Why not connect a database like MySQL or SQL Server (Big God) on Android or iOS?

Many students have the idea of directly connected to these databases, if I say the following two questions you still want to connect directly, then I can't

A database is the most important part of a server, the most vulnerable part, and the most sensitive part.

If the direct connection will cause the following problems

1. Security issues, your connection database account password exposed to the client, so it is easy to get caught, whether it is the anti-compilation of Android, or capture the package, it is easy to obtain this information, and if the need for direct connection, the database is bound to listen to 0.0.0.0, so that anyone can try your database, What a horrible thing.

2. Performance issues, we know that a database of concurrent connection is very low, MySQL default configuration only 100, directly connected to the database, resulting in no use of any cache, such as Memcache,

There should be other questions, let's all add up.

Why not connect a database like MySQL or SQL Server (Big God) on Android or iOS?