通过消除条件显示结果的 SQL 查询

Question

我正在尝试为此示例输入和输出编写 SQL 查询： 具有以下条件：层次结构将与以下相同

1. 删除 code = PK 且 filter 不为空的记录
2. 更新代码 = PK 的记录并将空白过滤为“XXXX”

有人可以帮我吗？

对于第一个条件，我只能尝试以下代码，但这会删除所有记录并给出不正确的结果：

 select *,
 from table t1
 where code='PK' and length('filter')=0

Answer 1

SELECT T1.KEY,
       T1.CODE,
       CASE
           WHEN CODE = 'PK' AND COALESCE(FILTER,0) = 0  THEN 'XXXX'
           ELSE T1.FILTER 
       END AS FILTER
FROM TABLE T1
WHERE CODE != 'PK' OR (CODE='PK' AND COALESCE(FILTER,0) = 0)

在您的查询中length('filter')返回6

Answer 2

select key, code, IF(code = 'PK', 'XXXX', filter) as filter
from table_t1
where (
    code != 'PK' 
    OR 
    code = 'PK' AND (filter IS NULL OR filter = '')
)