Add initial API interaction functions and database schema

DB/schema.sql

@@ -0,0 +1,27 @@
+CREATE TABLE USER (
+    Uid INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL,
+    username TEXT NOT NULL,
+    password TEXT NOT NULL
+);
+
+CREATE TABLE "GROUP" (
+    Gid INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL,
+    GroupName TEXT NOT NULL
+);
+
+CREATE TABLE SUBGROUP (
+    SGid INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL,
+    SubGroupName TEXT NOT NULL,
+    Gid INTEGER NOT NULL,
+    FOREIGN KEY (Gid) REFERENCES "GROUP"(Gid)
+);
+
+CREATE TABLE USER_GROUP (
+    Uid INTEGER NOT NULL,
+    Gid INTEGER NOT NULL,
+    SGid INTEGER,
+    PRIMARY KEY (Uid, Gid),
+    FOREIGN KEY (Uid) REFERENCES USER(Uid),
+    FOREIGN KEY (Gid) REFERENCES "GROUP"(Gid),
+    FOREIGN KEY (SGid) REFERENCES SUBGROUP(SGid)
+);