更新MySql字段（如果字段不为空，请转到下一个）

Question

Hello I have table with 10 columns (columnA, ColumnB, ColumnC ...), ColumnA is for ID.

I want to make an update in php in this way: IF ColumnB is not empty then update ColumnC, if ColumnC is not empty update ColumnD ...until it finds an empty column.

I know how to update an empty column but i can`t figurit out how to implement the IF statement.

Until now i have this:

mysql_query("UPDATE tabel_name SET ColumnB = 'example' WHERE ColumnA = '$ID' "); 

Thank You

Answer 1

You have to use case to update only selective columns.

Example:

update table_name set  
    col1 = ( case when col1 is null then ? else col1 end )  
    , col2 = ( case when col2 is null then ? else col2 end )  
--  , col3 = ...
;

Use mysql prepare to replace query parameters.

Also refer to Quassnoi's answer to a similar posting.

Update:

IF ColumnB is not empty then update ColumnC, if ColumnC is not empty update ColumnD ...until it finds an empty column.

You need to set column values checking in the reverse order.

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 )  
;

Example Table:

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)

You can also use if function as an alternative to case statement.

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)