Database migrations in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens after a database migration?

You have an Android app using Room database. You add a new column to a table and write a migration from version 1 to 2. What will the app do after updating if the migration is correct?

A The app crashes because the schema version is higher than before.

B The app deletes all existing data and creates a new database.

C The app updates the database schema without losing existing data.

D The app ignores the migration and uses the old schema.

💡 Hint

Think about what a migration is supposed to do to keep data safe.

A correct migration updates the database schema to the new version while preserving existing data. It applies changes like adding columns or tables without deleting data.

📝 Syntax

intermediate

Identify the correct Room migration code snippet

Which Kotlin code snippet correctly defines a migration from version 1 to 2 that adds a new column 'age' to the 'User' table with a default value 0?

Android Kotlin

val MIGRATION_1_2 = object : Migration(1, 2) {
  override fun migrate(database: SupportSQLiteDatabase) {
    // migration code here
  }
}

A database.execSQL("DROP TABLE User")

B database.execSQL("ALTER TABLE User ADD COLUMN age INTEGER NOT NULL DEFAULT 0")

C database.execSQL("CREATE TABLE User(age INTEGER)")

D database.execSQL("ALTER TABLE User ADD COLUMN age TEXT")

💡 Hint

Adding a non-null column requires a default value in SQLite.

Option B correctly adds a new non-null INTEGER column with a default value, which is required to avoid errors during migration.

❓ lifecycle

advanced

When is the migration code executed in an Android app?

At what point during the app lifecycle does Room execute the migration code when upgrading the database version?

A When the app is uninstalled and reinstalled.

B Immediately when the app starts, before any UI loads.

C Only when the user triggers a manual database refresh.

D When the database is first opened or accessed after the app update.

💡 Hint

Think about when the app needs to access the database after an update.

Room runs migrations when the database is opened for the first time after an update, ensuring the schema is current before queries run.

🔧 Debug

advanced

Why does this migration cause a crash?

You wrote this migration code: val MIGRATION_2_3 = object : Migration(2, 3) { override fun migrate(database: SupportSQLiteDatabase) { database.execSQL("ALTER TABLE User ADD COLUMN email TEXT NOT NULL") } } The app crashes on upgrade. Why?

A Adding a NOT NULL column without a default value causes a constraint error.

B The SQL syntax for ALTER TABLE is incorrect.

C The migration version numbers are reversed.

D The User table does not exist in version 2.

💡 Hint

SQLite requires a default value when adding NOT NULL columns.

SQLite cannot add a NOT NULL column without a default value because existing rows would violate the constraint, causing a crash.

🧠 Conceptual

expert

What is the best practice for handling complex schema changes in Room migrations?

You need to rename a column and change its type in a Room database migration. SQLite does not support renaming columns directly. What is the recommended approach?

A Create a new table with the desired schema, copy data from the old table, drop the old table, then rename the new table.

B Delete the database and recreate it with the new schema.

C Use ALTER TABLE to rename the column and change its type directly.

D Ignore the change and keep the old schema.

💡 Hint

Think about how to preserve data when SQLite lacks direct support for renaming columns.

Since SQLite does not support renaming columns or changing types directly, the best practice is to create a new table with the new schema, copy data over, drop the old table, and rename the new one.

Practice

1.

What is the main purpose of a database migration in Android apps?

easy

A. To update the database schema without losing existing data

B. To delete all data and recreate the database from scratch

C. To backup the database to external storage

D. To optimize database queries for faster performance

Solution

1. Step 1: Understand database migration purpose

   Database migrations are used to change the structure of the database safely.

2. Step 2: Identify data preservation goal

   Migrations update tables or columns without deleting existing user data.

3. Final Answer:

   To update the database schema without losing existing data -> Option A

4. Quick Check:

   Migration = Safe schema update [OK]

Hint: Migrations keep data safe while changing schema [OK]

Common Mistakes:

• Thinking migrations delete all data
• Confusing migrations with backups
• Believing migrations only optimize queries

2.

