Question

我正在寻找通过变量动态选择不同列名的查询。

假设我有包含我想要选择的“动态列”，如下所示：

SELECT `ObjectID`,`ObjectLabel` FROM `modules_forms_objects`

+----------+---------------+
| ObjectID | ObjectLabel   |
+----------+---------------+
|       71 | Join Date     |
|       72 | Active        |
+----------+---------------+

现在，我还有一个表，它是由上面的“ObjectID”调用的列。这里：

SELECT `data_id`,`71`,`72` FROM `7`

+---------+---------------------+------+
| data_id | 71                  | 72   |
+---------+---------------------+------+
|       1 | 0000-00-00 00:00:00 | NULL |
+---------+---------------------+------+

我想在第一个表中加入一个“值”列，这个'包含来自的值匹配第二个表中的列。（例如：对于ObjectID＃72，该值将为NULL）。

最终，我希望我的结果是这样的：

+----------+---------------+--------------------+
| ObjectID | ObjectLabel   |       Value    |
+----------+---------------+--------------------+
|       71 | Join Date     |0000-00-00 00:00:00 |
|       72 | Active        |        NULL    |
+----------+---------------+--------------------+

当我使用传统的“JOIN”时，我得到了每行的所有列，看起来有点沉重。

有什么想法吗？非常感谢！

Answer 1

基本静态查询可能如下所示

SELECT m.ObjectID, m.ObjectLabel, q.Value
  FROM modules_forms_objects m LEFT JOIN
(
  SELECT objectid,
         CASE objectid 
            WHEN 71 THEN `71`
            WHEN 72 THEN `72`
         END value
    FROM `7` t CROSS JOIN
  (
    SELECT 71 objectid UNION ALL
    SELECT 72 
  ) c
) q 
    ON m.objectid = q.objectid

我们的想法是先展开您的7表格，然后将其加入modules_forms_objects

输出：

| OBJECTID | OBJECTLABEL |               VALUE |
-----------|-------------|---------------------|
|       71 |   Join Date | 0000-00-00 00:00:00 |
|       72 |      Active |              (null) |

这是 SQLFiddle 演示

如果7表中没有几十列，我建议坚持使用静态查询。

现在与动态SQL相同

SET @sql = NULL, @sql1 = NULL, @sql2 = NULL;

SELECT GROUP_CONCAT(
           CONCAT('WHEN ''',  column_name, ''' THEN `', column_name, '`')
       SEPARATOR ' ')
  INTO @sql1
  FROM INFORMATION_SCHEMA.COLUMNS
 WHERE table_schema = SCHEMA()
   AND table_name = '7'
   AND column_name LIKE '7%'
 GROUP BY table_name;

SELECT GROUP_CONCAT(
           CONCAT('SELECT ''',  column_name, ''' objectid' )
       SEPARATOR ' UNION ALL ')
  INTO @sql2
  FROM INFORMATION_SCHEMA.COLUMNS
 WHERE table_schema = SCHEMA()
   AND table_name = '7'
   AND column_name LIKE '7%'
 GROUP BY table_name;

SET @sql = CONCAT(
'SELECT m.ObjectID, m.ObjectLabel, q.Value
  FROM modules_forms_objects m LEFT JOIN
(
  SELECT objectid,
         CASE objectid ', @sql1, ' END value 
    FROM `7` t CROSS JOIN
  (', @sql2, '
  ) c
) q
    ON m.objectid = q.objectid');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

这是 SQLFiddle 演示

Answer 2

USE SHOW COLUMNS FROM tab_name并使用FIELD属性获取相应的列。

Answer 3

这将在特定表中提供所有列名称

SELECT 
        GROUP_CONCAT(COLUMN_NAME)
FROM 
        INFORMATION_SCHEMA.COLUMNS
WHERE 
        table_name = 'mytable'

MySQL动态SELECT COLUMN

3 个答案: