选择具有值的列名

Question

考虑以下 table：

+-----------+---+------+---+
| Telephone | A |  B   | C |
+-----------+---+------+---+
|  12345    | x | NULL | y |
|  32456    | z | NULL | a |
+-----------+---+------+---+

假设这个 table 不限于 4 列（即，我不知道列数，因为它会变化）我如何 select 不为空的列名？（在此示例中，我希望结果为 A 和 C，因为它们已填充。）

Answer 1

如果需要列名，可以使用 case:

的聚合查询

select concat_ws(',',
                 (case when count(telephone) > 0 then 'telephone' end),
                 (case when count(A) > 0 then 'A' end),
                 (case when count(B) > 0 then 'B' end),
                 (case when count(C) > 0 then 'C' end)
                ) as ColumnsWithVowels
from t;

Answer 2

试试这些选项：
1- 存储过程：丢弃 NULLS 列

DELIMITER $$

    DROP PROCEDURE IF EXISTS `eliminateNull` $$
        CREATE PROCEDURE `eliminateNull`()
        BEGIN

        declare colName_v varchar(20);
        declare done int;
        declare cur1 cursor for 
        SELECT colName,group_concat(value) 
        FROM(SELECT 'telephone' AS ColName, p.telephone  as value FROM `table`  p WHERE p.telephone IS NOT NULL
        UNION
        SELECT 'A' AS ColName FROM `table`  p WHERE p.A IS NOT NULL
        UNION
        SELECT 'B' AS ColName FROM `table`  p WHERE p.B IS NOT NULL
        UNION
        SELECT 'C' AS ColName FROM `table`  p WHERE p.C IS NOT NULL
        )t
       group by t.colName;
       declare continue handler for not found set done=1;

    SET @ddl='';
    set done = 0;
    open cur1;
    igmLoop: loop   
    fetch cur1 into colName_v;
     if done = 1 then leave igmLoop; end if;
     if(@ddl='')THEN
      SET @ddl = CONCAT(@ddl,'p.',colName_v);
      ELSE 
        SET @ddl = CONCAT(@ddl,',p.',colName_v);
        END IF;
      end loop igmLoop;
    close cur1;
      SET @ddl = CONCAT('CREATE TEMPORARY TABLE IF NOT EXISTS table2 AS SELECT  ',@ddl, '  FROM `table` p;');

        PREPARE STMT FROM @ddl;
      EXECUTE STMT;  
      DEALLOCATE PREPARE STMT;
         select * from table2;
        DROP TEMPORARY TABLE table2;
END $$
    DELIMITER ;

程序调用：

call  eliminateNull();

2- Select：丢弃空列并分组结果

SELECT colName,group_concat(value) 
FROM(SELECT 'Telephone' AS ColName, p.telephone  as value FROM table  p WHERE p.telephone IS NOT NULL
UNION
SELECT 'A' AS ColName, p.A  as value FROM table  p WHERE p.A IS NOT NULL
UNION
SELECT 'B' AS ColName, p.B as value FROM table  p WHERE p.B IS NOT NULL
UNION
SELECT 'C' AS ColName, p.C as value FROM table  p WHERE p.C IS NOT NULL
)t
group by t.colName

Answer 3

这是诀窍，您只需更改架构名称和 table名称，它就会生成一个查询以进行检查。

1) 执行这个查询

2) 将结果作为准备好的语句执行

3) 执行结果

完成:-)

SELECT
    -- CREATE a SELECT 
    CONCAT("SELECT CONCAT( 'SELECT ', CONCAT_WS(' , ' , "
    -- CONCAT only FIELD WHERE NOT NULL
    , GROUP_CONCAT('IF(Sum(IF(`',ic.COLUMN_NAME,'` IS NOT NULL ,1,0)) > 0,\''
        ,ic.COLUMN_NAME,'\',NULL)  '
        SEPARATOR ',\n'
    )
    , "),' FROM ",ic.TABLE_NAME, "') FROM ",ic.TABLE_NAME) INTO @tmpsql
FROM information_schema.COLUMNS ic
WHERE
    ic.TABLE_SCHEMA = 'test'
AND
    ic.TABLE_NAME = 'test_table' ;

