我有两个表与1到n的关系。 表爱好为表用户提供了一个名为＆＃34; userid＆＃34;
的外键SELECT * FROM hobbies WHERE userid = 7
可以有多个结果。 表用户包含用户配置文件数据列表，表爱好包含用户爱好列表。我想打印多个用户的个人资料信息列表，其中包含每个用户的爱好。 （业余爱好连成一个字符串） 我目前有以下sql查询:(伪代码）：
ResultSet result = executeQuery("SELECT * FROM users;");
for each returned row: executeQuery("SELECT * FROM hobbies WHERE userid =" + result.getInt("ref"));
如何从循环中获取SQL查询以优化大量用户的性能？
我正在尝试执行LEFT JOIN然后在java中而不是在SQL中执行WHERE ref = check 问题是我然后得到重复的用户，我只想在处理结果集时为每个用户打印一行 我也不确定JOIN是否真的是性能上的提升，因为我必须处理更多的行。
table users
+--------+------+---------+--------+
| userid | name | country | telno |
+--------+------+---------+--------+
| 1 | John | USA | 123456 |
| 2 | Max | Germany | 345678 |
+--------+------+---------+--------+
+--------------+------------+
| userid |hobby |
+--------------+------------+
| 1 | football |
| 1 | basketball |
| 2 | TV |
| 2 | Music |
| 2 | football |
+--------------+------------+
example output:
John, USA, 123456, {football, basketball}
Max, Germany, 345678, {TV, Music, football}
答案 0 :(得分：0)
您可以尝试：
SELECT h.userid, h.allyouneed FROM hobbies h, users u WHERE h.userid =u.userid
并在一个查询中获取信息
答案 1 :(得分：0)
"SELECT * FROM hobbies h join users u on h.userid = u.userid WHERE userid = ?"
preparedStatement.setInt(result.getInt("ref"));
然后，您可以将所有爱好保存到类似于下面“书籍”示例的列表中。
public static List<String> selectAll() throws SQLException {
PreparedStatement ps = null;
ResultSet rs = null;
// THE LIST OF BOOKS YOU WILL RETURN
List<String> books = new ArrayList<>();
String sql = "SELECT BOOK FROM BOOKs";
try(Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/bookshop", "root", "");){
ps = conn.prepareStatement(sql);
rs = ps.executeQuery();
while(rs.next()){
String book= rs.getString ("BOOK");
// ADD BOOK TO THE LIST
books.add(book);
}
}
finally{
ps.close();
rs.close();
}
// RETURN LIST OF ALL BOOKS
return books;
}
答案 2 :(得分：0)
List<TableItem> tableItems;
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
// Set our view from the "main" layout resource
SetContentView(Resource.Layout.Projlist);
mToolbar = FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
SetSupportActionBar(mToolbar);
SupportActionBar.Title = "قائمة طلبات المشاريع";
Selling.WebServiceDB ws = new Selling.WebServiceDB();
ws.MajorAListCompleted += Ws_MajorAListCompleted;
ws.MajorAListAsync("المرحلة الجامعية");
}
private void Ws_MajorAListCompleted(object sender, Selling.MajorAListCompletedEventArgs e)
{
if (e.Result.ToString().Equals("0"))
{
}
else
{
Spinner spinmajer = FindViewById<Spinner>(Resource.Id.spinmajer);
List<String> datalist = new List<String>();
foreach (var item in e.Result)
{
datalist.Add(item.MajorA);
}
var ArrayAdpter1 = new ArrayAdapter<String>(
this, Android.Resource.Layout.SimpleSpinnerItem, datalist);
spinmajer.Adapter = ArrayAdpter1;
//spinmajer.ItemSelected += new EventHandler<AdapterView.ItemSelectedEventArgs>(spinmater3_ItemSelected);
spinmajer.ItemSelected += spinmater3_ItemSelected ;
}
}
private void spinmater3_ItemSelected(object sender, AdapterView.ItemSelectedEventArgs e)
{
Spinner spinmajer = (Spinner)sender;
string input2 = Convert.ToString(spinmajer.GetItemAtPosition(e.Position));
Selling.WebServiceDB ws2 = new Selling.WebServiceDB();
ws2.MaterDataLvlCompleted += Ws2_MaterDataLvlCompleted;
ws2.MaterDataLvlAsync("المرحلة الجامعية", input2);
}
private void Ws2_MaterDataLvlCompleted(object sender, Selling.MaterDataLvlCompletedEventArgs e)
{
if (e.Result.ToString().Equals("0"))
{
}
else
{
Spinner spinmater3 = FindViewById<Spinner>(Resource.Id.spinmater3);
