Question

如何检查特定列是否为null然后忽略查询的某些部分

我想得到的结果包括这两行，但如果user_id为null则无法查询

SELECT 
  mc.* FROM media_credit mc
  LEFT JOIN credit c ON c.id = mc.credit_id
  LEFT JOIN "user" u ON u.id = mc.user_id
  LEFT JOIN "user" _u ON u.id = c.create_by_user_id
  WHERE mc.media_id = $1
    c.status = $2
    AND u.status = $2   // how to make if mc.user_id is null then dont use this 
    AND _u.status = $2

表

media_credit
id | media_id | credit_id | username | user_id
1  | 1        | 1         | xx       | null
2  | 1        | 1         | null     |  1

user
id | status
1  | 0

credit
id | status | create_by_user_id
1  | 0      | 32

Answer 1

您应该将WHERE子句中的其他条件添加到ON子句中：

SELECT mc.* 
FROM media_credit mc
LEFT JOIN credit c 
    ON c.id = mc.credit_id
    AND c.status = $2
LEFT JOIN "user" u 
    ON u.id = mc.user_id
    AND u.status = $2
LEFT JOIN "user" _u 
    ON u.id = c.create_by_user_id
    AND _u.status = $2
WHERE 
    mc.media_id = $1

当您过滤WHERE子句中的右表时，您的LEFT JOIN将转换为INNER JOIN，从而删除不匹配的行。

Answer 2

您可以在where子句中使用CASE语句...如果mc.user_id为NULL，则$ 2 = $ 2有效删除AND子句

SELECT 
  mc.* FROM media_credit mc
  LEFT JOIN credit c ON c.id = mc.credit_id
  LEFT JOIN "user" u ON u.id = mc.user_id
  LEFT JOIN "user" _u ON u.id = c.create_by_user_id
  WHERE mc.media_id = $1
    c.status = $2
    AND _u.status = $2
    AND CASE WHEN mc.user_id is NULL THEN $2
        ELSE u.status END = $2

如何检查特定列是否为null然后忽略查询的某些部分

2 个答案: