Question

MERGE INTO TABLE1 t1
USING TABLE2 t2
ON (t1.ID = t2.ID) 
WHEN MATCHED THEN UPDATE 
  SET t1.PHONE_NUMBER = CASE 
                          WHEN t1.type in ('A','B') THEN  t2.phone_number 
                          ELSE NVL(t2.phone_number, t1.phone_number)
                        END

需要将MERGE以上的内容转换为UPDATE语句。

我尝试了以下方法：

UPDATE TABLE1 t1 
   SET t1.PHONE_NUMBER = (
                          SELECT t2.PHONE_NUMBER 
                            FROM TABLE2 t2, TABLE1 t3 
                           WHERE t3.type in ('A','B') 
                             AND t3.ID = t2.ID
                         )

如何在以上实现中合并CASE？

Answer 1

MERGE语句将更加高效，您应该使用它；但是，您可以使用：

UPDATE table1 t1
SET PHONE_NUMBER = NVL(
                     ( SELECT t2.phone_number
                       FROM   table2 t2
                       WHERE  t1.id = t2.id ),
                     CASE
                     WHEN t1.type IN ( 'A', 'B' )
                     THEN NULL
                     ELSE t1.phone_number
                     END
                   )
WHERE EXISTS ( SELECT 1 FROM table2 t2 WHERE t1.id = t2.id );

Oracle设置：

CREATE TABLE table1 ( id, phone_number, type ) AS
SELECT 1, 123456, 'A' FROM DUAL UNION ALL
SELECT 2, 123456, 'B' FROM DUAL UNION ALL
SELECT 3, 123456, 'C' FROM DUAL UNION ALL
SELECT 4, 123456, 'D' FROM DUAL UNION ALL
SELECT 5, 123456, 'E' FROM DUAL;

CREATE TABLE table2 ( id, phone_number ) AS
SELECT 1, 234567 FROM DUAL UNION ALL
SELECT 2, NULL   FROM DUAL UNION ALL
SELECT 3, 345678 FROM DUAL UNION ALL
SELECT 4, NULL   FROM DUAL;

输出：

运行更新后，然后：

SELECT * FROM table1

与MERGE语句输出相同：

ID | PHONE_NUMBER | TYPE
-: | -----------: | :---
 1 |       234567 | A   
 2 |         null | B   
 3 |       345678 | C   
 4 |       123456 | D   
 5 |       123456 | E

db <>提琴here

更新：

如果您查看EXPLAIN PLAN语句的MERGE：

| PLAN_TABLE_OUTPUT                                                               |
| :------------------------------------------------------------------------------ |
| Plan hash value: 3423411568                                                     |
|                                                                                 |
| ------------------------------------------------------------------------------- |
| | Id  | Operation            | Name   | Rows  | Bytes | Cost (%CPU)| Time     | |
| ------------------------------------------------------------------------------- |
| |   0 | MERGE STATEMENT      |        |     4 |   168 |     7  (15)| 00:00:01 | |
| |   1 |  MERGE               | TABLE1 |       |       |            |          | |
| |   2 |   VIEW               |        |       |       |            |          | |
| |*  3 |    HASH JOIN         |        |     4 |   268 |     7  (15)| 00:00:01 | |
| |   4 |     TABLE ACCESS FULL| TABLE2 |     4 |   104 |     3   (0)| 00:00:01 | |
| |   5 |     TABLE ACCESS FULL| TABLE1 |     5 |   205 |     3   (0)| 00:00:01 | |
| ------------------------------------------------------------------------------- >

然后，它仅读取一次TABLE1和TABLE2。

将其与EXPLAIN PLAN语句的UPDATE比较：

| PLAN_TABLE_OUTPUT                                                              |
| :----------------------------------------------------------------------------- |
| Plan hash value: 735598124                                                     |
|                                                                                |
| ------------------------------------------------------------------------------ |
| | Id  | Operation           | Name   | Rows  | Bytes | Cost (%CPU)| Time     | |
| ------------------------------------------------------------------------------ |
| |   0 | UPDATE STATEMENT    |        |     4 |   168 |    23  (22)| 00:00:01 | |
| |   1 |  UPDATE             | TABLE1 |       |       |            |          | |
| |*  2 |   HASH JOIN SEMI    |        |     4 |   168 |     7  (15)| 00:00:01 | |
| |   3 |    TABLE ACCESS FULL| TABLE1 |     5 |   145 |     3   (0)| 00:00:01 | |
| |   4 |    TABLE ACCESS FULL| TABLE2 |     4 |    52 |     3   (0)| 00:00:01 | |
| |*  5 |   TABLE ACCESS FULL | TABLE2 |     1 |    26 |     3   (0)| 00:00:01 | |
| ------------------------------------------------------------------------------ |

然后它将从TABLE1读取一次，并从TABLE2读取两次；因此MERGE可能是一个性能更高的查询...。但是，如果需要，您可以使用UPDATE进行查询。

db <>提琴here

Answer 2

您可以在RIGHT JOIN中使用这样的嵌套子查询：

update table1 t 
   set t.phone_number = (
                          select tt.phone_number
                            from( 
                                 select case 
                                        when t1.type in ('A','B') then  t2.phone_number 
                                        else nvl(t2.phone_number, t1.phone_number)
                                        end as phone_number, 
                                        nvl(t2.ID,t1.ID) as ID
                                   from table2 t2
                                  right join table1 t1 
                                     on t1.ID = nvl(t2.ID,t1.ID)
                                 ) tt 
                            where tt.ID = t.ID
                           );

Demo

感谢您的小提琴@MTO

将MERGE语句转换为UPDATE语句

2 个答案: