HQL is the abbreviation of Hibernate Query Language, which provides more flexible and powerful query capabilities; HQL is closer to SQL query syntax.

query

Query all (persistent data)

//  s: aliases must be aliased to objects, not *. Not in hibernate*
 String hql="select s from Student s";
 //The Query object is the Query object
  Query query=session.createQuery(hql);
//. list a collection of lists found by. List
  List<Student> list=query.list();
  for (Student s : list) {
     System.out.println(s.getName());
  }

Specify column name to query all (non persistent data)
Sometimes there may be more than ten columns of data in the database, but only a few columns are needed, so you need to specify the column name to query.

//Query the non persistent state of the specified column name
//Non persistent: because the data queried does not correspond to the database
//s.sid,s.name the alias of the specified column name. Column name
  String hql="select s.sid,s.name from Student s";
  Query query=session.createQuery(hql);
  //The query is an object array, not a student object
  List<Object[]> list = query.list();
  for (Object[] objects : list) {
   System.out.println(objects[0]+"\t"+objects[1]);
  }

Specify column name to query all (persistent data)

//Query the persistent state of the specified column name
//In the database, there are corresponding, in the student object
//new Student(s.sid,s.name) ------ the constructor corresponding to it in entity class
  String hql="select new Student(s.sid,name) from Student s";
  Query query=session.createQuery(hql);
  //The Student object is found.
  List<Student> list=query.list();
  for (Student student : list) {
   System.out.println(student.getName());
  }

Function query

//Function query - Maximum
//Max (alias. Column name)
  String hql="select max(s.sid) from Student s";
  Query query=session.createQuery(hql);
  //Return is the only result
  int max=(Integer) query.uniqueResult();
  System.out.println("Maximum value"+max);

Note: when querying the total number of rows, you need to use the long type to receive

Placeholder query

The first way is to get the placeholder Name: user defined name.

//: name custom name 
String hql="select s from Student s where s.name like :name";
//Set the placeholder's value setparameter (placeholder name (not required:, assignment)); no order
  Query query=session.createQuery(hql).setParameter("name", "%xx%");
  List<Student> list = query.list();
  for (Student student : list) {
   System.out.println(student.getName());
  }

The second: through? placeholder

//If the hibernate version is higher than 5, the order of placeholders should be added after?
//For example, s.name like? 0 and s.age >? 1
String hql="select s from Student s where s.name like ?0";
//When assigning a value, you need to start from 0.
  Query query=session.createQuery(hql).setParameter(0, "%x%");
  List<Student> list = query.list();
  for (Student student : list) {
   System.out.println(student.getName());
  }

Paging query

//Find all
  String hql="select s from Student s ";
  int pages=1;//Current number of pages
  int pageSize=5;//How many lines are displayed on one page
  //setMaxResults sets the maximum number of pages to display setfirstresult sets which page to start from 
  Query query=session.createQuery(hql).setMaxResults(pageSize).setFirstResult((pages-1)*pageSize);
  List<Student> list = query.list();
  for (Student a : list) {
   System.out.println(a.getName());
  }

There are connection queries below (I use a one to many relationship)

//Connection query (full connection)
//C.province.pid PID PID in the province object in C and PID in the p object
  String sql="select c from City c inner join Province p on c.province.pid=p.pid";
  Query query = session.createQuery(sql);
  List<City> list = query.list();
  for (City c : list) {
   System.out.println(c.getCname());
  }

Child connection

 //Query the names of all provinces with Han in the city
  String hql="select p from Province p where p.pid in(select c.province.pid from City c where c.cname like :cname)";
  Query query = session.createQuery(hql).setParameter("cname", "%Chinese%");
  List<Province> list=query.list();
  for (Province province : list) {
   System.out.println(province.getPname());
  }

Child connection 2

 //Query the names of all provinces with lakes in cities - the query criteria of one table is the query result of another table
  String hql="select p from Province p where p.pid in(select c.province.pid from City c where c.cname like :cname)";
  Query query = session.createQuery(hql).setParameter("cname", "%Chinese%");
  List<Province> list=query.list();
  for (Province province : list) {
   System.out.println(province.getPname());
  }