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

Question

我对用Oracle联接（+）符号正确找到左外部联接和右外部联接感到困惑。 检查这个与这个 。 我觉得两者矛盾。 我的理解是，第一个链接说，如果（+）号在右侧，它将是“ Right Outer Join”。

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找到左右外部连接？

Answer 1

左外部联接只是意味着您要返回联接左侧的所有表，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语法中将是： from t1 left outer join t2 on (t1.col1 = t2.col1)

右外部联接意味着您想返回联接右侧的所有表，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，只需反转表的顺序即可将右外部联接转换为左外部联接。 大多数外部联接都是左联接，这可能是因为更容易想到“我想要所有第一个表以及该另一个表中的所有匹配行”，而不是相反。 YMMV，当然！

---

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   

Answer 2

请举例说明如何正确找到左右外部连接

我将尝试展示Oracle外部联接语法和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);

外部连接-

在11gR1中对散列完全外部联接的本地支持之前，Oracle将以以下方式在内部转换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);

看看这个 。