SQL 多对多查询

1、创建测试表，

create table test_task(taskID varchar2(20),taskName varchar2(200));

create table test_person(personID varchar2(20),personName varchar2(200));

create table test_task_person(taskID varchar2(20),personID varchar2(200));

2、插入测试数据；

insert into test_task values (101,'Task_101');

insert into test_task values (102,'Task_102');

insert into test_task values (103,'Task_103');

insert into test_person values (1001,'Person_1001');

insert into test_person values (1002,'Person_1002');

insert into test_person values (1003,'Person_1003');

insert into test_task_person values (101,1001);

insert into test_task_person values (101,1002);

insert into test_task_person values (102,1001);

insert into test_task_person values (102,1003);

insert into test_task_person values (103,1001);

insert into test_task_person values (103,1003);

commit;

3、查询1，查询每个任务对应的担当者；select t.*, b.personName from test_task_person t, test_person b where t.personID = b.personID;

4、查询2，查询每个人承担的任务；select t.*, b.taskName from test_task_person t, test_task b where t.taskID = b.taskID;

防止出现有任务没有对应的人担当，因此用left join防止漏掉任务

查询1：每个任务对应的担当者
SELECT t.taskID, t.taskName, tp.personID, p.personName
FROM task t
LEFT JOIN TaskPerson tp ON t.taskID = tp.taskID
LEFT JOIN Person p ON p.personID = tp.personID

查询2：每个人承担的任务
SELECT p.personID, p.personName, tp.taskID, t.taskName
FROM Person p
LEFT JOIN TaskPerson tp ON p.personID = tp.personID
LEFT JOIN Task t ON t.taskID = tp.taskID

查询1：每个任务对应的担当者
select taskName, personName from Task a,Person b,TaskPerson c
where a.taskID=c.taskID and b.personID=c.personID
group by taskName, personName
查询2：每个人承担的任务
select personName,taskName from Task a,Person b,TaskPerson c
where a.taskID=c.taskID and b.personID=c.personID
group by taskName, personName

两个结果好象是一样的

两个SQL差不多，第二个加上groupby就可以了。
SQL1: select a.taskName, c.personName from task a, TaskPerson b, Person c
where a. taskID = b.taskID and b.personID = c.personID

SQL2: select a.taskName, c.personName from task a, TaskPerson b, Person c
where a. taskID = b.taskID and b.personID = c.personID gourpby c.personName

第一个：
select taskID,taskName,personName from
TaskPerson,Person,Task
where TaskPerson.personID=Person.personID and
Task.taskID=TaskPerson.taskID
第二个：
select personName,taskID,taskName from
TaskPerson,Person,task
where TaskPerson.personID=Person.personID and
Task.taskID=TaskPerson.taskID