考虑下表：
+-----------+---+------+---+
| Telephone | A | B | C |
+-----------+---+------+---+
| 12345 | x | NULL | y |
| 32456 | z | NULL | a |
+-----------+---+------+---+
假设此表不限于 4 列（即，我不知道列数，因为它会有所不同）如何选择不为空的列名？ （在此示例中，我希望结果为 A 和 C，因为它们已填充。）
【问题讨论】：
如果你想要列名，你可以使用聚合查询case：
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;
【讨论】：
desc test_table 显示所有列，然后使用我的文本编辑器创建此查询。
这是诀窍，您只需要更改模式名和表名，它就会生成一个查询来检查这个。
1) 执行这个查询
2) 将结果作为预处理语句执行
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' ;
示例表
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)
按准备执行结果
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>
【讨论】：
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- 选择：丢弃空列和分组结果
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
【讨论】：