如何使用 ORACLE JOIN (+) 查找 LEFT OUTER JOIN 或 RIGHT OUTER JOIN

Question

我对使用 Oracle 连接 (+) 符号正确查找左外连接和右外连接感到困惑。检查this 与this。我觉得两者矛盾。我的理解是，第一个链接说如果（+）号在右手边，它将是右外连接。

而第二个链接，我的理解是完全错误的。

请举例说明如何正确查找左右外连接？

Answer 1

请举例说明如何正确查找左右外连接

我将尝试展示 Oracle 外连接语法 和 ANSI/ISO 语法 之间的区别。

左外连接 -

SELECT e.last_name,
  d.department_name
FROM employees e,
  departments d
WHERE e.department_id = d.department_id(+);

SELECT e.last_name,
  d.department_name
FROM employees e
LEFT OUTER JOIN departments d
ON (e.department_id = d.department_id);

右外连接 -

SELECT e.last_name,
  d.department_name
FROM employees e,
  departments d
WHERE e.department_id(+) = d.department_id;

SELECT e.last_name,
  d.department_name
FROM employees e
RIGHT OUTER JOIN departments d
ON (e.department_id = d.department_id);

全外连接 -

在 11gR1 原生支持 hash full outerjoin 之前，Oracle 会在内部通过以下方式转换 FULL OUTER JOIN -

SELECT e.last_name,
  d.department_name
FROM employees e,
  departments d
WHERE e.department_id = d.department_id(+)
UNION ALL
SELECT NULL,
  d.department_name
FROM departments d
WHERE NOT EXISTS
  (SELECT 1 FROM employees e WHERE e.department_id = d.department_id
  );

SELECT e.last_name,
  d.department_name
FROM employees e
FULL OUTER JOIN departments d
ON (e.department_id = d.department_id);

看看this。

Answer 2

左外连接只是意味着您要返回连接左侧的所有表，以及右侧表中的所有匹配行。

在旧式 Oracle 语法中，这将是：where t1.col1 = t2.col1 (+) 在 ANSI 语法中，这将是：from t1 left outer join t2 on (t1.col1 = t2.col1)

右外连接意味着您要返回连接右侧的所有表，以及左侧表中的所有匹配行。

在旧式 Oracle 语法中，这将是：where t2.col1 (+) = t1.col1 在 ANSI 语法中，这将是：from t2 right outer join t1 on (t2.col1 = t1.col1)

您当然会发现，只需颠倒表的顺序，您就可以将右外连接变成左外连接。大多数外连接都是左连接，可能是因为更容易想到“我想要所有第一个表，以及来自另一个表的任何匹配行”，而不是相反。当然是 YMMV！

---

ETA 以下示例：

左外连接：

with t1 as (select 1 col1, 10 col2 from dual union all
            select 2 col1, 20 col2 from dual union all
            select 3 col1, 30 col2 from dual),
     t2 as (select 1 col1, 100 col2 from dual)
select t1.*, t2.*
from   t1, t2
where  t1.col1 = t2.col1 (+)
order by t1.col1;

      COL1       COL2     COL1_1     COL2_1
---------- ---------- ---------- ----------
         1         10          1        100
         2         20                      
         3         30   

with t1 as (select 1 col1, 10 col2 from dual union all
            select 2 col1, 20 col2 from dual union all
            select 3 col1, 30 col2 from dual),
     t2 as (select 1 col1, 100 col2 from dual)
select t1.*, t2.*
from t1 left outer join t2 on (t1.col1 = t2.col1)
order by t1.col1;

      COL1       COL2     COL1_1     COL2_1
---------- ---------- ---------- ----------
         1         10          1        100
         2         20                      
         3         30   

右外连接：

with t1 as (select 1 col1, 10 col2 from dual union all
            select 2 col1, 20 col2 from dual union all
            select 3 col1, 30 col2 from dual),
     t2 as (select 1 col1, 100 col2 from dual)
select t1.*, t2.*
from   t1, t2
where t2.col1 (+) = t1.col1
order by t1.col1;

      COL1       COL2     COL1_1     COL2_1
---------- ---------- ---------- ----------
         1         10          1        100
         2         20                      
         3         30   

with t1 as (select 1 col1, 10 col2 from dual union all
            select 2 col1, 20 col2 from dual union all
            select 3 col1, 30 col2 from dual),
     t2 as (select 1 col1, 100 col2 from dual)
select t1.*, t2.*
from t2 right outer join t1 on (t2.col1 = t1.col1)
order by t1.col1;

      COL1       COL2     COL1_1     COL2_1
---------- ---------- ---------- ----------
         1         10          1        100
         2         20                      
         3         30