How to add a condition for an associated table in an SQL statement [null value required or not required]

Catalog

1. Unwanted null value

 2. need null value   

Didn't understand the message , Baidu search for a long time basically wants few

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1. Unwanted null value

hold and Put the condition at the end

SELECT
            ad.dept_id,
            ad.dept_name,
            au.id,
            au.realname
        FROM
             user_post aup 
                LEFT JOIN dept ad ON aup.dept_id = ad.dept_id
				left join build_rec_user abru 
on abru.order_id=ad.dept_id    
                LEFT JOIN user au  ON au.id = abru.user_id
        WHERE ad.company_id = 100   and abru.type_id = 0

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 2. need null value   

In fact, in the left join ... on Add later   and Conditions = ? Screening Associated library condition

Example : left join build_rec_user abru 
            on abru.order_id=ad.dept_id  and  abru.type_id = 0 

SELECT
            ad.dept_id,
            ad.dept_name,
            au.id,
            au.realname
        FROM
             user_post aup 
                LEFT JOIN dept ad ON aup.dept_id = ad.dept_id
				left join build_rec_user abru 
            on abru.order_id=ad.dept_id  
and abru.type_id = 0  
                LEFT JOIN user au  ON au.id = abru.user_id
        WHERE ad.company_id = 100  

Didn't understand the message , Baidu search for a long time basically wants few