如何做检查，如果特定列null，则忽略查询

Question

的某些部分如何做检查，如果特定列null，则忽略查询的某些部分

我想要得到的结果包括这两个行，但低于查询，如果user_id为null，则不行

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

你应该把其他条件从WHERE条款到ON条款：

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 

当过滤WHERE子句中的右表，您LEFT JOIN将被改造成一个INNER JOIN，从而丢弃不匹配的行。

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有效去除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