List<String> datalist = new List<String>();
foreach (var item in e.Result)
{
datalist.Add(item.MaterNameA);
}
var ArrayAdpter1 = new ArrayAdapter<String>(
this, Android.Resource.Layout.SimpleSpinnerItem, datalist);
spinmater3.Adapter = ArrayAdpter1;
//spinmater3.ItemSelected += new EventHandler<AdapterView.ItemSelectedEventArgs>(spinner1_ItemSelected);
spinmater3.ItemSelected += spinner1_ItemSelected;
}
}
private void spinner1_ItemSelected(object sender, AdapterView.ItemSelectedEventArgs e)
{
Spinner spinmajer = (Spinner)sender;
string sub = Convert.ToString(spinmajer.GetItemAtPosition(e.Position));
Selling.WebServiceDB ws3 = new Selling.WebServiceDB();
ws3.ProjectListCompleted += Ws3_ProjectListCompleted;
ws3.ProjectListAsync(sub);
}
private void Ws3_ProjectListCompleted(object sender, Selling.ProjectListCompletedEventArgs e)
{
ListView listprojall = FindViewById<ListView>(Resource.Id.listprojall);
string msg = "";
if (e.Result.ToString().Equals("0"))
{
var callDialog = new AlertDialog.Builder(this);
callDialog.SetTitle("Notify");
callDialog.SetMessage(msg);
callDialog.SetNeutralButton("The Customer Code is not correct", delegate { });
callDialog.Show();
}
else
{
// full class
tableItems = new List<TableItem>();
foreach (var item in e.Result)
{
tableItems.Add(new TableItem(item.ProjName, item.Projansr, Convert.ToString(item.Projid)));
}
listprojall.Adapter = new HomeScreenAdapter(this, tableItems);
//listprojall.ItemClick += OnlistItemClick;
listprojall.ItemClick += new EventHandler<AdapterView.ItemClickEventArgs>(OnlistItemClick);
}
}
void OnlistItemClick(object sender, AdapterView.ItemClickEventArgs e)
{
var listprojall = sender as ListView;
var t = tableItems[e.Position];
Globals.GlobalAskId = t.SubHeading1;
StartActivity(typeof(Teansrproj));
}
// adpater manage
public class HomeScreenAdapter : BaseAdapter<TableItem>
{
List<TableItem> items;
Activity context;
public HomeScreenAdapter(Activity context, List<TableItem> items)
: base()
{
this.context = context;
this.items = items;
}
public override long GetItemId(int position)
{
return position;
}
public override TableItem this[int position]
{
get { return items[position]; }
}
public override int Count
{
get { return items.Count; }
}
public override View GetView(int position, View convertView, ViewGroup parent)
{
var item = items[position];
View view = convertView;
if (view == null) // no view to re-use, create new
view = context.LayoutInflater.Inflate(Resource.Layout.Ticket_News, null);
view.FindViewById<TextView>(Resource.Id.Text1).Text = item.Heading;
view.FindViewById<TextView>(Resource.Id.Text2).Text = item.SubHeading;
view.FindViewById<TextView>(Resource.Id.Text3).Text = item.SubHeading1;
return view;
}
}
public class TableItem
{
public string Heading;
public string SubHeading;
public string SubHeading1;
public TableItem(string Heading, string SubHeading, string SubHeading1)
{
this.Heading = Heading;
this.SubHeading = SubHeading;
this.SubHeading1 = SubHeading1;
}
}
}
假设您与SELECT u.userid,u.name,u.country GROUP_CONCAT(hobby SEPARATOR ',')
FROM hobbies as h join users as u on h.userid=u.userid
where h.userid = ? GROUP BY u.userid,u.name,u.country;
和hobbies有关系。查询将连接连接表的users列，每行将有一个唯一用户。
答案 3 :(得分：0)
这很可能是最快的解决方案
SELECT name, country, telNo, GROUP_CONCAT(hobby)
FROM users u LEFT JOIN hobbies h ON u.id = h.userId
GROUP BY name, country, telNo
有关格式化选项，请参阅https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat。
它只对最终输出有用，例如，如果您的爱好包含逗号，则您无法唯一地解析它们。