diff --git a/internal/database/mysql.go b/internal/database/mysql.go
index a86af89..ff343b8 100644
--- a/internal/database/mysql.go
+++ b/internal/database/mysql.go
@@ -1,194 +1,246 @@
-// Copyright (c) 2024 Seoyoung Cho and Carlos Andres Cotera Jurado.
-
-package database
-
-import (
-	"database/sql"
-	"errors"
-	"log"
-	"os"
-	"strconv"
-	"strings"
-
-	_ "github.com/go-sql-driver/mysql"
-	"github.com/matcha-devs/matcha/internal"
-	"golang.org/x/crypto/bcrypt"
-)
-
-type MySQLDatabase struct {
-	rootDSN      string
-	dbName       string
-	underlyingDB *sql.DB
-}
-
-func New(dbName string, username string, password string) (mysql *MySQLDatabase) {
-	// Initialized struct to be returned.
-	mysql = &MySQLDatabase{
-		rootDSN:      username + ":" + password + "@tcp(localhost:3306)/",
-		dbName:       dbName,
-		underlyingDB: nil,
-	}
-
-	// Open a separate connection to the root DSN and create the database if it does not exist
-	initDB, err := sql.Open("mysql", mysql.rootDSN+"?multiStatements=true")
-	if err != nil {
-		log.Fatalln("Error opening MySQL root DSN -", err)
-	}
-	if err = initDB.Ping(); err != nil {
-		if err = initDB.Close(); err != nil {
-			log.Fatalln("Error closing broken MySQL root DSN -", err)
-		}
-		log.Fatalln("Error connecting to MySQL root DSN -", err)
-	}
-	_, err = initDB.Exec("CREATE DATABASE IF NOT EXISTS " + dbName)
-	if err != nil {
-		log.Fatalln("Error creating database -", err)
-	}
-
-	// Move to the database and run 'init_tables.sql' script.
-	_, err = initDB.Exec("USE " + dbName)
-	if err != nil {
-		log.Fatalln("Error using", dbName, "-", err)
-	}
-	initScript, err := os.ReadFile("internal/database/queries/init_tables.sql")
-	if err != nil {
-		log.Fatalln("Error reading init_tables.sql file -", err)
-	}
-	if _, err = initDB.Exec(string(initScript)); err != nil {
-		log.Fatalln("Error executing init_tables.sql -", err)
-	}
-	if err := initDB.Close(); err != nil {
-		log.Fatalln("Error closing init DB -", err)
-	}
-	return
-}
-
-func (db *MySQLDatabase) Open() (err error) {
-	if db.underlyingDB, err = sql.Open("mysql", db.rootDSN+db.dbName+"?parseTime=true"); err != nil {
-		log.Println("Error opening database -", err)
-		return
-	}
-	log.Println("MySQL Database connecting to", db.rootDSN[strings.Index(db.rootDSN, "@"):]+db.dbName, "🫡")
-	if err = db.underlyingDB.Ping(); err != nil {
-		if err = db.underlyingDB.Close(); err != nil {
-			log.Println("Error closing broken database -", err)
-		}
-		log.Println("Error connecting to database -", err)
-	}
-	return
-}
-
-func (db *MySQLDatabase) Close() (err error) {
-	if err = db.underlyingDB.Close(); err != nil {
-		log.Println("underlying database close failure -", err)
-	} else {
-		log.Println("MySQL database has closed 👋🏽")
-	}
-	return
-}
-
-func (db *MySQLDatabase) AuthenticateLogin(email, password string) (id uint64, err error) {
-	var hash []byte
-	err = db.underlyingDB.QueryRow("SELECT id, password FROM users WHERE BINARY email = ?", email).Scan(
-		&id, &hash,
-	)
-	if errors.Is(err, sql.ErrNoRows) {
-		return 0, errors.New("invalid email")
-	}
-	if err = bcrypt.CompareHashAndPassword(hash, []byte(password)); err != nil {
-		return 0, errors.New("invalid password")
-	}
-	return
-}
-
-func (db *MySQLDatabase) GetUser(id uint64) (user *internal.User) {
-	user = &internal.User{}
-	err := db.underlyingDB.QueryRow("SELECT * FROM users WHERE id = ?", id).Scan(
-		&user.ID, &user.FirstName, &user.MiddleName, &user.LastName, &user.Email, &user.Password, &user.DateOfBirth,
-		&user.CreatedOn)
-	if errors.Is(err, sql.ErrNoRows) {
-		log.Println("No user with ID:", id, "-", err)
-		return nil
-	} else if err != nil {
-		log.Println("Failed to query users for ID:", id, "-", err)
-		return nil
-	} else if !user.IsValid() {
-		log.Println("Malformed user with ID:", id, "-", user)
-		return nil
-	}
-	return
-}
-
-func (db *MySQLDatabase) getOpenID() (id uint64, err error) {
-	err = db.underlyingDB.QueryRow("SELECT id FROM openid LIMIT 1").Scan(&id)
-	if err != nil && !errors.Is(err, sql.ErrNoRows) {
-		log.Println("Failed to query for an openid -", err)
-		return 0, errors.New("invalid openid")
-	}
-	return
-}
-
-func (db *MySQLDatabase) AddUser(firstName, middleName, lastName, email, password, dateOfBirth string) (
-	id uint64, err error) {
-	if len(firstName) == 0 || len(lastName) == 0 || len(email) == 0 || len(password) == 0 || len(dateOfBirth) == 0 {
-		return 0, errors.New("empty fields")
-	}
-	query := "INSERT INTO users (first_name, middle_name, last_name, email, password, date_of_birth"
-	if openID, err := db.getOpenID(); err != nil && !errors.Is(err, sql.ErrNoRows) {
-		log.Println("Error getting open id -", err)
-		return id, errors.New("internal server error")
-	} else if openID == 0 {
-		query += `) VALUES (?, ?, ?, ?, ?, ?)`
-	} else {
-		log.Println("Re-using open id:", openID, "for {"+email+"}")
-		id = openID
-		query += `, id) VALUES (?, ?, ?, ?, ?, ?, ` + strconv.FormatUint(id, 10) + `)`
-		if _, err = db.underlyingDB.Exec("DELETE FROM openid WHERE id = ?", id); err != nil {
-			log.Println("Error deleting open id -", err)
-			return id, errors.New("internal server error")
-		}
-	}
-	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
-	if err != nil {
-		log.Println("Error hashing password -", err)
-		return id, errors.New("internal server error")
-	}
-	result, err := db.underlyingDB.Exec(query, firstName, middleName, lastName, email, hashedPassword, dateOfBirth)
-	if err != nil {
-		log.Println("Error adding user -", err)
-		return 0, errors.New("internal server error")
-	}
-	if id == 0 {
-		userid, err := result.LastInsertId()
-		if err != nil {
-			log.Println("Error getting user ID -", err)
-			return 0, errors.New("internal server error")
-		}
-		id = uint64(userid)
-		log.Println("All existing IDs in use, assigning new ID:", id, "to {"+email+"}")
-	}
-	return
-}
-
-func (db *MySQLDatabase) GetUserID(email string) (id uint64) {
-	// TODO(@seoyoungcho213): might not use this anymore cuz of cookie
-	if err := db.underlyingDB.QueryRow(
-		"SELECT id FROM users WHERE BINARY email = ?", email,
-	).Scan(&id); err != nil && !errors.Is(err, sql.ErrNoRows) {
-		log.Println("Error querying users for email:"+email, "-", err)
-		return 0
-	}
-	return id
-}
-
-func (db *MySQLDatabase) DeleteUser(id uint64) (err error) {
-	if _, err = db.underlyingDB.Exec("INSERT INTO openid (id) VALUES(?)", id); err != nil {
-		log.Println("Error inserting openID", id, " to the table -", err)
-		return errors.New("internal server error")
-	}
-	if _, err = db.underlyingDB.Exec("DELETE FROM users WHERE BINARY id = ?", id); err != nil {
-		log.Println("Error deleting the user id", id, " -", err)
-		return errors.New("internal server error")
-	}
-	return err
-}
+// Copyright (c) 2024 Seoyoung Cho and Carlos Andres Cotera Jurado.
+
+package database
+
+import (
+	"database/sql"
+	"errors"
+	"log"
+	"os"
+	"strconv"
+	"strings"
+
+	_ "github.com/go-sql-driver/mysql"
+	"github.com/matcha-devs/matcha/internal"
+	"golang.org/x/crypto/bcrypt"
+)
+
+type MySQLDatabase struct {
+	rootDSN      string
+	dbName       string
+	underlyingDB *sql.DB
+}
+
+func New(dbName string, username string, password string) (mysql *MySQLDatabase) {
+	// Initialized struct to be returned.
+	mysql = &MySQLDatabase{
+		rootDSN:      username + ":" + password + "@tcp(localhost:3306)/",
+		dbName:       dbName,
+		underlyingDB: nil,
+	}
+
+	// Open a separate connection to the root DSN and create the database if it does not exist
+	initDB, err := sql.Open("mysql", mysql.rootDSN+"?multiStatements=true")
+	if err != nil {
+		log.Fatalln("Error opening MySQL root DSN -", err)
+	}
+	if err = initDB.Ping(); err != nil {
+		if err = initDB.Close(); err != nil {
+			log.Fatalln("Error closing broken MySQL root DSN -", err)
+		}
+		log.Fatalln("Error connecting to MySQL root DSN -", err)
+	}
+	_, err = initDB.Exec("CREATE DATABASE IF NOT EXISTS " + dbName)
+	if err != nil {
+		log.Fatalln("Error creating database -", err)
+	}
+
+	// Move to the database and run 'init_tables.sql' script.
+	_, err = initDB.Exec("USE " + dbName)
+	if err != nil {
+		log.Fatalln("Error using", dbName, "-", err)
+	}
+	initScript, err := os.ReadFile("internal/database/queries/init_tables.sql")
+	if err != nil {
+		log.Fatalln("Error reading init_tables.sql file -", err)
+	}
+	if _, err = initDB.Exec(string(initScript)); err != nil {
+		log.Fatalln("Error executing init_tables.sql -", err)
+	}
+	if err := initDB.Close(); err != nil {
+		log.Fatalln("Error closing init DB -", err)
+	}
+	return
+}
+
+func (db *MySQLDatabase) Open() (err error) {
+	if db.underlyingDB, err = sql.Open("mysql", db.rootDSN+db.dbName+"?parseTime=true"); err != nil {
+		log.Println("Error opening database -", err)
+		return
+	}
+	log.Println("MySQL Database connecting to", db.rootDSN[strings.Index(db.rootDSN, "@"):]+db.dbName, "🫡")
+	if err = db.underlyingDB.Ping(); err != nil {
+		if err = db.underlyingDB.Close(); err != nil {
+			log.Println("Error closing broken database -", err)
+		}
+		log.Println("Error connecting to database -", err)
+	}
+	return
+}
+
+func (db *MySQLDatabase) Close() (err error) {
+	if err = db.underlyingDB.Close(); err != nil {
+		log.Println("underlying database close failure -", err)
+	} else {
+		log.Println("MySQL database has closed 👋🏽")
+	}
+	return
+}
+
+func (db *MySQLDatabase) AuthenticateLogin(email, password string) (id uint64, err error) {
+	var hash []byte
+	err = db.underlyingDB.QueryRow("SELECT id, password FROM users WHERE BINARY email = ?", email).Scan(
+		&id, &hash,
+	)
+	if errors.Is(err, sql.ErrNoRows) {
+		return 0, errors.New("invalid email")
+	}
+	if err = bcrypt.CompareHashAndPassword(hash, []byte(password)); err != nil {
+		return 0, errors.New("invalid password")
+	}
+	return
+}
+
+func (db *MySQLDatabase) GetUser(id uint64) (user *internal.User) {
+	user = &internal.User{}
+	err := db.underlyingDB.QueryRow("SELECT * FROM users WHERE id = ?", id).Scan(
+		&user.ID, &user.FirstName, &user.MiddleName, &user.LastName, &user.Email, &user.Password, &user.DateOfBirth,
+		&user.CreatedOn)
+	if errors.Is(err, sql.ErrNoRows) {
+		log.Println("No user with ID:", id, "-", err)
+		return nil
+	} else if err != nil {
+		log.Println("Failed to query users for ID:", id, "-", err)
+		return nil
+	} else if !user.IsValid() {
+		log.Println("Malformed user with ID:", id, "-", user)
+		return nil
+	}
+	return
+}
+
+// EmailExists checks if an email already exists in the users table.
