Question

我有一张名为fee_category

的表格

+------------+--------------+-----------+
| fee_cat_id | fee_cat_name | parent_id |
+------------+--------------+-----------+
|          1 | days scholar |         0 |
|          2 | tuition      |         1 |
|          3 | exam         |         1 |
|          4 | bus          |         1 |
+------------+--------------+-----------+

我想基于parent_id fee_type得到如下

如果parent_id=1

然后

feetype=days scholar

我试过如下

SELECT fee_cat_id
     , fee_cat_name
     , fee_cat_name IN 
         ( select fee_cat_name  
             from fee_category  
            where parent_id !=0
         ) fee_type 
 from fee_category;

我的输出看起来像这样

+------------+--------------+----------+
| fee_cat_id | fee_cat_name | fee_type |
+------------+--------------+----------+
|          1 | days scholar |        0 |
|          2 | tuition      |        1 |
|          3 | exam         |        1 |
|          4 | bus          |        1 |
+------------+--------------+----------+

我想要输出如下

+------------+--------------+----------  +
| fee_cat_id | fee_cat_name | fee_type   |
+------------+--------------+----------  +
|          1 | days scholar |  none      |
|          2 | tuition      |days scholar|
|          3 | exam         |days scholar|
|          4 | bus          |days scholar|
+------------+--------------+----------+

Answer 1

您可以通过表的LEFT JOIN获得此结果，以便将parent_id与fee_cat_name匹配：

SELECT fc1.fee_cat_id, fc1.fee_cat_name, IFNULL(fc2.fee_cat_name, 'none') AS fee_type
FROM fee_category fc1
LEFT JOIN fee_category fc2
ON fc2.fee_cat_id = fc1.parent_id
ORDER BY fc1.fee_cat_id

输出：

fee_cat_id  fee_cat_name    fee_type
1           days scholar    none
2           tuition         days scholar
3           exam            days scholar
4           bus             days scholar

Answer 2

您可以为表名添加别名。

SELECT a.fee_cat_id, a.fee_cat_name, COALESCE(b.fee_cat_name, 'NONE') as fee_type 
FROM fee_category a
LEFT OUTER JOIN fee_category b
ON a.fee_type = b.fee_cat_id;

Answer 3

您可以使用select列表中的子查询来实现：

select fee_cat_id,
       fee_cat_name,
       coalesce((select fee_cat_name from fee_category where fee_cat_id = FC.parent_id), 'none') fee_type
from fee_category FC

SQL fiddle

如何用相同的表colums写sql条件

3 个答案: