### Database Schema
CREATE TABLE Customers (
customer_id INTEGER NOT NULL,
customer_details VARCHAR(255),
PRIMARY KEY (customer_id)
)

CREATE TABLE Properties (
property_id INTEGER NOT NULL,
property_type_code CHAR(15) NOT NULL,
property_address VARCHAR(255),
other_details VARCHAR(255),
PRIMARY KEY (property_id)
)

CREATE TABLE Residents (
resident_id INTEGER NOT NULL,
property_id INTEGER NOT NULL,
date_moved_in DATETIME NOT NULL,
date_moved_out DATETIME NOT NULL,
other_details VARCHAR(255),
PRIMARY KEY (resident_id, property_id, date_moved_in),
FOREIGN KEY (property_id) REFERENCES Properties (property_id)
)

CREATE TABLE Organizations (
organization_id INTEGER NOT NULL,
parent_organization_id INTEGER,
organization_details VARCHAR(255),
PRIMARY KEY (organization_id)
)

CREATE TABLE Services (
service_id INTEGER NOT NULL,
organization_id INTEGER NOT NULL,
service_type_code CHAR(15) NOT NULL,
service_details VARCHAR(255),
PRIMARY KEY (service_id),
FOREIGN KEY (organization_id) REFERENCES Organizations (organization_id)
)

CREATE TABLE Residents_Services (
resident_id INTEGER NOT NULL,
service_id INTEGER NOT NULL,
date_moved_in DATETIME,
property_id INTEGER,
date_requested DATETIME,
date_provided DATETIME,
other_details VARCHAR(255),
PRIMARY KEY (resident_id, service_id),
FOREIGN KEY (service_id) REFERENCES Services (service_id),
FOREIGN KEY (resident_id, property_id, date_moved_in) REFERENCES Residents (resident_id,property_id,date_moved_in)
)

CREATE TABLE Things (
thing_id INTEGER NOT NULL,
organization_id INTEGER NOT NULL,
Type_of_Thing_Code CHAR(15) NOT NULL,
service_type_code CHAR(10) NOT NULL,
service_details VARCHAR(255),
PRIMARY KEY (thing_id),
FOREIGN KEY (organization_id) REFERENCES Organizations (organization_id)
)

CREATE TABLE Customer_Events (
Customer_Event_ID INTEGER NOT NULL,
customer_id INTEGER,
date_moved_in DATETIME,
property_id INTEGER,
resident_id INTEGER,
thing_id INTEGER NOT NULL,
PRIMARY KEY (Customer_Event_ID),
FOREIGN KEY (thing_id) REFERENCES Things (thing_id),
FOREIGN KEY (customer_id) REFERENCES Customers (customer_id),
FOREIGN KEY (resident_id, property_id, date_moved_in) REFERENCES Residents (resident_id,property_id,date_moved_in)
)

CREATE TABLE Customer_Event_Notes (
Customer_Event_Note_ID INTEGER NOT NULL,
Customer_Event_ID INTEGER NOT NULL,
service_type_code CHAR(15) NOT NULL,
resident_id INTEGER NOT NULL,
property_id INTEGER NOT NULL,
date_moved_in DATETIME NOT NULL,
PRIMARY KEY (Customer_Event_Note_ID),
FOREIGN KEY (Customer_Event_ID) REFERENCES Customer_Events (Customer_Event_ID)
)

CREATE TABLE Timed_Status_of_Things (
thing_id INTEGER NOT NULL,
Date_and_Date DATETIME NOT NULL,
Status_of_Thing_Code CHAR(15) NOT NULL,
PRIMARY KEY (thing_id, Date_and_Date, Status_of_Thing_Code),
FOREIGN KEY (thing_id) REFERENCES Things (thing_id)
)

CREATE TABLE Timed_Locations_of_Things (
thing_id INTEGER NOT NULL,
Date_and_Time DATETIME NOT NULL,
Location_Code CHAR(15) NOT NULL,
PRIMARY KEY (thing_id, Date_and_Time, Location_Code),
FOREIGN KEY (thing_id) REFERENCES Things (thing_id))

### Question
Which organizations are not a parent organization of others? List the organization id.

### SQL
SELECT organization_id FROM organizations EXCEPT SELECT parent_organization_id FROM organizations