+func (db *MySQLDatabase) EmailExists(email string) (bool, error) {
+	var exists bool
+	query := "SELECT EXISTS (SELECT 1 FROM users WHERE email = ?)"
+	err := db.underlyingDB.QueryRow(query, email).Scan(&exists)
+	if err != nil {
+		log.Println("Error checking email existence -", err)
+		return false, err
+	}
+	return exists, nil
+}
+
+func (db *MySQLDatabase) getOpenID(tx *sql.Tx) (uint64, error) {
+	var id uint64
+	err := tx.QueryRow("SELECT id FROM openid LIMIT 1").Scan(&id)
+	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+		log.Println("Failed to query for an openid -", err)
+		return 0, errors.New("invalid openid")
+	}
+	return id, nil
+}
+
+func (db *MySQLDatabase) AddUser(firstName, middleName, lastName, email, password, dateOfBirth string) (id uint64, err error) {
+	if len(firstName) == 0 || len(lastName) == 0 || len(email) == 0 || len(password) == 0 || len(dateOfBirth) == 0 {
+		return 0, errors.New("empty fields")
+	}
+	//Check if email already exists
+	if emailExists, err := db.EmailExists(email); err != nil {
+		return 0, errors.New("internal server error")
+	} else if emailExists {
+		return 0, errors.New("email already exists")
+	} 
+	
+	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		log.Println("Error hashing password -", err)
+		return id, errors.New("internal server error")
+	}
+
+	// Start transaction
+	tx, err := db.underlyingDB.Begin()
+	if err != nil {
+		log.Println("Error starting transaction -", err)
+		return 0, errors.New("internal server error")
+	}
+	defer func() {
+		if err != nil {
+			tx.Rollback()
+		}
+	}()
+
+	// Get open ID
+	openID, err := db.getOpenID(tx)
+	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+		log.Println("Error getting open ID -", err)
+		return 0, errors.New("internal server error")
+	}
+
+	// Build query
+	var query string
+	if openID == 0 {
+		query = "INSERT INTO users (first_name, middle_name, last_name, email, password, date_of_birth) VALUES (?, ?, ?, ?, ?, ?)"
