Average number books issued per student grouped by

Calculate the average number of books currently issued per student, grouped by the student's degree.

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Database schema

CREATE TABLE "public"."staff" (
    "staff_fname" varchar(80) NOT NULL,
    "staff_lname" varchar(80) NOT NULL,
    "id" varchar(20) NOT NULL,
    "gender" varchar(1) NOT NULL,
    "mobile_no" numeric(10,0) NOT NULL,
    "doj" date NOT NULL,
    PRIMARY KEY ("id")
);

CREATE TABLE "public"."book_catalogue" (
    "isbn_no" varchar(50) NOT NULL,
    "title" varchar(256) NOT NULL,
    "publisher" varchar(80) NOT NULL,
    "year" int4 NOT NULL,
    PRIMARY KEY ("isbn_no")
);

CREATE TABLE "public"."book_authors" (
    "isbn_no" varchar(13) NOT NULL,
    "author_fname" varchar(80) NOT NULL,
    "author_lname" varchar(80) NOT NULL,
    PRIMARY KEY ("isbn_no","author_fname","author_lname")
);

CREATE TABLE "public"."book_copies" (
    "isbn_no" varchar(13) NOT NULL,
    "accession_no" varchar(20) NOT NULL,
    PRIMARY KEY ("accession_no")
);

CREATE TABLE "public"."members" (
    "member_no" varchar(20) NOT NULL,
    "member_class" varchar(20) NOT NULL,
    "member_type" varchar(2) NOT NULL,
    "roll_no" varchar(20),
    "id" varchar(20),
    PRIMARY KEY ("member_no")
);

CREATE TABLE "public"."book_issue" (
    "member_no" varchar(20) NOT NULL,
    "accession_no" varchar(20) NOT NULL,
    "doi" date NOT NULL,
    PRIMARY KEY ("member_no","accession_no")
);

CREATE TABLE "public"."departments" (
    "department_code" varchar(80) NOT NULL,
    "department_name" varchar(80),
    "department_building" varchar(80),
    PRIMARY KEY ("department_code")
);

CREATE TABLE "public"."faculty" (
    "faculty_fname" varchar(80) NOT NULL,
    "faculty_lname" varchar(80) NOT NULL,
    "id" varchar(20) NOT NULL,
    "department_code" varchar(80) NOT NULL,
    "gender" varchar(1) NOT NULL,
    "mobile_no" numeric(10,0) NOT NULL,
    "doj" date NOT NULL,
    PRIMARY KEY ("id")
);

CREATE TABLE "public"."students" (
    "student_fname" varchar(80) NOT NULL,
    "student_lname" varchar(80) NOT NULL,
    "roll_no" varchar(20) NOT NULL,
    "department_code" varchar(80) NOT NULL,
    "gender" varchar(1) NOT NULL,
    "mobile_no" numeric(10,0) NOT NULL,
    "dob" date NOT NULL,
    "degree" varchar(80) NOT NULL,
    PRIMARY KEY ("roll_no")
);

CREATE TABLE "public"."quota" (
    "member_type" varchar(2) NOT NULL,
    "max_books" int4 NOT NULL,
    "max_duration" int4 NOT NULL,
    PRIMARY KEY ("member_type")
);