Which of the following is the correct way to define a migration from version 1 to 2 in Room database?

val MIGRATION_1_2 = object : Migration(1, 2) {
    override fun migrate(database: SupportSQLiteDatabase) {
        // ?
    }
}

easy

A. database.execSQL("ALTER TABLE users ADD COLUMN age INTEGER DEFAULT 0")

B. database.update("users", "age", 0)

C. database.insert("users", ContentValues())

D. database.delete("users")

Solution

1. Step 1: Understand Migration class usage

   Migration requires SQL commands inside migrate() to change schema.

2. Step 2: Identify correct SQL command

   ALTER TABLE adds a new column safely; execSQL runs raw SQL.

3. Final Answer:

   database.execSQL("ALTER TABLE users ADD COLUMN age INTEGER DEFAULT 0") -> Option A

4. Quick Check:

   Migration uses execSQL with ALTER TABLE [OK]

Hint: Use execSQL with ALTER TABLE to add columns [OK]

Common Mistakes:

• Using update() instead of execSQL
• Trying to insert or delete rows in migration
• Not providing default value for new column

3.

Given this migration code, what will be the result after migrating from version 1 to 2?

val MIGRATION_1_2 = object : Migration(1, 2) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("CREATE TABLE IF NOT EXISTS books (id INTEGER PRIMARY KEY, title TEXT)")
    }
}

Assuming version 1 had no books table.

medium

A. No changes happen because CREATE TABLE is ignored

B. Migration fails because table 'books' already exists

C. A new table 'books' is created with columns 'id' and 'title'

D. Existing tables are deleted and only 'books' remains

Solution

1. Step 1: Analyze migration SQL command

   CREATE TABLE IF NOT EXISTS creates the table only if it doesn't exist.

2. Step 2: Check initial database state

   Version 1 has no 'books' table, so it will be created.

3. Final Answer:

   A new table 'books' is created with columns 'id' and 'title' -> Option C

4. Quick Check:

   CREATE TABLE IF NOT EXISTS creates missing table [OK]

Hint: IF NOT EXISTS avoids errors if table exists [OK]

Common Mistakes:

• Assuming migration fails if table exists
• Thinking CREATE TABLE is ignored
• Believing existing tables get deleted

4.

What is wrong with this migration code from version 2 to 3?

val MIGRATION_2_3 = object : Migration(2, 3) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("ALTER TABLE users ADD COLUMN email TEXT")
        database.execSQL("ALTER TABLE users ADD COLUMN email TEXT")
    }
}

medium

A. The migration does not update the database version

B. The same column 'email' is added twice causing an error

C. The SQL syntax for ALTER TABLE is incorrect

D. The migration should delete the 'users' table first

Solution

1. Step 1: Review SQL commands in migration

   The code tries to add the 'email' column twice to the same table.

2. Step 2: Understand SQLite behavior

   Adding the same column twice causes a runtime error during migration.

3. Final Answer:

   The same column 'email' is added twice causing an error -> Option B

4. Quick Check:

   Duplicate column addition causes migration failure [OK]

Hint: Avoid adding same column twice in one migration [OK]

Common Mistakes:

• Ignoring duplicate column errors
• Thinking migration version is updated automatically
• Believing ALTER TABLE syntax is wrong here

5.

You want to rename a column username to user_name in your Room database from version 3 to 4. SQLite does not support renaming columns directly. Which migration strategy is correct?

hard

A. Add a new column and leave the old column as is

B. Use ALTER TABLE to rename the column directly

C. Delete the old table and create a new one with the new column name

D. Create a new table with the new column name, copy data, drop old table, rename new table

Solution

1. Step 1: Understand SQLite column rename limitation

   SQLite does not support direct column renaming with ALTER TABLE.

2. Step 2: Apply recommended migration pattern

   Create a new table with desired schema, copy data, drop old table, rename new table.

3. Final Answer:

   Create a new table with the new column name, copy data, drop old table, rename new table -> Option D

4. Quick Check:

   Rename column = new table + copy data [OK]

Hint: Rename columns by new table + copy data [OK]

Common Mistakes:

• Trying to rename column with ALTER TABLE
• Deleting old table without copying data
• Adding new column but not removing old one