如果列为空，请使用不同的where子句选择相同的列

Question

如果前一个返回空字符串，是否有办法使用不同的WHERE子句选择rpd.name？

我调查了CASE，但我不确定如何使用它。

table replacement_part

part_id href
1 url_1
2 url_2
3 url_3

table replacement_part_description

part_id language_id 名称
1 2
1 1软管
2 2
2 1控制模块
3 2 vonkajsi kryt
3 1外壳

预期产出

part_id href 名称
1 url_1软管
2 url_2控制模块
3 url_3 vonkajsi kryt

SELECT *
FROM replacement_part AS rp
LEFT JOIN  replacement_part_description AS rpd
    ON (rp.part_id = rpd.part_id)
WHERE language_id = :id

类似

if rpd.name = ''
    WHERE language_id = a,
else
    WHERE language_id = b    

Answer 1

这个？

SELECT *
FROM replacement_part AS rp
LEFT JOIN  replacement_part_description AS rpd
    ON (rp.part_id = rpd.part_id)
WHERE 
    (
        language_id = :id 
        AND (rpd.name = '' OR rpd.name IS NULL)
    )
    OR language_id = b   

Answer 2

您可以使用order by和limit获得一行优先级：

select . . .
from . . .
where (language_id = a and rpd.name = '') or
      language_id = b
order by (language_id = a) desc -- put the "a" rows first
limit 1;

Answer 3

一种方法是两次加入rpd表，在ON条件上移动where子句，然后使用COALESCE：

SELECT
  ...,
  COALESCE(rpd1.name, rpd2.name) AS name
FROM
  replacement_part rp LEFT JOIN replacement_part_description rpd1
  ON rp.part_id = rpd1.part_id AND rpd1.language=a
  LEFT JOIN replacement_part_description rpd2
  ON rp.part_id = prd2.part_id AND rpd2.language=b

这里我假设语言在描述表上。如果name是一个空字符串，而不是合并，则可以使用CASE WHEN：

CASE WHEN rpd1.name='' OR rpd1.name IS NULL THEN rpd2.name ELSE rpd1.name END AS Name