3

我已经创建了表格并插入了如下值。

create table mytable (id INT, col1 INT, col2 INT, col3 INT);

insert into mytable values
(1,1,1,NULL),
(2,1,NULL,NULL);

我想要做的是如果 col1 不为 null 则更新 col2，如果 col2 不为 null 则更新 col3 等等......但只有一列要更新。

考虑我想更新 id=2 的数据，那么应该只更新 col2 而不是 col2，col3，因为两者都是空的。

当我尝试使用以下查询时，所有列都会更新。

update myTable set  
      col1 = ( IF (col1 is null, 9, col1) ),
      col2 = ( IF (col2 is null, 9, col2) ),
      col3 = ( IF (col3 is null, 9, col3) );

应该怎么做才能只更新一列。

4

2 回答 2

0

看来您的查询是更新 MySql 字段的扩展（如果字段不为空，请转到下一个）

您需要以相反的顺序设置列值检查。 

update table_name set  
--      more next ( 4 to n ) columns here if required       
    col3 = ( case when ( col2 is not null and col3 is null ) then 7 else col3 end )  
    , col2 = ( case when ( col1 is not null and col2 is null ) then 8 else col2 end )  
    , col1 = ( case when ( col1 is null ) then 9 else col1 end )  
;

示例表：

mysql> create table col_values ( id INT, col1 INT, col2 INT, col3 INT );
Query OK, 0 rows affected (0.06 sec)

mysql> insert into col_values values ( 1, 1, 1, NULL ), ( 2, 1, NULL, NULL ), ( 3, NULL, NULL, NULL );
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from col_values;
+------+------+------+------+
| id   | col1 | col2 | col3 |
+------+------+------+------+
|    1 |    1 |    1 | NULL |
|    2 |    1 | NULL | NULL |
|    3 | NULL | NULL | NULL |
+------+------+------+------+
3 rows in set (0.00 sec)

mysql> update col_values set
    ->     col3 = ( case when ( col2 is not NULL and col3 is NULL ) then 7 else col3 end )
    ->     , col2 = ( case when ( col1 is not NULL and col2 is NULL ) then 8 else col2 end )
    ->     , col1 = ( case when ( col1 is NULL ) then 9 else col1 end )
    -> ;
Query OK, 3 rows affected (0.02 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from col_values;
+------+------+------+------+
| id   | col1 | col2 | col3 |
+------+------+------+------+
|    1 |    1 |    1 |    7 |
|    2 |    1 |    8 | NULL |
|    3 |    9 | NULL | NULL |
+------+------+------+------+
3 rows in set (0.00 sec)

您还可以使用if函数作为case语句的替代方法。

mysql> truncate table col_values;
Query OK, 3 rows affected (0.01 sec)

mysql> insert into col_values values ( 1, 1, 1, NULL ), ( 2, 1, NULL, NULL ), ( 3, NULL, NULL, NULL );
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from col_values;
+------+------+------+------+
| id   | col1 | col2 | col3 |
+------+------+------+------+
|    1 |    1 |    1 | NULL |
|    2 |    1 | NULL | NULL |
|    3 | NULL | NULL | NULL |
+------+------+------+------+
3 rows in set (0.00 sec)

mysql> update col_values set
    ->     col3 = if( ( col2 is not NULL and col3 is NULL ), 7, col3 )
    ->     , col2 = if( ( col1 is not NULL and col2 is NULL ), 8, col2 )
    ->     , col1 = if( col1 is NULL, 9, col1 )
    -> ;
Query OK, 3 rows affected (0.01 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from col_values;
+------+------+------+------+
| id   | col1 | col2 | col3 |
+------+------+------+------+
|    1 |    1 |    1 |    7 |
|    2 |    1 |    8 | NULL |
|    3 |    9 | NULL | NULL |
+------+------+------+------+
3 rows in set (0.00 sec)
于 2012-06-24T15:09:38.270 回答
0

您可以通过两种不同的方式进行操作，即串行更新方式：

UPDATE mytable SET col3 = 9 WHERE col2 IS NOT NULL;
UPDATE mytable SET col2 = 9 WHERE col1 IS NOT NULL;

按此顺序执行此操作可防止跨所有列级联更新。

于 2012-06-24T12:55:33.770 回答