+	} else {
+		log.Println("Re-using open ID:", openID, "for {"+email+"}")
+		query = "INSERT INTO users (first_name, middle_name, last_name, email, password, date_of_birth, id) VALUES (?, ?, ?, ?, ?, ?, " + strconv.FormatUint(openID, 10) + ")"
+		if _, err = tx.Exec("DELETE FROM openid WHERE id = ?", openID); err != nil {
+			log.Println("Error deleting open ID -", err)
+			return 0, errors.New("internal server error")
+		}
+	}
+
+	// Execute query
+	result, err := tx.Exec(query, firstName, middleName, lastName, email, string(hashedPassword), dateOfBirth)
+	if err != nil {
+		log.Println("Error adding user -", err)
+		if openID != 0 {
+			tx.Exec("INSERT INTO openid (id) VALUES (?)", openID)
+		}
+		return 0, errors.New("internal server error")
+	}
+
+	// Get inserted user ID
+	if openID == 0 {
+		userid, err := result.LastInsertId()
+		if err != nil {
+			log.Println("Error getting user ID -", err)
+			return 0, errors.New("internal server error")
+		}
+		id = uint64(userid)
+		log.Println("All existing IDs in use, assigning new ID:", id, "to {"+email+"}")
+	} else {
+		id = openID
+	}
+
+	// Commit transaction
+	if err := tx.Commit(); err != nil {
+		log.Println("Error committing transaction -", err)
+		return 0, errors.New("internal server error")
+	}
+
+	return id, nil
+}
+
+func (db *MySQLDatabase) GetUserID(email string) (id uint64) {
+	// TODO(@seoyoungcho213): might not use this anymore cuz of cookie
+	if err := db.underlyingDB.QueryRow(
+		"SELECT id FROM users WHERE BINARY email = ?", email,
+	).Scan(&id); err != nil && !errors.Is(err, sql.ErrNoRows) {
+		log.Println("Error querying users for email:"+email, "-", err)
+		return 0
+	}
+	return id
+}
+
+func (db *MySQLDatabase) DeleteUser(id uint64) (err error) {
+	if _, err = db.underlyingDB.Exec("INSERT INTO openid (id) VALUES(?)", id); err != nil {
+		log.Println("Error inserting openID", id, " to the table -", err)
+		return errors.New("internal server error")
+	}
+	if _, err = db.underlyingDB.Exec("DELETE FROM users WHERE BINARY id = ?", id); err != nil {
+		log.Println("Error deleting the user id", id, " -", err)
+		return errors.New("internal server error")
+	}
+	return err
+}
\ No newline at end of file
diff --git a/internal/database/mysql_test.go b/internal/database/mysql_test.go
index 6439355..53a6dd6 100644
--- a/internal/database/mysql_test.go
+++ b/internal/database/mysql_test.go
@@ -163,30 +163,27 @@ func TestAddUser(t *testing.T) {
 		expectedID    uint64
 		expectedError bool
 	}{
-		{"AddFirstUser", "test", "", "user", "test_user@example.com",
-			"test_pass", "2004-12-22", 1, false},
-		{"AddSecondUser", "test", "", "user2", "test_user@example2.com",
-			"test_pass2", "2024-12-22", 2, false},
-		{"AddDuplicateName", "test", "", "user", "dupl_name@example.com",
-			"test_pass3", "2024-10-22", 3, false},
-		{"AddDuplicateEmail", "unique", "", "user", "test_user@example2.com",
-			"test_pass4", "2024-10-22", 0, true},
-		{"AddEmptyFirstname", "", "", "user", "empty_first@example.com",
-			"test_pass5", "2024-08-22", 0, true},
-		// TODO : This supposed to be ID:3, but duplicate email increment ID by one. Fix this error.
-		{"AddEmptyMiddlename", "empty", "", "middle", "empty_mid@example.com",
-			"test_pass6", "2024-07-22", 5, false},
-		{"AddEmptyLastname", "empty", "email", "", "empty_last@example.com",
-			"test_pass7", "2024-07-22", 0, true},
-		{"AddEmptyEmail", "empty", "", "email", "", "test_pass7",
-			"2024-07-22", 0, true},
-		{"AddEmptyPassword", "empty", "pass", "user",
-			"empty_pass_user@example.com", "", "2021-07-22", 0, true},
-		{"AddEmptyDateOfBirth", "empty", "", "DOB", "empty_dob@example.com",
-			"test_pass8", "", 0, true},
-
+		{"AddFirstUser", "testname", "", "user", "test_user@example.com","test_pass", "2004-12-22", 1, false},