样本table

mysql> SELECT * FROM test_table;
+----+------+------+------+------+------+------+
| id | col1 | col2 | col3 | col4 | col5 | col6 |
+----+------+------+------+------+------+------+
|  1 |    1 |    3 | NULL |    5 |    4 | NULL |
|  2 |    7 |    8 | NULL |    3 |    2 | NULL |
+----+------+------+------+------+------+------+
2 rows in set (0,00 sec)

示例 - 执行查询并查看输出

mysql>
mysql> SELECT
    ->     -- CREATE a SELECT
    ->     CONCAT("SELECT CONCAT( 'SELECT ', CONCAT_WS(' , ' , "
    ->     -- CONCAT only FIELD WHERE NOT NULL
    ->     , GROUP_CONCAT('IF(Sum(IF(`',ic.COLUMN_NAME,'` IS NOT NULL ,1,0)) > 0,\''
    ->         ,ic.COLUMN_NAME,'\',NULL)  '
    ->         SEPARATOR ',\n'
    ->     )
    ->     , "),' FROM ",ic.TABLE_NAME, "') FROM ",ic.TABLE_NAME) INTO @tmpsql
    -> FROM information_schema.COLUMNS ic
    -> WHERE
    ->     ic.TABLE_SCHEMA = 'test'
    -> AND
    ->     ic.TABLE_NAME = 'test_table' ;
Query OK, 1 row affected (0,00 sec)

mysql>
mysql>
mysql> -- only to verify
mysql> SELECT @tmpsql\G
*************************** 1. row ***************************
@tmpsql: SELECT CONCAT( 'SELECT ', CONCAT_WS(' , ' , IF(Sum(IF(`id` IS NOT NULL ,1,0)) > 0,'id',NULL)  ,
IF(Sum(IF(`col1` IS NOT NULL ,1,0)) > 0,'col1',NULL)  ,
IF(Sum(IF(`col2` IS NOT NULL ,1,0)) > 0,'col2',NULL)  ,
IF(Sum(IF(`col3` IS NOT NULL ,1,0)) > 0,'col3',NULL)  ,
IF(Sum(IF(`col4` IS NOT NULL ,1,0)) > 0,'col4',NULL)  ,
IF(Sum(IF(`col5` IS NOT NULL ,1,0)) > 0,'col5',NULL)  ,
IF(Sum(IF(`col6` IS NOT NULL ,1,0)) > 0,'col6',NULL)  ),' FROM test_table') FROM test_table
1 row in set (0,00 sec)

执行准备好的结果

mysql>
mysql> PREPARE stmt FROM @tmpsql;
Query OK, 0 rows affected (0,00 sec)
Statement prepared

mysql> EXECUTE stmt;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| CONCAT( 'SELECT ', CONCAT_WS(' , ' , IF(Sum(IF(`id` IS NOT NULL ,1,0)) > 0,'id',NULL)  ,
IF(Sum(IF(`col1` IS NOT NULL ,1,0)) > 0,'col1',NULL)  ,
IF(Sum(IF(`col2` IS NOT NULL ,1,0)) > 0,'col2',NULL)  ,
IF(Sum(IF(`col3` IS NOT NULL ,1,0)) > 0,'col3',NULL)  , |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| SELECT id , col1 , col2 , col4 , col5 FROM test_table                                                                                                                                                                                                            |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0,00 sec)

mysql> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0,00 sec)

这与你的说法不符

mysql>
mysql> SELECT id , col1 , col2 , col4 , col5 FROM test_table;
+----+------+------+------+------+
| id | col1 | col2 | col4 | col5 |
+----+------+------+------+------+
|  1 |    1 |    3 |    5 |    4 |
|  2 |    7 |    8 |    3 |    2 |
+----+------+------+------+------+
2 rows in set (0,00 sec)

mysql>

选择具有值的列名

Selecting column names which have values

mysql

select

notnull