### Database Schema
CREATE TABLE `Ref_Address_Types` (
`address_type_code` VARCHAR(15) PRIMARY KEY,
`address_type_description` VARCHAR(80)
)

CREATE TABLE `Ref_Detention_Type` (
`detention_type_code` VARCHAR(10) PRIMARY KEY,
`detention_type_description` VARCHAR(80)
)

CREATE TABLE `Ref_Incident_Type` (
`incident_type_code` VARCHAR(10) PRIMARY KEY,
`incident_type_description` VARCHAR(80)
)

CREATE TABLE `Addresses` (
`address_id` INTEGER PRIMARY KEY,
`line_1` VARCHAR(120),
`line_2` VARCHAR(120),
`line_3` VARCHAR(120),
`city` VARCHAR(80),
`zip_postcode` VARCHAR(20),
`state_province_county` VARCHAR(50),
`country` VARCHAR(50),
`other_address_details` VARCHAR(255)
)

CREATE TABLE `Students` (
`student_id` INTEGER PRIMARY KEY,
`address_id` INTEGER NOT NULL,
`first_name` VARCHAR(80),
`middle_name` VARCHAR(40),
`last_name` VARCHAR(40),
`cell_mobile_number` VARCHAR(40),
`email_address` VARCHAR(40),
`date_first_rental` DATETIME,
`date_left_university` DATETIME,
`other_student_details` VARCHAR(255),
FOREIGN KEY (`address_id` ) REFERENCES `Addresses`(`address_id` )
)

CREATE TABLE `Teachers` (
`teacher_id` INTEGER PRIMARY KEY,
`address_id` INTEGER NOT NULL,
`first_name` VARCHAR(80),
`middle_name` VARCHAR(80),
`last_name` VARCHAR(80),
`gender` VARCHAR(1),
`cell_mobile_number` VARCHAR(40),
`email_address` VARCHAR(40),
`other_details` VARCHAR(255),
FOREIGN KEY (`address_id` ) REFERENCES `Addresses`(`address_id` )
)

CREATE TABLE `Assessment_Notes` (
`notes_id` INTEGER NOT NULL ,
`student_id` INTEGER,
`teacher_id` INTEGER NOT NULL,
`date_of_notes` DATETIME,
`text_of_notes` VARCHAR(255),
`other_details` VARCHAR(255),
FOREIGN KEY (`student_id` ) REFERENCES `Students`(`student_id` ),
FOREIGN KEY (`teacher_id` ) REFERENCES `Teachers`(`teacher_id` )
)

CREATE TABLE `Behavior_Incident` (
`incident_id` INTEGER PRIMARY KEY,
`incident_type_code` VARCHAR(10) NOT NULL,
`student_id` INTEGER NOT NULL,
`date_incident_start` DATETIME,
`date_incident_end` DATETIME,
`incident_summary` VARCHAR(255),
`recommendations` VARCHAR(255),
`other_details` VARCHAR(255),
FOREIGN KEY (`incident_type_code` ) REFERENCES `Ref_Incident_Type`(`incident_type_code` ),
FOREIGN KEY (`student_id` ) REFERENCES `Students`(`student_id` )
)

CREATE TABLE `Detention` (
`detention_id` INTEGER PRIMARY KEY,
`detention_type_code` VARCHAR(10) NOT NULL,
`teacher_id` INTEGER,
`datetime_detention_start` DATETIME,
`datetime_detention_end` DATETIME,
`detention_summary` VARCHAR(255),
`other_details` VARCHAR(255),
FOREIGN KEY (`detention_type_code` ) REFERENCES `Ref_Detention_Type`(`detention_type_code` ),
FOREIGN KEY (`teacher_id` ) REFERENCES `Teachers`(`teacher_id` )
)

CREATE TABLE `Student_Addresses` (
`student_id` INTEGER NOT NULL,
`address_id` INTEGER NOT NULL,
`date_address_from` DATETIME NOT NULL,
`date_address_to` DATETIME,
`monthly_rental` DECIMAL(19,4),
`other_details` VARCHAR(255),
FOREIGN KEY (`address_id` ) REFERENCES `Addresses`(`address_id` ),
FOREIGN KEY (`student_id` ) REFERENCES `Students`(`student_id` )
)

CREATE TABLE `Students_in_Detention` (
`student_id` INTEGER NOT NULL,
`detention_id` INTEGER NOT NULL,
`incident_id` INTEGER NOT NULL,
FOREIGN KEY (`incident_id` ) REFERENCES `Behavior_Incident`(`incident_id` ),
FOREIGN KEY (`detention_id` ) REFERENCES `Detention`(`detention_id` ),
FOREIGN KEY (`student_id` ) REFERENCES `Students`(`student_id` )
)

### Question
How many distinct students have been in detention?

### SQL
SELECT count(DISTINCT student_id) FROM Students_in_Detention