+		{"AddSecondUser", "test2", "", "user2", "test_user@example2.com","test_pass2", "2024-12-22", 2, false},
+		{"AddDuplicateName", "testname", "", "user", "dupl_name@example.com","test_pass3", "2024-10-22", 3, false},
+		{"AddDuplicateEmail", "dupl_email", "", "user", "test_user@example2.com","test_pass4", "2024-10-22", 0, true},
+		{"AddEmptyFirstname", "", "", "user", "empty_first@example.com","test_pass5", "2024-08-22", 0, true},
+		{"AddEmptyMiddlename", "empty", "", "middle", "empty_mid@example.com","test_pass6", "2024-07-22", 4, false},
+		{"AddEmptyLastname", "empty", "email", "", "empty_last@example.com","test_pass7", "2024-07-22", 0, true},
+		{"AddEmptyEmail", "empty", "", "email", "", "test_pass7","2024-07-22", 0, true},
+		{"AddEmptyPassword", "empty", "pass", "user","empty_pass_user@example.com", "", "2021-07-22", 0, true},
+		{"AddEmptyDateOfBirth", "empty", "", "DOB", "empty_dob@example.com","test_pass8", "", 0, true},
+
+		{"MultipleFailures1", "user_fail_1","middle","last", "user_fail@example.com","","2024-10-22", 0, true}, // Should fail
+		{"MultipleFailures2", "user_fail","middle","last", "","password","2024-10-22", 0, true}, // Should fail
+		{"MultipleFailures3", "","middle","last", "user_fail@example.com","password", "2024-10-22", 0, true}, // Should fail
+		{"ValidAfterFailures", "valid_user", "middle","last", "valid_user@example.com","password", "2024-10-22", 5, false}, // Should be ID 5 after failures
+		
+		// WIP: These test cases to Re-use open id are not implemented yet!!! This DOES NOT WORK ATM!!!:
+		//{"CreateAndReuseOpenID1", "temp_user", "middle","last", "temp_user@example.com","password", "2024-10-22", 6, false}, // Should be ID 6
+		//{"ReuseOpenID", "new_user", "middle","last", "new_user@example.com", "password","2024-10-22", 6, false}, // Should reuse ID 6
 		// TODO: The functionality for this test need to be implemented
-		// {"AddInvalidEmail", "invalid_email_user", "invalid_email.com", "test_pass7", 0, true},
+		// {"AddInvalidEmail", "invalid_email_user","middle","last", "invalidemail.com", "test_pass7", "2024-10-22", 0, true}, // TODO: The functionality for this test need to be implemented
 	}
 
 	for _, tc := range testCases {
@@ -205,6 +202,15 @@ func TestAddUser(t *testing.T) {
 					if id != tc.expectedID {
 						t.Fatalf("Expected user id %d but got %d for case: %s", tc.expectedID, id, tc.name)
 					}
+					// Specific case to delete the user to test reusing open ID
+					if tc.name == "CreateAndReuseOpenID1" {
+						_, err = probe.Exec("DELETE FROM users WHERE id = ?", id)
+						if err != nil {
+							t.Fatalf("Failed to delete user - %v for case: %s", err, tc.name)
+						} else {
+							t.Log("Deleted user for case: ", tc.name)
+						}
+					}
 				}
 			},
 		)
@@ -229,11 +235,11 @@ func TestAuthenticateLogin(t *testing.T) {
 		password   string
 		expectedID uint64
 		happyPath  bool
-	}{
-		{name: "valid_login", email: happyEmail, password: happyPass, expectedID: 1, happyPath: true},
-		{name: "bad_email", email: "im a mistake", password: happyPass, expectedID: 0, happyPath: false},
-		{name: "bad_pass", email: happyEmail, password: "im a mistake", expectedID: 0, happyPath: false},
-		{name: "bad_email_and_pass", email: "we're both", password: "mistakes", expectedID: 0, happyPath: false},
+	}{		
+		{name:"ValidLogin", email: happyEmail, password: happyPass, expectedID: 1, happyPath: true},
+		{name:"BadUser", email: "im a mistake", password: happyPass, expectedID: 0, happyPath: false},
+		{name:"BadPass", email: happyEmail, password: "im a mistake", expectedID: 0, happyPath: false},
+		{name:"BadUser_and_BadPass", email: "we're both", password: "mistakes", expectedID: 0, happyPath: false},
 	}
 
 	for _, testCase := range testCases {