one-to-many

问题 What's the best way to query one-to-many in MySQL? This is a simplified version of the database I am working on (if anything doesn't look right tell me): CREATE TABLE Tenant( tenant_id int NOT NULL, first_name varchar(20), last_name varchar(20), PRIMARY KEY (tenant_id) ); CREATE TABLE Rent( tenant_id int NOT NULL, month enum('JAN', 'FEB', ...), date_paid date NOT NULL, amount_paid int NOT NULL, FOREIGN KEY (tenant_id) REFERENCES Tenant(tenant_id) ); (The reason that there is month and date

问题 I have two tables A and Band the relation between A to B is A--->one-to-Many--->B Normally i have one record of B for every record of A. I am trying to write a query which would give me list of ONLY records of A which have more than ONE(MULTIPLE) record(s) in B. I am very confused as I have only done basic sql queries and this one seems complex to me. Could some one please guide me to correct answer or give me the solution. edited: ok i tried something like below and it gave me an error

问题 I have a User (parent) and a Home (child) entities related following a unidirectional one-to-many relationship. My problem is that, when adding a new Home to a User , the newly created and persisted Home doesn't have the id . Is this normal? Do I need to manually persist the child if I want the id? These are my entities: @Entity @Table(name = "user") public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Column(name = "id") private Long id; @NotNull @Column(name =

问题 I have a User (parent) and a Home (child) entities related following a unidirectional one-to-many relationship. My problem is that, when adding a new Home to a User , the newly created and persisted Home doesn't have the id . Is this normal? Do I need to manually persist the child if I want the id? These are my entities: @Entity @Table(name = "user") public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Column(name = "id") private Long id; @NotNull @Column(name =

问题 I use Core Data database. I have Entity Album, that has many entities SpecificImage (key to get set of these images is @"specificImages") I want to get Album with id = 1 and to sort attached SpecificImages by id. I tried NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"specificImages.id" ascending:YES]; but I get the error Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here' Here is my code: NSString

问题 OneToMany + MapKeyJoinColumn doesn't work for me, please suggest what I'm doing wrong. I'm using JPA 2.0 + Hibernate 3.6.1 And want to map following tables: To Classes: @Entity public class Question { // id and other fields @OneToMany(mappedBy="question", cascade = CascadeType.ALL) @MapKeyJoinColumn(name="language_id") private Map<Language, Statement> statements = new HashMap<Language, Statement>(); } @Entity public class Statement { @Id private Long id; @ManyToOne @JoinColumn(name =

问题 I am seeking some advice on the best way to retrieve and display my data using MySQL and PHP. I have 3 tables, all 1 to many relationships as follows: Each SCHEDULE has many OVERRIDES and each override has many LOCATIONS . I would like to retrieve this data so that it can all be displayed on a single PHP page e.g. list out my SCHEDULES. Within each schedule list the OVERRIDES, and within each override list the LOCATIONS. Option1 - Is the best way to do this make 3 separate SQL queries and

问题 I want to persist parent entity with 20 child entities, my code is below Parent Class @OneToMany(mappedBy = "parentId") private Collection<Child> childCollection; Child Class @JoinColumn(name = "parent_id", referencedColumnName = "parent_id") @ManyToOne(optional=false) private Parent parent; String jsonString = "json string containing parent properties and child collection" ObjectMapper mapper = new ObjectMapper(); Parent parent = mapper.readValue(jsonString, Parent.class); public void save

问题 I've been trying to find the answer to this question here. Several people seem to ask similar things, but I don't get the answers. I have an EF entity with a bunch of child entities (one-to-many relationship). I want to be able to delete the "parent" entity and have all the child entities deleted at the same time. Some people mention "Cascade Delete" should be set on both EF model and database (Sql Server in my case). The problem is: I have absolutely no idea how to do this (seems to be

问题 I have the data model you can see below, and a nested SUBQUERY predicate, but in somehow it just not works. Any idea how to correct it? I figured out, this here down is working finally: [NSPredicate predicateWithFormat:@"SUBQUERY(bs, $B, SUBQUERY($B.cs, $C, $C.ds.name != \"xxx\").@count > 0).@count > 0"]; 回答1: Ok, so here is the working solution: [NSPredicate predicateWithFormat:@"SUBQUERY(bs, $B, SUBQUERY($B.cs, $C, $C.ds.name != \"xxx\").@count > 0).@count > 0"]; 来源： https://stackoverflow