blocks|key|991809|text|当您使用利用游标的增强型适配器(例如ResourceCursorAdapter)时，_id非常有用。它被这些适配器用来提供一个ID，该ID可用于指向表中的特定行，该特定行与适配器用于任何用途(例如，ListView中的一行)中的项相关。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|991810|如果您不打算使用在游标中需要_id列的类，则不需要这样做，而且您还可以使用"as+_id“使另一列在游标中显示为_id。|991811|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

_id is useful when you are using the enhanced Adapters which make use of a Cursor (e.g. ResourceCursorAdapter). It's used by these adapters to provide an ID which can be used to refer to the specific row in the table which relates the the item in whatever the adapter is being used for (e.g. a row in a ListView).

It's not necessary if you're not going to be using classes which need an _id column in a cursor, and you can also use "as _id" to make another column appear as though it's called _id in your cursor.

blocks|key|1206415|text|为什么不使用_ROWID_呢？|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1206416|SQLite为每一行都提供了别名，所以只需在select语句中将其别名为_id即可。|1206417|entityMap|0|LINK|mutability|MUTABLE|url|http://www.sqlite.org/autoinc.html^0|6|7|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

Why not make use of <a href="http://www.sqlite.org/autoinc.html" rel="noreferrer">_ROWID_</a>? 

SQLite provides this anyway for every row, so you can just alias it to _id in your select statement.

blocks|key|991880|text|在许多情况下，拥有id字段是非常方便的。我更喜欢我的是自动递增的(如下所示)。我总是在寻找id字段的新用途:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|991881|当需要将数据附加到适配器时，我喜欢使用表名别名将id字段查询为_id。示例：SELECT+id+_id,+msg+from+message+order+by+id。这样适配器就会看到一个名为_id的字段，每个人都会很高兴。|offset|length|style|CODE|991882|下面是我如何定义我的表的示例：|991883|CREATE+TABLE+message+(_id+INTEGER+PRIMARY+KEY+AUTOINCREMENT,+timestamp+INTEGER,+tripID+TEXT,+msg+TEXT);|code-block|syntax|javascript|991884|entityMap^0|0|V|3|12|17|2N|3|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]|$D|W|E|X|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|Z|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|P|$]]

It's quite convenient in many cases to have an id field. I prefer mine to be auto-incrementing (as shown below). I'm always finding new uses for the id field :)

When it comes time to attach the data to an adapter, I like to use a table name alias to query the id field as <code>_id</code>. Example: <code>SELECT id _id, msg from message order by id</code>. That way the adapter sees a field called <code>_id</code> and everybody's happy. 

Here's a sample of how I define my tables: 

<pre><code>CREATE TABLE message (_id INTEGER PRIMARY KEY AUTOINCREMENT, timestamp INTEGER, tripID TEXT, msg TEXT);
</code></pre>

blocks|key|1206576|text|如果将ROWID列定义为一个自动递增的整数，那么它实际上是SQLite默认提供的_id列(https://www.sqlite.org/lang_createtable.html#rowid)的别名。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1206577|您的create语句需要采用以下形式...|1206578|CREATE+TABLE+t(_id+INTEGER+PRIMARY+KEY+ASC,+y,+z);|code-block|syntax|javascript|1206579|为了证明这是有效的..。|1206580|UPDATE+t+SET+_id=22+WHERE+_id=11;|1206581|然后|1206582|SELECT+ROWID,+_id+FROM+t;|1206583|你会发现_id和ROWID的值是一样的。|style|CODE|1206584|请注意，如果在CREATE中使用DESC，则会创建一个新列，并且不会为ROWID设置别名。|1206585|entityMap|0|LINK|mutability|MUTABLE|url|https://www.sqlite.org/lang_createtable.html#rowid^0|19|1E|0|0|0|0|0|0|0|0|4|3|8|5|0|Z|5|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]]|C|$]]|$1|D|3|E|5|6|7|1A|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|1B|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|1C|8|@]|9|@]|C|$]]|$1|M|3|N|5|H|7|1D|8|@]|9|@]|C|$I|J]]|$1|O|3|P|5|6|7|1E|8|@]|9|@]|C|$]]|$1|Q|3|R|5|H|7|1F|8|@]|9|@]|C|$I|J]]|$1|S|3|T|5|6|7|1G|8|@$A|1H|B|1I|U|V]|$A|1J|B|1K|U|V]]|9|@]|C|$]]|$1|W|3|X|5|6|7|1L|8|@$A|1M|B|1N|U|V]]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|1O|8|@]|9|@]|C|$]]]|Z|$10|$5|11|12|13|C|$14|15]]]]

