Question

我有4个表：t1，t2，t3，t4。我需要显示t1.event_id，t1.event_name，其中t1.event_id仅存在于3个表中的2个（t2，t3，t4）中。我尝试使用join，但我能做的就是返回所有表中存在的event_ids。

这是DDL.sql。

CREATE TABLE t1 
(event_id       NUMBER(15) NOT NULL,
event_name       VARCHAR2(80),
CONSTRAINT pk_event PRIMARY KEY (event_id));

CREATE TABLE t2 
(eid       NUMBER(15) NOT NULL,
equipment      VARCHAR2(100),
CONSTRAINT fk_eid2 FOREIGN KEY (eid)
REFERENCES t1 (event_id) ON DELETE CASCADE);

CREATE TABLE t3
(eid        NUMBER(15) NOT NULL,
security       VARCHAR2(100),
CONSTRAINT fk_eid3 FOREIGN KEY (eid)
REFERENCES t1 (event_id) ON DELETE CASCADE);

CREATE TABLE t4 
(eid       NUMBER(15) NOT NULL,
setup_by      TIMESTAMP,
CONSTRAINT fk_eid4 FOREIGN KEY (eid)
REFERENCES t1 (event_id) ON DELETE CASCADE);

Answer 1

不幸的是，我目前没有方便的Oracle（或任何数据库）副本，但我的第一个想法是LEFT OUTER JOIN到每个表，然后通过解码id的存在来过滤掉每张桌子？

SELECT t1.event_id, t1.event_name
  FROM t1
       LEFT OUTER JOIN t2 ON t1.event_id = t2.eid
       LEFT OUTER JOIN t3 ON t1.event_id = t3.eid
       LEFT OUTER JOIN t4 ON t1.event_id = t4.eid
 WHERE (
        DECODE(t2.eid, null, 0, 1) + 
        DECODE(t3.eid, null, 0, 1) + 
        DECODE(t4.eid, null, 0, 1)
       ) >= 2

Answer 2

如果你需要3个表中的2个，那么这个查询在没有更多逻辑的情况下将无法工作......但是如果你正在寻找缺少3个关系之一的情况，它会。但是，它应该让你开始准备好3个案例中的2个。

select t1.event_id, t1.event_name from t1 left outer join t2 on t1.event_id = t2.eid where t2.eid is null
union
select t1.event_id, t1.event_name from t1 left outer join t3 on t1.event_id = t3.eid where t3.eid is null
union
select t1.event_id, t1.event_name from t1 left outer join t4 on t1.event_id = t4.eid where t4.eid is null

基本上，左外连接将子表的id与null进行比较将有效地获取每个表中缺少的行，然后您可以将结果合并在一起。如果要确保只在一个表中缺少数据，则需要对每个子查询上的其他两个表执行内部联接，以确保它可以正常工作。类似的东西：

select t1.event_id, t1.event_name from t1 left outer join t2 on t1.event_id = t2.eid 
inner join t3 on t1.event_id = t3.eid inner join t4 on t1.event_id = t4.eid 
where t2.eid is null
union
select t1.event_id, t1.event_name from t1 left outer join t3 on t1.event_id = t3.eid 
inner join t2 on t1.event_id = t2.eid inner join t4 on t1.event_id = t4.eid
where t3.eid is null
union
select t1.event_id, t1.event_name from t1 left outer join t4 on t1.event_id = t4.eid 
inner join t3 on t1.event_id = t3.eid inner join t2 on t1.event_id = t2.eid
where t4.eid is null

如何连接4个表，条件是数据仅存在于3个表中的2个表中

2 个答案: