使用自联接进行更新

Question

我想更新一个表，表明某些行是其他行的父级，所以我在表中添加了一个“parentid”列。 以下查询查找所有父项：

SELECT ca1.id, ca2.id 
FROM contactassociations ca1
JOIN contactassociations ca2 ON (ca1.contactid = ca2.contactid)
where ca1.entitytable = 'EMPLOYER' AND
ca2.entitytable = 'CLIENT';

但是当我尝试调整该语法来进行更新时，它不起作用：

UPDATE contactassociations ca1
SET    ca1.parentid = ca2.id
JOIN  contactassociations ca2 ON (ca1.contactid = ca2.contactid)
WHERE ca1.entitytable = 'EMPLOYER' AND ca2.entitytable = 'CLIENT';

我明白了：

Error starting at line 6 in command:
UPDATE contactassociations ca1
SET    ca1.parentid = ca2.id
JOIN  contactassociations ca2 ON (ca1.contactid = ca2.contactid)
WHERE ca1.entitytable = 'EMPLOYER' AND ca2.entitytable = 'CLIENT'
Error at Command Line:7 Column:28
Error report:
SQL Error: ORA-00933: SQL command not properly ended
00933. 00000 -  "SQL command not properly ended"
*Cause:    
*Action:

注意，第7行第28列是“SET”行的结尾。

Answer 1

Oracle不支持UPDATE语句中的JOIN子句。

用这个：

MERGE
INTO    contactassociations ca1
USING   contactassociations ca2
ON      (
        ca1.contactid = ca2.contactid
        AND ca1.entitytable = 'EMPLOYER'
        AND  ca2.entitytable = 'CLIENT'
        )
WHEN MATCHED THEN
UPDATE
SET     parentid = ca2.id

Answer 2

我发现以下样式更易于阅读，但您需要在UPDATE关键字之后使用别名，而不是表名：

UPDATE ca1
SET    ca1.parentid = ca2.id
FROM contactassociations ca1
LEFT JOIN contactassociations ca2 ON (ca1.contactid = ca2.contactid)
WHERE ca1.entitytable = 'EMPLOYER' AND ca2.entitytable = 'CLIENT'

Answer 3

-- Method #1
update emp set MANAGERNAME= mgr.EMPNAME
FROM SelfJoinTable emp , SelfJoinTable mgr where emp.MANAGERID = mgr.EMPID

-- Method #2
update emp 
set  MANAGERNAME= mgr.EMPNAME  
FROM SelfJoinTable emp 
   LEFT OUTER JOIN SelfJoinTable mgr 
   ON emp.MANAGERID = mgr.EMPID

-- Method #3
update emp 
set  MANAGERNAME= mgr.EMPNAME  
FROM SelfJoinTable emp 
   JOIN SelfJoinTable mgr 
   ON emp.MANAGERID = mgr.EMPID

-- Method #4
update emp 
set  MANAGERNAME= mgr.EMPNAME 
 FROM SelfJoinTable emp 
   inner JOIN SelfJoinTable mgr 
   ON emp.MANAGERID = mgr.EMPID