如何在 SQLite 中连接三个表？

Question

How to join three tables in SQLite? I have three tables, one is Info , second is workForce and third is workDetails .

Table Info: id(PK),name,status,date,weather

Table WorkForce: id1(PK), subContractors,noOfPeople,noOfHours

Table WorkDetails: id2(PK),project,workDescription,TableInfo_id(FK) //contains multiple row

Table Info

  ID          NAME        Weather        Date     Status
 ----------  ----------  ----------  ----------  ----------
    1           Paul        Sunny         15/10      MC
    2           Allen       Rainy         15/10      Working

Table WorkForce

ID1          SubContractors   NoOfPeople      NoOfHours
----------  --------------   ----------       ----------
1           AAA                2                 2
2           BBB                3                 1

Table WorkDetails

ID2         Project       WorkDescription        TableInfo_id
----------  ----------     --------------          ----------
1              A               B                       1
2                                                      1
3                                                      1
4                                                      1
5               C               D                      2
6                                                      2
7                                                      2
8                                                      2

Assume the name is Paul, so all the row with ID 1 and TableInfo_id 1 will be retrieved.

Here is what I tried so far

public Cursor readEntry(String name) {

        String selectQuery = ("SELECT Weather,Date,Status,SubContractors,NumberOfPeople,NumberOfHours,TimeIn,TimeOut FROM "+TABLE_INFO+TABLE_WORKFORCE+TABLE_WORKDETAILS+ "WHERE Name= ? AND"+ID=ID1+ "AND"+ID=TableInfo_id);
        Cursor c = database.query(TABLE_INFO,TABLE_WORKFORCE,TABLE_WORKDETAILS,new String[]{id,name,weather,date,status,iD1,subcontractors,numberOfPerson,numberOfHours,id2project,workDescription,TableInfo_id},MyDatabaseHelper.Name+"=?",
                new String[] { String.valueOf(name)}, null, null, null, null,null,null,null,null,null,null,null,null);

        if (c != null) {
            c.moveToFirst();
        }
        return c;

    }

My code seems like not working..how can I achieve this? Thanks!

Answer 1

Fist thing you need to do is to add foreign key of Table Info into Table WorkForce and foreign key of Table WorkForce into Table WorkDetails

Then write modify your query like this

Select * from Table Info tf
LEFT JOIN Table WorkForce twf ON twf.tf_id = tf.id
LEFT JOIN Table WorkDetails twd ON twd.tw_id = twf.id

Modify your query based on requirement after joining the three tables.

Check out the tutorials for adding a foreign key.

Answer 2

1. Add a foreign key to the workforce table the point to ID within the info table
2. select * from Info inner join Workforce inner join Workdetails ON Info.ID = Wordforce.SOME_FOREIGN_KEY AND Info.ID = Workdetails.TableInfo_id Where NAME = 'Paul'

Think that should work have not tried it tho