sql - SQL语句中的 "set+0"是做什么用的？

Question

我对这条 SQL 语句的含义感到困惑:

SELECT exhibitor_categories+0 from exhibitor_registry

什么是 exhibitor_categories+0 ？它为返回的每一行返回一个数字。

exhibitor_categories 定义为:

set('contemporary', 'classical impression / transitional', 'outdoor', 'home accessories')

感谢您的宝贵时间:)

Answer 1

它将设置值隐式转换为 INTEGER。

集合被视为位图，因此第一个值设置位 0，第二个值设置位 1 等。

mysql> CREATE TABLE exhibitor_registry(exhibitor_categories set('contemporary',
'classical impression / transitional', 'outdoor', 'home accessories') NOT NULL);

Query OK, 0 rows affected (0.08 sec)

mysql> INSERT
    -> INTO    exhibitor_registry
    -> VALUES  ('contemporary,classical impression / transitional,outdoor');
Query OK, 1 row affected (0.03 sec)

mysql> SELECT  exhibitor_categories
    -> FROM    exhibitor_registry;
+----------------------------------------------------------+
| exhibitor_categories                                     |
+----------------------------------------------------------+
| contemporary,classical impression / transitional,outdoor |
+----------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT  exhibitor_categories + 0
    -> FROM    exhibitor_registry;
+--------------------------+
| exhibitor_categories + 0 |
+--------------------------+
|                        7 |
+--------------------------+
1 row in set (0.00 sec)