If you define your _id column as an autoincrementing integer it is actually an alias for the ROWID column that SQLite provides by default (<a href="https://www.sqlite.org/lang_createtable.html#rowid" rel="noreferrer">https://www.sqlite.org/lang_createtable.html#rowid</a>). 

Your create statement needs take the form...

<pre><code>CREATE TABLE t(_id INTEGER PRIMARY KEY ASC, y, z);
</code></pre>

To prove this works...

<pre><code>UPDATE t SET _id=22 WHERE _id=11;
</code></pre>

then

<pre><code>SELECT ROWID, _id FROM t;
</code></pre>

and you'll find both <code>_id</code> and <code>ROWID</code> have the same value.

Note, that if you use DESC in the CREATE a new column is created and <code>ROWID</code> is not aliased.

blocks|key|992010|text|当然不是。这是一些像ListView这样的小部件用来填充数据的方便字段。看看这篇好文章：http://www.casarini.org/blog/2009/android-contentprovider-on-sqlite-tables-without-the-_id-column/|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|992011|entityMap|0|LINK|mutability|MUTABLE|url|http://www.casarini.org/blog/2009/android-contentprovider-on-sqlite-tables-without-the-_id-column/^0|18|2Q|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Surely not.
Its a convenience field that some widgets like ListView uses to populate data. See this good article:
<a href="http://www.casarini.org/blog/2009/android-contentprovider-on-sqlite-tables-without-the-_id-column/" rel="nofollow">http://www.casarini.org/blog/2009/android-contentprovider-on-sqlite-tables-without-the-_id-column/</a>

blocks|key|992049|text|当然，如果您正在创建自己的UI小部件和适配器，则不必将主键命名为"_id“。它可以是你想要的任何名字。但您将负责管理UI小部件的集合，并将它们绑定到数据库中的正确行。正如布拉德所指出的，"_id“只对ListView有用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|992050|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Of course if you are creating your own UI widget and your own adapter, you don't have to name your primary key as "_id". It can be any name you want. But you would be responsible for managing your collections of UI widgets and binding them to the right row in your database. "_id" is only useful for ListView as Brad has pointed out.

blocks|key|1206625|text|_id字段在sqlite中确实是必需的，它将帮助您从sqlite中选择特定的数据。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1206626|SELECT+name+from+table_name+where+_id+=+?|code-block|syntax|javascript|1206627|如果您正在创建一个recyclerview/+listview，并且您想要该列表项的详细活动，那么您确实需要一个id来获取该列表项的数据。|1206628|如果你正在为常量创建一个类，在android中有一个BaseColumn接口，它为常量类提供了_ID字段。|1206629|//from+android+documentation..
++public+static+class+FeedEntry+implements+BaseColumns+{
++++++++public+static+final+String+TABLE_NAME+=+"entry";
++++++++public+static+final+String+COLUMN_NAME_TITLE+=+"title";
++++++++public+static+final+String+COLUMN_NAME_SUBTITLE+=+"subtitle";
++++}|1206630|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|S|8|@]|9|@]|A|$E|F]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The _id field is indeed necessary in sqlite, it will help you to select a particular data from sqlite.

<pre><code>SELECT name from table_name where _id = ?
</code></pre>

And if your are creating a recyclerview/ listview and you want a detailed activity for that list item you indeed need an id for this to fetch data of that item.

if you are creating a class for constants there is a BaseColumn interface in android, 
which provide _ID field to that constant class.

<pre><code>//from android documentation..
 public static class FeedEntry implements BaseColumns {
 public static final String TABLE_NAME = "entry";
 public static final String COLUMN_NAME_TITLE = "title";
 public static final String COLUMN_NAME_SUBTITLE = "subtitle";
 }
</code></pre>

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