将 MERGE 语句转换为 UPDATE 语句

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

需要将以上 MERGE 转换为 UPDATE 语句。

我试过以下方法：

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
                         )

如何将CASE合并到上面的实现中？

Answer 1

MERGE 语句会更有效率，您或许应该使用它；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:

运行更新之后：

SELECT * FROM table1

输出与 MERGE 语句相同：

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

db<>fiddle here

更新:

如果您查看 EXPLAIN PLAN 中的 MERGE 语句：

| 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 | |
| ------------------------------------------------------------------------------- >

然后它只读取 TABLE1 和 TABLE2 各一次。

将其与 UPDATE 语句的 EXPLAIN PLAN 进行比较：

| 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 | |
| ------------------------------------------------------------------------------ |

然后它会从TABLE1读取一次，从TABLE2读取两次；所以 MERGE 可能是一个性能更高的查询....但如果你愿意，你可以用 UPDATE 来做到这一点。

db<>fiddle here

Answer 2

您可以将这样的嵌套子查询与 RIGHT JOIN 一起使用：

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 fiddle @MTO

将 MERGE 语句转换为 UPDATE 语句

Convert MERGE statement to UPDATE statement

sql

oracle

sql-update

sql-merge