数据访问
SQL CE 特性
l 单个文件
l 支持添加密码和加密
l 支持事务
l 多列索引
l 支持聚合查询 (Nested Query)
l 支持 Having/Group By 子句

| All CE database resources are contained in a single file that typically has an .sdf extension.
| These files can be password protected and encrypted for security when they are created, and these security options can be changed during the lifetime of the database.
| You can create, commit, and abort transactions just as you would for SQL Server, although you cannot enlist in transactions created from another database.
| As you just saw, you can create multi-column indexes, and the use of these indexes is automatically managed by the CE engine based on statistics it maintains about the database.
| For your queries, you can use ANSI standard SQL including nested sub-queries and aggregation using Having and Group By clauses.