将d.userid = 1谓词放在ON子句中：
select c.onlinetestid, d.userid, d.status
from onlinetest c
left join userstatus d on d.onlinetestid = c.onlinetestid and d.userid = 1
where c.category = 'English'
这将返回的所有行onlinetest，其中谓词失败的列userstatus填充为。null``d.userid = 1
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