29-32`	REAL	NULL	DEFAULT NULL,
	`F33-42`	REAL	NULL	DEFAULT NULL,
	`F43+`	REAL	NULL	DEFAULT NULL,
	`M22-`	REAL	NULL	DEFAULT NULL,
	`M23-26`	REAL	NULL	DEFAULT NULL,
	`M27-28`	REAL	NULL	DEFAULT NULL,
	`M29-31`	REAL	NULL	DEFAULT NULL,
	`M32-38`	REAL	NULL	DEFAULT NULL,
	`M39+`	REAL	NULL	DEFAULT NULL
);

CREATE TABLE events_relevant (
	event_id	INTEGER	NOT NULL	PRIMARY KEY,
	device_id	INTEGER	NULL	DEFAULT NULL,
	timestamp	DATETIME	NOT NULL,
	longitude	REAL	NOT NULL,
	latitude	REAL	NOT NULL,
	foreign key (device_id) references gender_age (device_id)
);

CREATE TABLE app_all (
	app_id	INTEGER	NOT NULL	PRIMARY KEY
);

CREATE TABLE app_labels (
	app_id	INTEGER	NOT NULL,
	label_id	INTEGER	NOT NULL,
	foreign key (app_id) references app_all (app_id),
	foreign key (label_id) references label_categories (label_id)
);

CREATE TABLE gender_age_train (
	device_id	INTEGER	NOT NULL	PRIMARY KEY,
	gender	TEXT	NULL	DEFAULT NULL,
	age	INTEGER	NULL	DEFAULT NULL,
	group	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE events (
	event_id	INTEGER	NOT NULL	PRIMARY KEY,
	device_id	INTEGER	NULL	DEFAULT NULL,
	timestamp	DATETIME	NULL	DEFAULT NULL,
	longitude	REAL	NULL	DEFAULT NULL,
	latitude	REAL	NULL	DEFAULT NULL
);

CREATE TABLE gender_age (
	device_id	INTEGER	NOT NULL	PRIMARY KEY,
	gender	TEXT	NULL	DEFAULT NULL,
	age	INTEGER	NULL	DEFAULT NULL,
	group	TEXT	NULL	DEFAULT NULL,
	foreign key (device_id) references phone_brand_device_model2 (device_id)
);

CREATE TABLE phone_brand_device_model2 (
	device_id	INTEGER	NOT NULL	PRIMARY KEY,
	phone_brand	TEXT	NOT NULL	PRIMARY KEY,
	device_model	TEXT	NOT NULL	PRIMARY KEY
);

CREATE TABLE app_events_relevant (
	event_id	INTEGER	NOT NULL	PRIMARY KEY,
	app_id	INTEGER	NOT NULL	PRIMARY KEY,
	is_installed	INTEGER	NULL	DEFAULT NULL,
	is_active	INTEGER	NULL	DEFAULT NULL,
	foreign key (app_id) references app_all (app_id),
	foreign key (event_id) references events_relevant (event_id)
);

CREATE TABLE label_categories (
	label_id	INTEGER	NOT NULL	PRIMARY KEY,
	category	TEXT	NULL	DEFAULT NULL
);

CREATE TABLE app_events (
	event_id	INTEGER	NOT NULL	PRIMARY KEY,
	app_id	INTEGER	NOT NULL	PRIMARY KEY,
	is_installed	INTEGER	NOT NULL,
	is_active	INTEGER	NOT NULL,
	foreign key (event_id) references events (event_id)
);

### User's Query:
Which brand is most common among people in their twenties?

### Extra Knowledge:
brand refers to phone_brand; brand that is most common refers to MAX(COUNT(phone_brand)); twenties refers to age BETWEEN 20 AND 30;
SELECT T.phone_brand FROM ( SELECT T2.phone_brand, COUNT(T2.phone_brand) AS num FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.age BETWEEN 20 AND 30 GROUP BY T2.phone_brand ) AS T ORDER BY T.num DESC LIMIT 1