Choose any three.
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内存数据库
In-Memory Databases
SQLite数据库通常是存储在一个单独的普通磁盘文件中。不过，在某些情况下，数据库也可以存储在内存中。
An SQLite database is normally stored in a single ordinary disk
file. However, in certain circumstances, the database might be stored in
memory.
建立一个纯内存数据库最常用的方法是使用特殊的文件名":memory:"来打开数据库。也就是说，传入sqlite3_open()、sqlite3_open16()或sqlite3_open_v2()这几个函数的参数不是一个真正磁盘文件的名字，而是一个字符串":memory:"。例如：
The most common way to force an SQLite database to exist purely
in memory is to open the database using the special filename
":memory:". In other words, instead of passing the name of
a real disk file into one of the sqlite3_open(), sqlite3_open16(), or
sqlite3_open_v2() functions, pass in the string ":memory:". For
example:
rc = sqlite3_open(":memory:", &db);
当完成调用时，不会打开任何磁盘文件。而是创建了一个纯内存数据库。一旦数据库连接断开，这个数据库就消失了。每个:memory:数据库都是独立的。所以两个使用文件名":memory:"打开的数据库连接会创建两个独立的内存数据库。
When this is done, no disk file is opened.
Instead, a new database is created
purely in memory. The database ceases to exist as soon as the database
connection is closed. Every :memory: database is distinct from every
other. So, opening two database connections each with the filename
":memory:" will create two independent in-memory databases.
特殊文件名":memory:"可以用在任何允许使用数据库文件名的地方。例如，可以再ATTACH命令中当做文件名来使用：
The special filename ":memory:" can be used anywhere that a database
filename is permitted. For example, it can be used as the
filename in an ATTACH command:
ATTACH DATABASE ':memory:' AS aux1;
注意，创建纯内存数据库使用的特殊文件名":memory:"里面不能加入任何其他文本内容。因此，如果使用"./:memory:"这样的名字，则会在预定的目录中创建一个存储在磁盘文件中的磁盘数据库。
Note that in order for the special ":memory:" name to apply and to
create a pure in-memory database, there must be no additional text in the
filename. Thus, a disk-based database can be created in a file by prepending
a pathname, like this: "./:memory:".
特殊文件名":memory:"同样适用于URI 文件名。例如：
The special ":memory:" filename also works when using URI filenames.
For example:
rc = sqlite3_open("file::memory:", &db);
ATTACH DATABASE 'file::memory:' AS aux1;
内存数据库和共享缓存
In-memory Databases And Shared Cache
如果是使用URI 文件名打开的内存数据库，那么就允许使用共享缓存。如果使用原始的":memory:"名字来创建内存数据库，那么这个数据库只能使用私用的缓存，并且只有原始打开这个数据库的数据库连接才可以访问。不过可以使用如下方式来让多个数据库连接打开同一个内存数据库：
In-memory databases are allowed to use shared cache if they are
opened using a URI filename. If the unadorned ":memory:" name is used
to specify the in-memory database, then that database always has a private
cache and is this only visible to the database connection that originally
opened it. However, the same in-memory database can be opened by two or
more database connections as follows:
rc = sqlite3_open("file::memory:?cache=shared", &db);
ATTACH DATABASE 'file::memory:?cache=shared' AS aux1;
这个允许各个不同的数据库连接共享同一个内存数据库。当然，共享内存数据库的所有数据库连接必须在同一个进程中。当最后一个连接关闭时，响应的内存数据库会自动删除并释放内存。
This allows separate database connections to share the same
in-memory database. Of course, all database connections sharing the
in-memory database need to be in the same process. The database is
automatically deleted and memory is reclaimed when the last connection
to the database closes.
如果需要在一个进程中创建两个或多个可以共享访问的独立内存数据库，那么使用URI 文件名的mode=memory查询参数可以创建一个命名内存数据库：
If two or more distinct but shareable in-memory databases are needed
in a single process, then the mode=memory query parameter can
be used with a URI filename to create a named in-memory database:
rc = sqlite3_open("file:memdb1?mode=memory&cache=shared", &db);
ATTACH DATABASE 'file:memdb1?mode=memory&cache=shared' AS aux1;
当使用这种方式来命名内存数据库，那么这个数据库只会和其它使用相同名字的链接共享内存。
When an in-memory database is named in this way, it will only share its
cache with another connection that uses exactly the same name.
临时数据库
Temporary Databases
如果传入sqlite3_open()或ATTACH的文件名是个空字符串，那么就会创建一个新的临时数据库。
When the name of the database file handed to sqlite3_open() or to
ATTACH is an empty string, then a new temporary file is created to hold
the database.
rc = sqlite3_open("", &db);
ATTACH DATABASE '' AS aux2;
每次执行都会创建一个不同的临时文件，这与使用特定字符串":memory:"类似，两个不同的数据库连接创建的临时数据库分别是各自私有的。当创建临时库的链接关闭后临时库会自动删除。
A different temporary file is created each time, so that just like as
with the special ":memory:" string, two database connections to temporary
databases each have their own private database. Temporary databases are
automatically deleted when the connection that created them closes.
虽然每个临时数据库都会分配一个磁盘文件，但是在实际中，临时数据库通常位于内存中的页缓存中，因此，使用":memory:"创建的纯内存数据库与空文件名创建的临时库之间只有很小的区别。它们之间唯一的区别就是":memory:"数据库只能存在于内存中，而临时数据库当数据量变大或者受到内存压力时可以写入磁盘。
Even though a disk file is allocated for each temporary database, in
practice the temporary database usually resides in the in-memory pager
cache and hence is very little difference between a pure in-memory database
created by ":memory:" and a temporary database created by an empty filename.
The sole difference is that a ":memory:" database must remain in memory
at all times whereas parts of a temporary database might be flushed to
disk if database becomes large or if SQLite comes under memory pressure.
前一段描述的是默认SQLite配置下的临时数据库行为。如果需要，应用程序可以使用temp_store pragma和SQLITE_TEMP_STORE编译期参数来强制临时数据库的行为与纯内存数据库一致。
The previous paragraphs describe the behavior of temporary databases
under the default SQLite configuration. An application can use the
temp_store pragma and the SQLITE_TEMP_STORE compile-time parameter to
force temporary databases to behave as pure in-memory databases, if desired.