Oracle SQL条件联接
我有以下SQL语句:Oracle SQL条件联接,sql,oracle,join,Sql,Oracle,Join,我有以下SQL语句: select DOCUMENT.DOCUMENT_ID, (case when DOCUMENT.CLASSIFICATION_CODE is not null then DOCUMENT.CLASSIFICATION_CODE else TEMPLATE.CLASSIFICATION_CODE end) CLASSIFICATION_CODE, CLASSIFICATION.NAME CLASSIFI
select
DOCUMENT.DOCUMENT_ID,
(case
when DOCUMENT.CLASSIFICATION_CODE is not null
then DOCUMENT.CLASSIFICATION_CODE
else TEMPLATE.CLASSIFICATION_CODE end) CLASSIFICATION_CODE,
CLASSIFICATION.NAME CLASSIFICATION_NAME
from
DOCUMENT,
TEMPLATE,
CLASSIFICATION
where
DOCUMENT.TEMPLATE_ID = TEMPLATE.TEMPLATE_ID and
DOCUMENT.CLASSIFICATION_CODE = CLASSIFICATION.CLASSIFICATION_CODE(+)
我想做的是:
if DOCUMENT.CLASSIFICATION_CODE IS NOT NULL THEN
DOCUMENT.CLASSIFICATION_CODE = CLASSIFICATION.CLASSIFICATION_CODE(+),
ELSE
TEMPLATE.CLASSIFICATION_CODE = CLASSIFICATION.CLASSIFICATION_CODE(+)
这可能吗?您可以使用或更标准的:
COALESCE(DOCUMENT.CLASSIFICATION_CODE, TEMPLATE.CLASSIFICATION_CODE)
= CLASSIFICATION.CLASSIFICATION_CODE(+)
在ANSI语法中:
select
DOCUMENT.DOCUMENT_ID,
COALESCE(DOCUMENT.CLASSIFICATION_CODE, TEMPLATE.CLASSIFICATION_CODE)
CLASSIFICATION_CODE,
CLASSIFICATION.NAME CLASSIFICATION_NAME
from
DOCUMENT
inner join
TEMPLATE on DOCUMENT.TEMPLATE_ID = TEMPLATE.TEMPLATE_ID
left join
CLASSIFICATION on
COALESCE(DOCUMENT.CLASSIFICATION_CODE, TEMPLATE.CLASSIFICATION_CODE)
= CLASSIFICATION.CLASSIFICATION_CODE
您可以使用:
NVL(DOCUMENT.CLASSIFICATION\u code,TEMPLATE.CLASSIFICATION\u code)
合并(DOCUMENT.CLASSIFICATION\u代码、TEMPLATE.CLASSIFICATION\u代码)
- 或者
(与当DOCUMENT.CLASSIFICATION\u code不为空时,则DOCUMENT.CLASSIFICATION\u code ELSE TEMPLATE.CLASSIFICATION\u code END
子句中使用的完全相同)选择
SELECT d.DOCUMENT_ID,
NVL( d.CLASSIFICATION_CODE, t.CLASSIFICATION_CODE )
AS CLASSIFICATION_CODE,
c.NAME AS CLASSIFICATION_NAME
FROM DOCUMENT d
INNER JOIN TEMPLATE t
ON ( d.TEMPLATE_ID = t.TEMPLATE_ID )
LEFT OUTER JOIN CLASSIFICATION c
ON ( NVL( d.CLASSIFICATION_CODE, t.CLASSIFICATION_CODE )
= c.CLASSIFICATION_CODE )
虽然您可以使用
coalesce
或nvl
,但正如其他答案所建议的那样,我更愿意加入分类表两次。是的,引擎必须执行额外的连接,但是避免使用这些函数会允许引擎使用索引
SELECT document.document_id,
COALESCE (d.classification_code, t.classification_code)
AS classification_code,
COALESCE (d_c.name, t_c.name) AS classification_name
FROM document d
JOIN template t ON d.template_id = t.template_id
LEFT JOIN classification d_c
ON d.classification_code = d_c.classification_code
LEFT JOIN classification t_c
ON t.classification_code = t_c.classification_code
学习使用正确的显式JOIN
语法。谁能记住(+)
在WHERE
子句上的意思?啊!非ANSI外部联接语法?2016年!?!?!这个世界会变成什么样子!